CA1337852C

CA1337852C - Combination of photosensitive elements for use in radiography

Info

Publication number: CA1337852C
Application number: CA000605729A
Authority: CA
Inventors: Sergio Pesce
Original assignee: Minnesota Mining and Manufacturing Co
Current assignee: 3M Co
Priority date: 1988-07-14
Filing date: 1989-07-14
Publication date: 1996-01-02
Anticipated expiration: 2013-01-02
Also published as: IT8821361A0; IT1226917B; EP0350883A3; DE68925440D1; AU619309B2; EP0350883B1; JP2837878B2; DE68925440T2; JPH02110538A; AU3712189A; BR8903451A; EP0350883A2

Abstract

Combination of photosensitive elements for use in radiography consisting of a pair of radiographic fluores-cent screens and a double-side coated silver halide ra-diographic element. Each screen is arranged adjacent to each silver halide layer and each screen is capable of imagewise emitting radiation to which the adjacent silver halide layer is sensitive when imagewise exposed to X
radiation. The combination is characterized in that one fluorescent screen emits a first radiation in a first wavelength region of the electromagnetic spectrum and the other fluorescent screen emits a second radiation in a second wavelength region of the electromagnetic spectrum, and each silver nalide layer is substantially insensitive to the radiation emitted by the opposite screen.

Description

43309CANlA

Combination of Photosensitive Elements for Use in Radiography FIELD OF THE INVENTION

The present invention relates to a combination of photosensitive elements for use in radiography and, more in particular, to a combination of a fluorescent phosphor screen pair and a double-side coated silver halide radio-graphic element for use in industrial or medical radiog-raphy.

~ACKC~ JNI) OF THE INVENI'ION

In radiography, and particularly in medical radiog-raphy, light-sensitive elements having silver halide emulsion layers coated on both faces of a transparent support (called double-side coated silver halide ele-ments) are used. Said double coated silver halide ele-ments are generally used in combination with fluorescent phosphor screens in order to reduce the X radiation expo-sure necessary to obtain the required image. Generally, one fluorescent phosphor screen is used in association with each silver halide emulsion layer of the double coated element. The silver halide emulsions used in the double coated element are sensitized to a region of the electromagnetic spectrum corresponding to the wave length of the light emitted by the phosphor materials used in the fluorescent phosphor screens, thus obt~; n ing a sig-nificant amplification factor.
The quality of the image obtained upon X radiation exposure of said screen pair and double coated silver halide element combination and development of said double coated silver halide element is negatively affected by crossover exposure. Crossover exposure, which causes a *

1 3378~2 reductlon in imaqe sharpness, occurs in double coated silver halide elements when llght emltted by one fluorescent phosphor screen passes through the ad~acent silver halide emulslon layer and, the light havlng been spread by the support, image-wise exposes the sllver halide emulsion layer on the opposite face of the support.
The crossover exposure causes poor definition even if light-sensitive elements are used which employ reduced silver halide coverages to lower the costs or increase the processing speed of the element. In fact, the decrease of the emulsion turbidity increases the amount of light available for cross-over and therefore worsens the image.
To reduce the crossover exposure, dyes or pigments can be used within the radiographic element. The absorption of said dyes or pigments is in a region of the electromagnetic spectrum corresponding to the wavelength of the light emitted by the fluorescent phosphor screens. The dyes or pigments absorb some of the light emitted by the fluorescent phosphor screen so that imaging of a silver halide emulsion layer by the opposite screen is reduced by absorbance of the light from the opposite screen by the anticrossover dyes or pigments.
These dyes or pigments are eliminated during the photographic developing, fixing and washing process of the exposed mater-ial; they can be for instance washed away or, more preferably, bleached while processing the radiographic element.
The dyes can be incorporated in any layer of the light-sensltive element: in the emulsion layer, ln an inter-mediate layer between the emulslon and the base, or ln the subbing layer of the support base. It is preferred to incor-porate the dyes in a layer different from that containing theemulsion to avoid possible desensitization phenomena. Since 1978, Minnesota Mlning and Manufacturing Company has sold a radiographic element under the name of 3M Trimax M Type XUD X-Ray Film to be used in combination with 3M
Trimax Intensifying Screens. Such radiographic element comprises a transparent polyester base, each surface of which has a silver halide emulsion layer sensitized to the light emitted by the screens. Between the emulsion and the base is a gelatin layer containing water-soluble acid dyes, which dyes can be decolorized during processing and have an absorption in a region of the electromagnetic spectrum corresponding to the wavelength of the light emitted by the screens and of the spectral sensitivity of the emulsion. The dyes are anchored in the layer by means of a basic mordant consisting of polyvinylpyridine.
In the practical solution of reducing the crossover exposure by using a mordanted dye layer (as described for instance in the European Patent Application 101,295, published on February 22, 1984 in the name of Konishiroku Photo KK), some problems are created which up to now have not yet been solved properly. In fact, the improvement of image definition involves not only a natural decrease of the light-sensitive element sensitivity caused by the absorption of the transmitted light which otherwise would take part in the formation of a part of the image, but also the possibility of desensitization phenomena due to the migration of dye not firmly mordanted in the silver halide emulsion layer.
There is also a problem with residual stain even after processing, the retention of significant quantities of thiosulfate from the fixing bath which causes image yellowing upon long-time storage on shelf, and lengthening of the drying times after processing because of element thickening.
Other approaches have been suggested to reduce crossover, as reported hereinafter.
US Patent 3,923,515 discloses a relatively lower speed silver halide emulsion between the support and a higher speed silver halide emulsion layer to reduce crossover.

US Patent 4,639,411 discloses a photographic element, to be used with blue emitting intensifying screens, having reduced crossover, said element comprising coated on both sides of a transparent support a blue sensitive silver halide emulsion layer and, interposed between the support and the emulsion layer, a blue absorbing layer comprising bright yellow silver iodide grains of a specific crystal structure.
Japanese Patent Application 62-52546, published on March 7, 1987 in the name of Konishiroku Photo KK, discloses a radiographic element of improved image quality comprising coated on both sides of a transparent support a light sensitive silver halide emulsion layer and, interposed between the support and the emulsion layer, a layer containing water insoluble metal salt particles having adsorbed on their surface a dye. Said dye has a maximum absorption within the range of _ 20 nm of the maximum absorption of said silver halide and corresponds to the light emitted by intensifying screens. Silver halides are disclosed as preferred metal salt particles.
Japanese Patent Application 62-99748, published on May 9, 1987 in the name of Fuji Photographic Film KK, discloses a radiographic element of improved image quality comprising coated on both sides of a transparent support a light-sensitive silver halide emulsion layer and, interposed between the support and the emulsion layer, a silver halide emulsion layer having substantially no light-sensitivity.
The approaches of using light-insensitive silver halide layers as anticrossover layers interposed between the support and the light-sensitive silver halide emulsion layers, although preferred to using dyes or pigments, encounter some problems such as the increase of silver coverage and bad bleaching characteristics in photographic processing (residual stain).
The following are additional documents illustrating the state of the art.
BE 757,815 dlscloses a comblnation of a silver halide element and an intensifying screen comprlsing a fluorescent compound emittlng llght of wavelength less than 410 nm.
US 4,130,428 discloses a combination of two fluor-escent screens and a double coated silver halide element wherein the maximum emisslon of the screens is in the wave-length range of 450-570 nm and silver hallde layers are sensl-tlve to llght ln the same wavelength range.
US 3,795,814, 4,070,583 and GB 2,119,396 disclose rare earth oxyhalide phosphors activated wlth terbium and/or thulium employed ln fluorescent screens having UV emission.
FR 2,264,306 discloses a combination of a silver halide element and fluorescent screen comprising a rare earth actlvated rare earth oxyhalide phosphor having its maximum emlsslon in the wavelength range of 400-500 nm.
EP 88,820 discloses a radiographlc fluorescent screen comprlslng a flrst blue emltting phosphor layer and a second green emittlng phosphor layer to be combined with a silver halide element having spectral sensitivity in the blue-green region ~"ortho-type" elements).
JP 60175-000 discloses a combination of a double coated silver halide element and a screen pair wherein the fluorescent layers of the two screens have different wave-length region emisslons and each screen comprises an organic dye to absorb the light emitted by the opposite screen.
EP 232,888 discloses a combination of a double coated sllver halide element and a pair of front and back intensify-ing screens whereln sald front and back screens, emlttlng llght in the same wavelength region, have different modulatlon transfer factors to be used in low energy radlography.
US 4,480,024 dlscloses the combinatlon of a silver hallde photothermographic element and a rare-earth intensify-lng screen which are unlquely adapted to one another for the purpose of industrial radiographic lmaglng. The photothermo-graphlc element ls dye-sensltlzed to the spectral emlsslon of the screen and the combination of screen and element has an amplificatlon factor greater or equal to at least 50. Accord-ing to this lnvention preferably a single screen is used in combination wlth a slngle-side coated photothermographic ele-ment, or double screens wlth elther single-side or a double-slde coated photothermographic elements, the latter wlthoutany slgnlficant benefit and at increased cost of film.

SUMMARY OF THE INVENTION
The present invention provldes a combinatlon of a pair of radlographic fluorescent screens and a double coated silver hallde radlographic element. Each screen is arranged ad~acent to each silver hallde layer and each screen is capable of imagewlse emitting radlation to which the ad~acent sllver hallde layer ls sensltlve when lmagewise exposed to X radi-ation. The combination ls characterlzed ln that one fluor-escent screen emits a flrst radlatlon ln a first wavelengthregion of the electromagnetlc spectrum and the other fluor-escent screen emits a second radiation in a second wavelength region of the electromagnetlc spectrum, and each silver hallde layer ls substantlally lnsensltlve to the radlatlon emltted by the opposite screen.
In partlcular, the present lnventlon provldes for a comblnatlon of a palr of front and back radlographlc fluor-escent screens and a double coated silver hallde radlographlc element, whereln the flrst radlatlon emltted by sald front screen has a wavelength which dlffers from the second radl-ation emitted by sald back screen by at least 50 nm, and each sllver hallde layer ls substantlally lnsensltlve to the radl-ation emltted by the opposlte screen.
More ln partlcular, the present lnventlon provldes for a comblnatlon of a palr of front and back radlographlc fluorescent screens and a double coated sllver hallde radio-graphic element, wherein sald front screen comprlses a non-actinic radlatlon, preferably green radiatlon, emlttlng phosphor and is arranged ad~acent to a front silver hallde layer comprlsing a sllver hallde emulslon spectrally sensl-tlzed to the llght emltted by sald front screen, and sald backscreen comprlses an actlnlc radlatlon, preferably UV radl-atlon, emlttlng phosphor and ls arranged ad~acent to a back silver hallde layer comprlslng a sllver hallde emulslon lnsensltlzed to the vlsually observable reglons of the elec-tromagnetlc spectrum.
The comblnatlon of screen palr and double coated sllver halide element for use ln radlography accordlng to the present lnventlon provldes lmages havlng superlor lmage quallty, partlcularly less crossover, as compared to conven-tlonal radlographlc screen pair and double sllver hallde ele-ment comblnatlons wlthout causlng negatlve effects, such as slgnlflcant loss of sensltlvlty, resldual staln, lmage lnsta-blllty upon storage, excesslve element thickenlng and increas-ed sllver coverage. The comblnation may be used in lndustrlal or medical radlography.

BRIEF DESCRIPTION OF THE DRAWIN~S
Flgure 1 ls a schematlc dlagram of a radlographlc element and screen palr comblnatlon of the present lnventlon.
Flgures 2 and 4 are graphs lllustratlng emlsslon spectra of radiographic fluorescent screens of the present lnventlon.

Figures 3 and 5 are graphs lllustrating spectral sensitivity of a double-side coated silver hallde radlographlc element of the present lnvention.
Figures 6 and 7 are graphs illustrating sharpness and granularity versus sensitivity of radiographic double coated silver halide element and screen pair combinations.

DETAILED DESCRIPTION OF THE INVENTION
The present invention will be now described in detail.
Figure 1 shows in greater detail a combinatlon of the screen pair and the double silver halide element of this in-ventlon. The comblnatlon comprises three separate photosen-sitive elements a double coated silver halide radiographic element 10, a front screen 21 and a back screen 20.
As shown, the double coated silver halide radio-graphic element 10 comprises a support 11 and coated on its opposite faces are the subbing layers 12 and 13. A front silver halide emulsion layer 15 is coated over the subbing layer 13 and a back silver halide emulsion layer 14 is coated over the subbing layer 12 on the opposite face of the support.
Protective layers 16 and 17 are coated over the silver halide emulsion layers 14 and 15, respectively.
As shown, the front radiographic fluorescent screen 21 comprises a support 29, a reflectlve layer 27, a fluor-escent phosphor layer 25 and a protective layer Z3. Slmilar-ly, the back radlographlc fluorescent screen 20 comprlses a support 28, a reflectlve layer 26, a fluorescent phosphor layer 24 and a protectlve layer 22.
In practlcal use, the screen palr and the sllver halide element are compressed in a radlographic cassette wlth the front screen arranged ad~acent and ln close contact wlth the front sllver hallde emulslon layer and the back screen ls arranged ad~acent and ln close contact wlth the back sllver emulslon hallde layer. Imagewlse X radlatlon enters the screen palr and sllver hallde element comblnatlon through the front screen support 29 and reflectlve layer 27 and passes the front screen fluorescent phosphor layer 25. A portlon of the X radlatlon ls absorbed ln the phosphor layer 25. The remaln-lng X radlatlon passes through the protectlve layers 23 and 17. A small portlon of the X radlatlon ls absorbed ln the front sllver hallde emulslon layer 15, thereby contrlbutlng dlrectly to the formatlon of a latent lmage ln sald front sllver hallde emulslon layer 15. The ma~or portlon of the X
radlatlon passes through the subblng layer 13, the support 11 and the subblng layer 12. Agaln, a small portlon of the X
radlatlon ls absorbed ln the back sllver hallde emulslon layer 14, thereby contrlbutlng dlrectly to the formatlon of a latent lmage ln sald back sllver hallde emulslon layer 14. Agaln, the ma~or portlon of the X radlatlon passes through the pro-tectlve layers 16 and 22 and ls absorbed ln the back fluor-escent phosphor layer 24. The lmagewlse X radlatlon ls prln-clpally absorbed ln the fluorescent phosphor layers 24 and 25, thereby produclng the emlsslon of longer wavelength radlatlon.
Accordlng to the present lnventlon, the flrst radlatlon emltted by the front fluorescent phosphor layer 25 exposes the ad~acent front sllver hallde emulslon layer 15, and the second radiatlon emltted by the back fluorescent phosphor layer 24 exposes the adiacent back sllver hallde emulslon layer 14.
The sllver hallde emulslons are substantlally lnsensltlve to the radlation emltted by the opposlte fluorescent phosphor layer. Sald radlatlon emltted by a fluorescent phosphor layer passlng to at least some extent beyond the ad~acent sllver hallde emulslon layer penetrates the subblng layers and the support to expose the opposite silver halide emulslon layer.
Thls fact, while lncreaslng to some extent the speed of the screen pair and silver halide element combination, would have the effect of impairing the image sharpness by crossover exposure. The terms "actinlc" and "non-actinic" radiation according to the present invention are used to indicate, respectively, radiation of wavelength shorter than 500 nm (Ultravlolet and blue radiation), preferably from 300 to less than 500 nm, and radiation of wavelength from 500 nm upwards (green, red and Infrared radiation), preferably from 500 to 1200 nm. The term "insensitlve" as used herein, describes either primary or intrlnslc insensltlvlty of the sllver hallde grain emulsion (or layer includlng it) to a certain range of wavelengths, as defined, or secondary or induced insensitivity (or unreachability) of the silver halide emulsion (or layer lncludlng lt) in the double silver halide element because of filter action exercised by a further emulsion layer or layers interposed between the considered "lnsensitlve" layer and the radiation emitting screen or by filter dyes or agents included in the considered layer or in such interposed layers. Accord-ingly, the latent image formed by radiation (preferably com-prlsed between 300 and 1200 nm) exposure ln each sllver halide emulsion layer is primarily formed by exposure to the radi-ation emitted by the ad~acent fluorescent phosphor layer, with no significant contribution by opposite screen. Preferably, the radiation exposure necessary to produce a Dmax of 1.0 on said front silver halide layer will produce a Dmax of less than 0.2 on the back silver halide emulsion layer under the same development conditions. Conversely, an exposure at the ~max of the back layer that produces a Dmax of 1.0 will pro-duce a Dmax of less than 0.2 on the front layer.
The terms "front" and "back" ln the present inventlon X

are used to dlstlngulsh the dlfferent elements of the unsym-metrlcal screen palr and double coated sllver hallde element comblnatlon not thelr posltlon relatlng to exposlng X radl-atlon source. Accordlngly, X radlatlon may enter the unsym-metrlcal screen palr and double-slde coated sllver hallde element through elther the front fluorescent phosphor screen or the back fluorescent phosphor screen.
The term "sllver hallde element" ln the present inventlon lncludes both sllver hallde "photographlc" elements whlch use llquld development to produce the flnal lmage and sllver hallde "photothermographlc" elements, often referred to as "dry sllver" elements, whlch do not use llquld development to produce the flnal lmage, as descrlbed therelnafter. In both photographlc and photothermographlc sllver hallde ele-ments, exposure of the sllver hallde to radlatlon produces small clusters of sllver lons. The lmagewlse dlstrlbutlon of these clusters ls known ln the art as the latent lmage. Thls latent lmage generally ls not vlslble by ordlnary means and the llght-sensltlve element must be further processed ln order to produce a vlsual lmage. Thls vlsual lmage ls produced by the catalytlc reductlon of sllver whlch ls ln catalytlc prox-lmlty to the specks of the latent lmage. The photographlc sllver hallde element ls preferably used ln medlcal radlo-graphy and the photothermographlc sllver hallde element ls preferably used ln lndustrlal radlography.
Accordlngly, the present lnventlon relates to a comblnatlon of photosensltlve elements for use ln radiography comprlslng two separate front and back X-ray fluorescent screens and a sllver hallde radlographlc element comprlslng a support base and front and back sllver hallde emulslon layers each coated on one surface of sald support, whereln sald front screen ls arranged ad~acent to sald front sllver hallde layer - ~ 3 3 7 8 5 2 and sald back screen is arranged ad~acent to said back sllver halide layer, and wherein 1) said front screen comprises a flrst radiatlon emltting phosphor and sald front sllver halide layer comprises sllver halide grains sensltlve to sald flrst radlatlon emitted by sald front screen, and 2) sald back screen comprlses a second radlatlon emlttlng phosphor and sald back sllver hallde layer comprlses sllver hallde gralns sensltlve to sald second radlatlon emltted by sald back screen, characterlzed ln that a) sald flrst radlatlon emltted by sald front screen has a wavelength whlch dlffers from sald second radiatlon emltted by sald back screen by at least 50 nm, b) sald front sllver halide emulsion layer ls substan-tially lnsensitive to sald second radlatlon emltted by sald back screen, and c) sald back sllver hallde emulslon layer ls substan-tially lnsensltlve to sald flrst radlatlon emltted by sald front screen, the dlfference ln wavelength reglon of sald flrst and second radlatlons and the lnsensltlvlty of each sllver hallde layer to radlatlon emltted by opposite screen belng such to reduce crossover exposure of at least 10 percent when compared wlth a symmetrlcal combinatlon of a palr of green llght emlttlng fluorescent screens and a double coated green sensitlzed sllver hallde emulslon radlographlc element.
Accordlng to a preferred embodlment of thls lnven-tlon, ln the comblnatlon of a palr of front and back radlo-graphlc fluorescent screens and a double coated sllver hallde radlographlc element, sald front screen comprlses a non-actlnlc radlatlon, preferably green llght, emlttlng phosphorand ls arranged ad~acent to a front sllver hallde layer com-prlslng a sllver hallde emulslon spectrally sensltlzed to the 'X

radlatlon emitted by sald front screen, and said back screen comprlses an actlnlc radlatlon, preferably UV llght, emlttlng phosphor and ls arranged ad~acent to a back sllver hallde layer comprlslng a sllver hallde emulslon lnsensltlzed to the vlsually observable reglons of the electromagnetlc spectrum (that ls 410 to 750 nm).
Preferably, the phosphors used in the front fluor-escent screens applled ln the present lnventlon emlt radlatlon ln the green or red reglon of the vlslble spectrum. More preferably, sald phosphors emlt radiation in the green region of the visible spectrum. Most preferably, said phosphors emit radlation havlng more than about 80% of its spectral emission above 480 nm and lts maxlmum of emlssion ln the wavelength range of 530-570 nm. Green emlttlng phosphors whlch may be used ln the front fluorescent screens of the present inventlon lnclude rare earth actlvated rare earth oxysulflde phosphors of at least one rare earth element selected from yttrlum, lanthanum, gadollnlum and lutetlum, rare earth actlvated rare earth oxyhallde phosphors of the same rare earth elements, a phosphor composed of a borate of the above rare earth ele-ments, a phosphor composed of a phosphate of the above rare earth elements and a phosphor composed of tantalate of the above rare earth elements. These rare earth green emlttlng phosphors have been extenslvely descrlbed ln the Patent llt-erature, for example in US Patents 4,225,653, 3,418,246, 3,418,247, 3,725,704, 3,617,743, 3,974,389, 3,591,516, 3,607,770, 3,666,676, 3,795,814, 4,405,691, 4,311,487 and 4,387,141. These rare earth phosphors have a high X-ray stop-plng power and high efficlency of light emission when exclted wlth X radiation and enable radlologlsts to use substantially lower X radiatlon dosage levels. Particularly suitable phosphors for use in the front fluorescent screens of the present inventlon are terblum or terblum-thulium actlvated rare earth oxysulflde phosphors represented by the general formula (I) (Lnl a-b~ Tba' Tmb)22 (I) whereln Ln ls at least one rare earth element selected from lanthanum, gadollnlum and lutetlum, and a and b are numbers such as to meet the condltlons 0.0005 ~ a < 0.09 and 0 < b <
0.01, respectlvely, and terblum or terblum-thullum actlvated rare earth oxysulflde phosphors represented by the general formula (II) ( l-c-a-b~ Lnc~ Tba~ Tmb)2O2S (II) whereln Ln ls at least one rare earth element selected from lanthanum, gadollnlum and lutetlum, and a, b and c are numbers such as to meet the condltlons 0.0005 < a 5 0 . 09, 0 s b s 0.01 and 0.65 < c < 0.95, respectlvely.
Figure 2 shows an emisslon spectrum of a front fluor-escent screen comprlslng a fluorescent layer of (Gdl 0 05, Tbo 05)202S phosphor as green emlttlng phosphor, expressed as fluorescence (F) versus wavelengths (nm).
The front sllver hallde emulslon layer, to be arranged accordlng to thls lnventlon ad~acent to the front fluorescent screen, comprises sllver hallde gralns whlch are optlcally sensltlzed to the spectral reglon of the radlatlon emitted by the screens, preferably to a spectral region of an lnterval comprised wlthln 25 nm from the wavelength of the maxlmum emlsslon of the screen, more preferably within 15 nm, and most preferably wlthin 10 nm. The silver halide grains of the front silver halide layer have adsorbed on their surface spectral sensltizlng dyes that exhlblt absorptlon maxlma ln the regions of the visible spectrum where the front fluor-escent screen emits. Preferably, spectral sensitizing dyes according to this invention are those whlch exhibit J aggre-gates if adsorbed on the surface of the silver halide grains and a sharp absorption band (J-band) with a bathocromic shift-ing with respect to the absorption maxlmum of the free dye ln aqueous solutlon. Spectral sensitislng dyes produclng J
aggregates are well known ln the art, as illustrated by F.M.
Hamer, Cyanine Dyes and Related Compounds, John Wlley and Sons, 1964, Chapter XVII and by T.H. James, The Theory of the Photographic Process, 4th edition, Macmillan, 1977, Chapter 8.
In a preferred form, J-band exhiblting dyes are cyanine dyes. Such dyes comprise two baslc heterocycllc nuclel ~olned by a linkage of methine groups. The hetero-cyclic nuclel preferably lnclude fused benzene rings to enhance J aggregatlon.
The heterocycllc nuclei are preferably quinolinium, benzoxazolium, benzothiazolium, benzoselenazollum, benzlmlda-zolium, naphthoxazolium, naphthothlazolium and naphthoselenazolium quaternary salts.
J-band type dyes preferably used in the present invention have the followlng general formula (III):

I Zl ~ IR3 R4R~5 l-----z2-----~
Rl-N(-CH=CH)p~C=C(~C=C)m~C=(=CH-CH=)g=N -R2 (III) (A )k (B-) whereln Zl and Z2 may be the same or different and each represents the elements necessary to complete a cycllc nucleus derlved from baslc heterocycllc nitrogen compounds such as oxazoline, oxazole, benzoxazole, the naphthoxazoles (e.g., naphth{2,1-d}oxazole, naphth{2,3-d}oxazole, and naphth{1,2-d}
oxazole), thlazoline, thiazole, benzothiazole, the naphtho-thiazoles (e.g., naphtho{2,1-d}thiazole), the thiazoloquino-lines (e.g., thlazolo{4,5-b}quinoline), selenazoline, selen-azole, benzoselenazole, the naphthoselenazoles (e.g., naphtho {1,2-d}selenazole, 3H-indole (e.g., 3,3-dimethyl-3H-indole), the benzlndoles (e.g., l,1-dlmethylbenzlndole), imldazollne, lmldazole, benzlmidazole, the naphthlmidazoles (e.g., naphth {2,3-d}-imidazole), pyridlne, and gulnoline, whlch nuclel may be substituetd on the rlng by one or more of a wlde variety of substituents such as hydroxy, the halogens (e.g., fluoro, bromo, chloro, and iodo), alkyl groups or substituted alkyl groups (e.g., methyl, ethyl, propyl, lsopropyl, butyl, octyl, dodecyl, 2-hydroxy-ethyl, 3-sulfopropyl, carboxymethyl, 2-cyanoethyl, and trifluoromethyl), aryl groups or substituted aryl groups (e.g., phenyl, l-naphthyl, 2-naphthyl, 4-sulfo-phenyl, 3-carboxy-phenyl, and 4-biphenyl), aralkyl groups (e.g., benzyl and phenethyl), alkoxy groups (e.g., methoxy, ethoxy, and isopropoxy), aryloxy groups (e.g., phenoxy and l-naphthoxy), alkylthlo groups (e.g., ethylthlo and methyl-thio), arylthio groups (e.g., phenylthio, p-tolythlo, and 2-naphthylthlo), methylenedloxy, cyano, 2-thlenyl, styryl, amlno or substltuted amlno groups (e.g., anlllno, dlmethyl-anllino, dlethylanlllno, and morphollno), acyl groups (e.g., acetyl and benzoyl), and sulfo groups, R1 and R2 can be the same or dlfferent and represent alkyl groups, aryl groups, alkenyl groups, or aralkyl groups, wlth or without substltuents, (e.g., carboxymethyl, 2-hydroxy-ethyl, 3-sulfopropyl, 3-sulfobutyl, 4-sulfobutyl, 2-methoxy-ethyl, 2-sulfatoethyl, 3-thlosulfatoethyl, 2-phosphonoethyl, chlorophenyl, and bromophenyl), R3 represents a hydrogen atom, 1 337852 R4 and R5 can be the same or different and represent a hydrogen atom or a lower alkyl group of from 1 to 4 carbon atoms, p and q are 0 or 1, except that both p and q prefer-ably are not 1, m ls 0 or 1 except that when m ls 1 both p and q are 0 and at least one of Zl and Z2 represents lmldazoline, oxazollne, thlazollne, or selenazollne, A ls an anlonlc group, B ls a cationlc group, and k and 1 may be 0 or 1, dependlng on whether lonlc substltuents are present. Varlants are, of course, posslble ln whlch Rl and R3, R2 and R5, or Rl and R2 together represent the atoms necessary to complete an alkylene brldge.
In the most preferred form of thls lnventlon, whereln the phosphors of the front fluorescent screen are rare earth phosphors emlttlng in the green reglon of the vlslble spec-trum, sald optical sensitizlng dyes adsorbed on said silver halide gralns of the front sllver hallde layer are represented by the following general formula (IV):

~-C~=C-C~= ~ (X )nl Rll R12 wher-eln Rlo represents a hydrogen atom or a lower alkyl group of from 1 to 4 carbon atoms (e.g. methyl, and ethyl), R6, R7, R8 and Rg each represents a hydrogen atom, a halogen atom (e.g. chloro, bromo, iodo, and fluoro), a hydroxy group, an alkoxy group ~e.g. methoxy and ethoxy), an amlno group (e.g. amino, methylamlno, and dlmethylamlno), an acyl-amino group (e.g. acetamido and proplonamido), an acyloxy group (e.g. acetoxy group), an alkoxycarbonyl group (e.g.
methoxycarbonyl, ethoxycarbonyl, and butoxycarbonyl), an alkyl group (eg. methyl, ethyl, and isopropyl), an alkoxycarbonyl-amino group (e.g. ethoxycarbonylamino) or an aryl group (e.g.
phenyl and tolyl), or, together, R6 and R7 and, respectively, R8 and Rg can be the atoms necessary to complete a benzene ring (so that the heterocyclic nucleus results to be, for example, an a-naphthoxazole nucleus, a ~-naphthoxazole or a ~,~'-naphthoxazole), R11 and R12 each represents an alkyl group (e.g. methyl, propyl, and butyl), a hydroxyalkyl group (e.g. 2-hydroxy-ethyl, 3-hydroxypropyl, and 4-hydroxybutyl), an acetoxyalkyl group (e.g. 2-acetoxyethyl and 4-acetoxybutyl), an alkoxyalkyl group le.g. 2-methoxyethyl and 3-methoxypropyl), a carboxyl group containing alkyl group (e.g. carboxymethyl, 2-carboxy-ethyl, 4-carboxybutyl, and 2-(2-carboxyethoxy)-ethyl), a sulfo group containing alkyl group (e.g. 2-sulfoethyl, 3-sulfo-propyl, 4-sulfobutyl, 2-hydroxy, 3-sulfopropyl, 2-(3-sulfo-propoxy)-propyl, p-sulfobenzyl, and p-sulfophenethyl), a benzyl group, a phenethyl group, a vinylmethyl group, and the like, X represents an acid anion (e.g. a chloride, bromide, iodide, thiocyanate, methylsulfate, ethylsulfate, perchlorate, and p-toluenesulfonate ion), and n represents 1 or 2.
The alkyl groups included in sald substltuents R6, R7, R8, Rg, R1ol and R11 and, more partlcularly, the alkyl portions of said alkoxy, alkoxycarbonyl, alkoxycarbonylamino, 1 3~7852 hydroxyalkyl, acetoxyalkyl groups and of the alkyl groups as-soclated with a carboxy or sulfo group each preferably contain from 1 to 12, more preferably from 1 to 4 carbon atoms, the total number of carbon atoms included in said groups preferab-ly being no more than 20.
The aryl groups included in said substituents R6, R7 R8 and Rg each preferably contain from 6 to 18, more prefer-ably from 6 to 10 carbon atoms, the total number of carbon atoms included ln sald groups arrlvlng up to 20 carbon atoms.
The following are specific examples of J-band sensl-tlzlng dyes belonglng to those represented by the general for-mula (IV) above:
Dye Rlo R6 R7 8 9 Rll R12 Xn A C2H5 H 5-Cl H 5'-Cl (CH2)3S03 ~CH2)3S03 -1 **
B C2H5 H 5-Cl H 5'-C6H5 (CH2)3S03 CH3 -1 ( 2)2 3 C CH3 H 5-OCH3 H 5'-OCH3 C2H5(CH2)3S03 -1 ~ C2H5 6-CH3 5-Cl H 5'-Cl (CH2)4S03 ( 2)3 3 -1 ____ C2H5 __ ____________ ________ ________ _________________ _ ___________ ________,___-_ * Triethylamine salt ** Sodium salt Figure 3 shows the sensitivity spectrum of a front silver halide layer comprising silver bromoiodlde gralns com-prlslng 2.3 mole percent lodlde and havlng adsorbed on thelr surface the optlcal sensltizlng dye A above, expressed as sensltivity (S) versus wavelengths (nm).
Preferably, the phosphors used ln the back fluor-escent screens applled in the present lnvention emit radiation in the ultraviolet region of the electromagnetic spectrum.
More preferably, sald phosphors emit radlatlon havlng more than about 80% of their spectral emission below 410 nm and their maximum of emission ln the wavelength range of 300-360 nm. Ultraviolet emitting phosphors which may be used in the back fluorescent screens of the present invention include ultraviolet emitting phosphors known in the art such as lead or lanthanum activated barium sulfate phosphors, barium fluorohalide phosphors, lead activated barium silicate phos-phors, gadolinium activated yttrium oxide phosphors, bariumfluoride phosphors, alkali metal activated rare earth niobate or tantalate phosphors etc. Ultravlolet emitting phosphors are described for example in BE 703,998 and 757,815, in EP
202,875 and by Buchanan et al., J. Applled Physics, vol. 9, 4342-4347, 1968,and by Clapp and Glnther, J. of the Optical Soc. of America, vol. 37, 355-362, 1947. Partlcularly sult-able ultravlolet emlttlng phosphors for use ln the back, fluorescent screens of the present lnventlon are those repre-sented by the general formula (IV) (Y1~2/3X-l/3yrsrxlLly) TaO4 wherein x and y are numbers such as to meet the condltions < x < 1 and 10 c y < 0.1 as described ln EP 202,875.
Figure 4 shows an emission spectrum of a back fluor-escent screen comprislng a fluorescent layer of (Y, Sr, Ll) TaO4 phosphor as ultraviolet emitting phosphor, expressed as fluorescence (F) versus wavelengths (nm).
The back silver hallde emulsion layer, arranged according to thls lnventlon adjacent to the back actlnlc llght emlttlng phosphor screen, comprises silver halide grains which are not optically sensitized but possess the inherent spectral sensltlvlty of the known types of photosensltlve sllver hal-ldes. Sald lnherent spectral sensltlvlty of the conventlonal sllver hallde emulslons used ln photographlc fllms as known ranges ln the ultravlolet and blue reglon of the electromag-netlc spectrum.
Flgure 5 shows the sensltlvlty spectrum of a back sllver hallde emulslon layer comprlslng sllver bromolodlde gralns comprlslng 2.3 percent mole lodlde and havlng no op-tlcal sensltlzlng dye adsorbed on thelr surface, expressed as sensltlvlty (S) versus wavelenghts (nm).
Accordlng to the present lnventlon, the non-actlnlc radiatlon (preferably green llght) emitted by the front screen lmagewlse exposes the ad~acent front sllver hallde layer com-prlslng sllver hallde gralns optlcally sensltlzed to the radlatlon emltted by sald screens. Part of sald non-actlnlc radlatlon reaches the opposlte back sllver hallde layer but does not crossover expose the sllver hallde gralns of the back sllver hallde layer as those gralns are not optlcally sensl-tlzed to the radlatlon emltted by sald front screen. The ultravlolet or blue emlsslon of the back fluorescent screen undergoes absorptlon by the ad~acent back sllver hallde layer and lmagewlse exposes the sllver hallde gralns whlch are not optlcally sensltlzed, rather than crossover passlng to the opposlte front sllver hallde layer. Thls ls due to the fact that sllver hallde gralns possess a good lnherent absorption of llght wlth a wavelength below 500 nm and a strong llght scatterlng of ultravlolet and blue llght through the dlspersed sllver hallde partlcles of the emulslon layer. The above lm-plles not having a large amount of ultraviolet or blue llght of the back fluorescent screen to expose the opposlte front sllver hallde layer. The crossover exposure reductlon attaln-ed wlth the screen palr and double sllver hallde element com-binatlon of this invention is preferably at least 10 percent, more preferably at least 20 percent in comparlson with a con-ventlonal comblnation of green emlttlng, fluorescent screens and double coated green sensltlzed sllver halide radlographlc element. Accordlngly, the lmage sharpness ls lmproved by re-duclng crossover exposure uslng a unlque comblnation of con-ventional silver halide emulsion layers and fluorescent screens.
The light-sensitive double-side coated silver hallde radiographic element comprises a transparent polymerlc base of the type commonly used ln radlography, for lnstance a poly-ester base, and ln particular a polyethylene terephthalate base.
In the sllver hallde photographlc elements of thls lnventlon, the sllver hallde emulslons coated on the two sur-faces of the support, the front optlcally sensltized sllver hallde emulslon and the back optlcally unsensltlzed sllver hallde emulsion, may be simllar or dlfferent and comprise emulsions commonly used ln photographlc elements, such as sllver chlorlde, sllver lodlde, sllver chloro-bromide, sllver chlorobromolodlde, sllver bromlde and sllver bromolodlde, the sllver bromolodide being particularly useful for radlographlc elements. The sllver hallde gralns may have dlfferent shapes, for lnstance cublc, octahedrlcal, tabular shapes, and may have epltaxlal growths; they generally have mean slzes ranglng from 0.1 to 3 ~m, more preferably from 0.4 to 1.5 ~m. The emulslons are coated on the support at a total sllver coverage comprlsed in the range from about 3 to 6 grams per square meter. The sllver hallde blndlng materlal used ls a water-permeable hydrophlllc collold, which preferably is gelatin, but other hydrophillc collolds, such as gelatln derlvatlves, albumln, polyvinyl alcohols, alglnates, hydrollzed cellulose esters, hydrophllic polyvlnyl polymers, dextrans, polyacrylamides, hydrophilic acrylamide copolymers and alkylacrylates can also be used alone or in combination with gelatin.
As regards the processes for silver halide emulsion preparation and use of particular ingredients in the emulsion and in the light-sensitive element, reference is made to Research Disclosure 18,431 published in August 1979, wherein the following chapters are dealt with in deeper details:
IA. Preparation, purification and concentration methods for silver, halide emulsions.
IB. Emulsion types.
IC. Crystal chemical sensitizatlon and doplng.
II. Stablllzers, antlfogglng and antlfolding agents.
IIA. Stabilizers and/or antifogging.
IIB. Stabillzatlon of emulslons chemically sensitized with gold compounds.
IIC. Stablllzation of emulsions containing polyalkylene oxides or plasticizers.
IID. Fog caused by metal contaminants.
IIE. Stabilization of materlals comprising agents to increase the covering power.
IIF. Antlfoggants for dichrolc fog.
IIG. Antlfoggants for hardeners and developers comprlsing hardeners.
IIH. Additions to minimize desensitization due to folding.
III. Antifoggants for emulsions coated on polyester bases.
IIJ. Methods to stabilize emulsions at safety lights.
IIK. Methods to stabilize X-ray materials used for high temperature, Rapid Access, roller processor transport processing.
III. Compounds and antistatic layers.
IV. Protective layers.

V. Direct positlve materials.
VI. Materlals for processing at room light.
IX. Spectral sensitization.
X. UV sensitive materlals.
XII. Bases.
The silver hallde photothermographic elements of thls inventlon basically comprlse a llght lnsensltlve, reduclble sllver source, a llght sensltive materlal whlch generates sllver when lrradlated, and a reduclng agent for the sllver .10 source. The llght sensltlve materlal ls generally photo-graphlc sllver hallde whlch must be ln catalytlc proxlmlty to the llght lnsensltlve sllver source. Catalytlc proxlmlty is an intlmate physical associatlon of these two materials so that when silver specks or nuclei are generated by the irradi~
atlon or light exposure of the photographic silver halide, those nuclei are able to catalyze the reduction of the silver source by the reducing agent. It has been long understood that silver is a catalyst for the reductlon of sllver lons and the silver-generating light sensitlve silver hallde catalyst progenitor may be placed into catalytic proximity with the silver source in a number of different fashions, such as partial metathesls of the silver source with a halogen-con-taining source (e.g. US Pat. No. 3,457,075), coprecipitation of the silver hallde and silver source materlal ~e.g. US Pat.
No. 3,839,049), and any other method whlch lntimately associ-ates the sllver hallde and the silver source.
The silver source used in thls area of technology ls a material which contalns a reducible source of silver ions.
The earllest and still preferred source comprises sllver salts of long chain fatty carboxylic acids, usually of from 10 to 30 carbon atoms. The silver salt of behenic acid or mixtures of acids of like molecular weight have been primarlly used. Salts of other organic acids or other organic materials such as sllver imldazolates have been proposed, and British Pat. No.
1,110,046 discloses the use of complexes of inorganic or organic silver salts as image source materials. Silver salts of long chain (10 to 30, preferably 15 to 28 carbon atoms) fatty carboxylic acids are preferred ln the practlce of the present invention.
Photothermographic emulsions are usually constructed as one or two layers per side of the support. Single layer construction must contain the silver source material, the silver hallde, the developer and binder as well as optlonal addltlonal materials such as toners, coating aids and other adjuvants. Two-layer constructlons must contaln the sllver source and the sllver halide ln an emulslon layer (usually the layer ad~acent the support) and the other lngredlents ln the second layer or both layers. The sllver source materlal should constitute from about 20 to 70 percent by weight of the imaging layer. Preferably it is present as 30 to 55 percent by weight. The second layer in a two-layer construction would not affect the percentage of the silver source material desired ln the single imaging layer.
The sllver halide may be any photosensitive silver halide as silver bromlde, sllver iodide, sllver chlorlde, silver bromoiodide, silver chlorobromoiodide, silver chlorobromlde, etc., and may be added to the emulsion layer in any fashion which places it in catalytlc proxlmlty to the sllver source. The silver halide ls generally present as 0.75 to 15 percent by weight of the imaging layer, although larger amounts are useful. It ls preferred to use from 1 to 10 percent by welght silver halide in the imaging layer and most preferred to use from 1.5 to 7.0 percent. The vast list of photographic adjuvants and processing alds may be used ln - 1 337~52 silver halide emulslon preparation. These materlals include chemlcal sensltlzers (lncludlng sulfur and gold compounds), development accelerators (e.g. onlum and polyonlum compounds), alkylene oxlde polymer accelerators, antlfoggant compounds, stablllzers (e.g. azalndenes, especlally the tetra- and pentaazalndenes), surface actlve agents (particularly fluor-lnated surfactants), antistatic agents (partlcularly fluor-inated compounds), plasticizers, matting agents, and the like.
The reducing agent for the silver ion may be any material, preferably organic materlal, whlch wlll reduce silver lon to metallic silver. Conventional photographlc developers such as phenldone, hydroqulnones, and cathecol are useful, but hlndered phenol reducing agents are preferred.
The reduclng agents should be present as 1 to 20 percent by welght of the imaglng layer. In a two-layer constructlon, if the reducing agent is in the second layer, slightly higher proportions, of from about 2 to 20 percent tend to be more deslrable.
Toners such as phthalazinone, phthalazlne and phthallc acld are not essential to the constructlon, but hlgh-ly desirable. These materlals may be present, for example, in amounts of from 0.2 to 5 percent by welght.
The binder may be selected from any of the well known natural and synthetic resins such as gelatin, polyvinyl ace-tals, polyvinyl chlorlde, polyvinyl acetate, cellulose acet-ate, polyolefins, polyesters, polystyrene, polyacrylonitrlle, polycarbonates, and the like. Copolymers and terpolymers are, of course, included ln these deflnltlons. The polyvlnyl ace-tals, such as polyvinyl butyral and polyvinyl formal, and vinyl copolymers, such as polyvinyl acetate/chloride are par-ticularly desirable. The binders are generally used in a range of 20 to 75 percent by weight of each layer, and prefer-1 33~5~

ably about 30 to 55 percent by welght.
As previously noted, various other adjuvants may be added to the photothermographic emulslons of the present inventlon. For example, toners, accelerators, acutance dyes, sensltizers, stabilizers, surfactants, lubricants, coating aids, antifoggants, leuco dyes, chelating agents, and various other well known additives may be usefully incorporated. The use of acutance dyes matched to the spectral emlssion of the intensifying screen is particularly desirable.
The balance in propertles of the photothermographlc emulsion must be precisely restricted by the proportions of materlals ln the emulslon. The proportlons of the sllver salt and organic acid are particularly critical in obtalnlng nec-essary sensltometrlc propertles in the photothermographlc ele-ment for radlographic use. In conventional photothermographlc emulslons, lt ls common to use approximately pure salts of organlc aclds (e.g., behenlc acld, stearlc acid and mlxtures of long chaln aclds) as the substantive component of the emulsion. Sometimes minor amounts or larger amounts of the acld component are included in the emulsion. In the practice of the present invention the molar ratio of organic silver salt to organic acid must be in the range of 1.5/1 to 6.2/1 (salt/acid). Below that range, the contrast has been found to be too low, and above that range the speed and background stability of the emulsion drop off unacceptably. It is pre-ferred that the ratlo be ln the range of 2.0/1 to 3.50/1.
The sllver halide may be provided by in sltu halidizatlon or by use of pre-formed silver hallde.
The process of lndustrlal radlography would be per-formed by uslng a conventional X-ray pro~ectlon source or other high energy partlcle radlation sources including gamma and neutron sources. As well known ln the art, the partlcular phosphors used have a hlgh absorptlon coefflclent for the radlatlon emltted from the source. Usually thls radlatlon ls hlgh energy partlcle radlatlon whlch ls deflned as any of X-rays, neutrons and gamma radlatlon. The lndustrlal materlal would be placed between the controllable source of X-rays and the lndustrlal radlographlc system of the present lnventlon.
A controlled exposure of X-rays would be dlrected from the source and through the lndustrlal materlal so as to enter and lmpact the radlographlc system at an angle approxlmately per-pendlcular to the plane or surface of the lntenslfylng screensand the photothermographlc fllm contlguous to the lnslde sur-face of the screens. The radlatlon absorbed by the phosphors of the screens would cause llght to be emltted by the screens whlch ln turn would generate a latent lmage ln the sllver hal-lde gralns of the two emulslon layers. Conventlonal thermal development would then be used on the exposed fllm.
The X radlatlon lmage convertlng screens of thls lnventlon have a fluorescent layer comprlslng a blnder and a phosphor dlspersed thereln. The fluorescent layer ls formed by dlsperslng the phosphor ln the blnder to prepare a coatlng dlsperslon, and then applylng the coatlng dlsperslon by a conventlonal coatlng method to form a unlform layer. Although the fluorescent layer ltself can be a radlatlon lmage convert-lng screen when the fluorescent layer ls self-supportlng, the fluorescent layer ls generally provlded on a substrate to form a radlatlon lmage convertlng screen. Further, a protectlve layer for physlcally and chemlcally protectlng the fluorescent layer ls usually provlded on the surface of the fluorescent layer. Furthermore, a prlmer layer ls sometlmes provlded between the fluorescent layer and the substrate to closely bond the fluorescent layer to the substrate, and a reflectlve layer ls sometlmes provlded between the substrate (or the 1 337~52 primer) and the fluorescent layer.
The blnder employed in the fluorescent layer of the X
radiatlon image converting screens of the present invention, can be, for example, one of the binders commonly used ln form-ing layers: gum arabic, protein such as gelatin, polysacchar-ides such as dextran, organic polymer blnders such as poly-vinylbutyral, polyvinylacetate, nitrocellulose, ethylcellu-lose, vinylidene-chloride-vinylchloride copolymer, polymethyl-methacrylate, polybutylmethacrylate, vinylchloride-vinyl-acetate copolymer, polyurethane, cellulose acetate butyrate,polyvinyl alcohol, and the like.
Generally, the binder is used in an amount of 0.01 to 1 part by weight per one part by weight of the phosphor.
However, from the viewpolnt of the sensltlvlty and the sharp-ness of the screen obtalned, the amount of the blnder should preferably be small. Accordlngly, ln conslderation of both the sensitivity and the sharpness of the screen and the easi-ness of application of the coating dispersion, the blnder is preferably used in an amount of 0.03 to 0.2 parts by weight per one part by weight of the phosphor. The thickness of the fluorescent layer is generally within the range of 10 ~m to 1 mm.
In the radiation image converting screens of the present inventlon, the fluorescent layer is generally coated on a substrate. As the substrate, various materials such as polymer material, glass, wool, cotton, paper, metal, or the llke can be used. From the viewpoint of handling the screen, the substrate should preferably be processed lnto a sheet or a roll having flexibility. In this connection, as the substrate is preferably either a plastic film (such as a cellulose triacetate film, polyester film, polyethylene terephthalate film, polyamide fllm, polycarbonate film, or the like), or -- I 33/'852 ordinary paper or processed paper (such as a photographic paper, baryta paper, resln-coated paper, pigment-contalnlng paper which contains a pigment such as titanium dioxide, or the like). The substrate may have a primer layer on one surface thereof (the surface on which the fluorescent layer is provlded) for the purpose of holding the fluorescent layer tlghtly. As the material of the primer layer, an ordinary adhesive can be used. In providing a fluorescent layer on the substrate (or on the primer layer or on the reflective layer), a coating dispersion comprising the phosphor dispersed in a binder may be directly applied to the substrate (or to the primer layer or to the reflective layer).
Further, in the X radiation image converting screens of the present invention, a protective layer for physically and chemically protecting the fluorescent layer is generally provided on the surface of the fluorescent layer lntended for exposure (on the side opposite the substrate). When, as men-tioned above, the fluorescent layer is self-supporting, the protective layer may be provided on both surfaces of the fluorescent layer. The protective layer may be provided on the fluorescent layer by directly applying thereto a coating dispersion to form the protective layer thereon, or may be provided thereon by bondlng thereto the protective layer form-ed beforehand. As the material of the protective layer, a conventional material for a protective layer such a nitro-cellulose, ethylcellulose, cellulose acetate, polyester, polyethylene terephthalate, and the like can be used.
The X radiatlon image convertlng screens of the present invention may be colored with a colorant. Further, the fluorescent layer of the radiation lmage convertlng screen of the present inventlon may contaln a white powder dispersed therein. By using a colorant or a white powder, a radiation image converting screen whlch provides an image of hlgh sharp-ness can be obtained.

EXAMPLES
The lnventlon can be better lllustrated by reference to the followlng specific examples and comparative investiga-tlons.

Green Emitting Phosphor Screen GRSl A high resolutlon green emlttlng phosphor screen, screen GRSl, was prepared conslstlng of a ~Gdl 0 05~
Tbo 05)2O2S phosphor with average partlcle grain size of 3 ~m coated in a hydrophobic polymer binder at a phosphor coverage of 270 g/m and a thickness of 70 ~m on a polyester support.
Between the phosphor layer and the support a reflective layer of TiO2 partlcles in a poly(urethane) binder was coated. The screen was overcoated with a cellulose triacetate layer.

Green Emittinq Phosphor Screen GRS2 A medium resolution green emitting phosphor screen, screen GRS2, was prepared conslsting of a (Gdl 0 05~
Tbo 05)2O2S phosphor wlth average partlcle graln slze of 4 um coated in a hydrophoblc polymer binder at a phosphor coverage of 480 g/m and a thickness of 120 ~m on a polyester support.
Between the phosphor layer and the support a reflective layer of TlO2 particles in a poly(urethane) binder was coated. The screen was overcoated with a cellulose triacetate layer.

UV Emittinq Phosphor Screen UVS3 An UV emitting phosphor screen, screen UVS3, was prepared consisting of the type NP-3040 (Y, Sr, Li)TaO4 phosphor of Nichia Kagaku Kogyo K.K. with average particle grain size of 5.1 ~m coated in a hydrophoblc polymer blnder at a phosphor coverage of 450 g/m and a thlckness of 110 1~m on a polyester support. Between the phosphor layer and the support a reflectlve layer of TlO2 partlcles ln a poly(urethane) blnder was coated. The screen was overcoated wlth a cellulose trlacetate layer.

Llght-sensltlve Photoqraphlc Fllm GRUVFl A llght-sensltlve fllm having a green sensitlzed silver halide emulslon layer (herelnafter deslgnated front layer) coated on one side of the support and a spectrally unsensitized silver halide emulsion layer (herelnafter deslg-nated back layer) coated on the other slde of the support, fllm ~RUVFl, was prepared in the followlng manner. On one slde of a polyester support was coated a green sensltlzed sllver hallde gelatln emulsion layer containlng cubic sllver bromoiodide grains (having 2.3 mole percent iodide and an average graln slze of 0.65 ~m) at 2.57 g/m Ag and 1.9 g/m gelatln. The emulslon was sulfur and gold chemlcally sensl-tlzed and spectrally sensitized with 1,070 mg/mole Ag of the green sensitizlng Dye A, anhydro-5,5'-dichloro-9-ethyl-3,3' -bis(3-sulfopropyl)-oxacarbocyanine hydroxyde triethylamine salt. A protective overcoat containing 0.9 g/m2 gelatln was applied to said silver bromoiodide front layer. On the other side of the polyester support was coated a spectrally unsen-sitized silver halide sllver hallde gelatin emulsion layer containing cublc sllver bromolodlde gralns (comprlsing a 1 1 by weight blend of silver bromoiodide grains having 2 mole percent iodide and an average grain size of 1.3 ~m and silver bromoiodide grains having 2.3 mole percent iodide and an average grain size of 0.65 ~m) at 2.51 g/m2 Ag and 1.8 g/m gelatin. The emulsion was sulfur and gold chemically sensl-tlzed. A protective overcoat containing 0.9 g/m2 gelatin was applled to said silver bromolodide back layer.

Liqht-sensltlve Photographlc Fllm GRUVF2 A llght-sensltlve fllm havlng a green sensltlzed sllver halide emulsion layer (hereinafter deslgnated front layer) coated on one slde of the support and a spectrally unsensltlzed silver hallde emulslon layer (herelnafter desig-nated back layer) coated on the other side of the support, film GRUVF2, was prepared in the following manner. On one side of a polyester support was coated the green sensitized silver halide gelatin emulslon layer contalnlng cublc sllver bromoiodide grains of the front layer of film GRUVFl (havlng 2.3 mole percent lodide and an average grain size of 0.65 ~m) at 2.60 g/m2 Ag and 1.9 g/m2 gelatln. The emulsion was sulfur and gold chemically sensitized and spectrally sensitized with 1,070 mg/Ag mole of the green sensltlzlng Dye A, anhydro-5,5'-dlchloro-9-ethyl-3,3'-bis(3-sulfopropyl)-oxacarbocyanlne hydroxyde trlethylamlne salt. A protectlve overcoat contaln-lng 0.9 g/m2 gelatln was applled to sald sllver bromolodlde front layer. On the other slde of the polyester support was coated a spectrally unsensltlzed sllver hallde sllver hallde gelatln emulslon layer contalnlng cublc sllver bromolodide grains of the front layer of fllm GRUVFl (havlng 2.3 mole percent lodlde and an average graln size of 0.65 ~m) at 2.49 g/m Ag and 1.9 g/m gelatin. The emulslon was sulfur and gold chemlcally sensltlzed. A protectlve overcoat contalnlng 0.9 g/m2 gelatln was applled to sald sllver bromolodlde back layer.

Liqht-sensltlve Photographlc Fllm UVUVF3 A llght-sensitive fllm having a spectrally unsen-sltlzed silver hallde emulslon layer coated on each slde of a support, fllm UVUVF3, was prepared ln the followlng manner.
On each side of a polyester support was coated a spectrally unsensltlzed sllver hallde sllver hallde gelatln emulslon layer contalnlng cubic sllver bromolodlde gralns of the front layer of fllm GRUVFl (havlng 2.3 mole percent lodlde and an average graln slze of 0.65 ym) at 2.60 and 2.49 g/m2 Ag, respectlvely, and 1.9 g/m2 gelatln. The emulslon was sulfur and gold chemically sensltlzed. A protective overcoat con-talnlng 0.9 g/m gelatln was applled to each sllver bromolodlde layer.

Llght-sensltlve Photographlc Fllm GRGRF4 A llght-sensltive film havlng a green sensltlzed sllver hallde emulslon layer coated on each slde of a support, fllm GRGRF4, was prepared ln the followlng manner. On each side of a polyester support was coated the green sensltlzed sllver hallde gelatln emulslon layer contalnlng cublc sllver bromoiodide gralns of the front layer of fllm GRUVFl ~havlng 2.3 mole percent lodlde and an average graln slze of 0.65 ym) at 2.18 g/m2 Ag and l.g g~m2 gelatin. The emulsion was sulfur and gold chemlcally sensltlzed and spectrally sensltlzed wlth 1,070 mg/mole Ag of the green sensltlzlng Dye A, anhydro-5,5'-dichloro-9-ethyl-3,3'-bls(3-sulfopropyl)-oxacarbocyanlne hydroxyde trlethylamlne salt. A protectlve overcoat contaln-ing 0.9 g/m gelatin was applled to each sllver bromolodide layer.

- 1 3378~2 Comparlson of Screen Palrs and Llqht-sensltlve PhotoqraPhlc Fllms Palrs of screens ln comblnatlon wlth double coated llght-sensltlve photographlc fllms descrlbed above were exposed as follows. Referrlng to Flgure l, fllm-screens comblnatlons were made ln whlch the front screen was ln con-tact wlth the front emulslon layer and the back screen was ln contact wlth the back emulslon layer. Each screen palr-fllm comblnatlon was exposed to X-rays from a tungsten target tube operated at 80 kVp and 25 mA from a dlstance of 120 cm. The X-rays passed through an alumlnlum step wedge before reachlng the screen-fllm comblnatlon. Followlng exposure the fllms were processed ln a 3M TrlmatlcTM XP507 processor uslng 3M
XAD~2 developer replenlsher and 3M XAF/2 flxer replenlsher.
The speed and the lmage quallty are reported ln the followlng table. Percent crossover was calculated by uslng the followlng equatlon:

Percent Crossover = x 100 antllog (6 logE) whereln ~ logE ls the dlfference ln speed between the two emulslon layers of the same fllm when exposed wlth a slngle screen (the lower the percent crossover, the better the lmage quallty).

TABLE

Comb. Screen Pair Fllm Rel. Speed % Crossover Front/Back log E
_ 1 1 GRGRF4 0.00 37 2UVS3/UVS3 UVUVF3 -0.39 9 3GRSl/UVS3 GRUVF2 -0.22 4 9 * **
4GRSl/UVS3 GRUVFl 0.00 9 5 * **
5GRS2/UVS3 GRUVFl +0.18 4 5 crossover measured ln the front emulslon layer crossover measured in the back emulslon layer Sharpness and granularlty of the screen palr-fllm comblnatlons were determlned as follows. Each screen palr-fllm comblnatlon was exposed to X-rays from a tungsten tube operated at 80 kVp and 25 mA from a dlstance of 150 cm. The X-rays passed through a 100 ~ thlck lead Funk target sold by Huttner Company before reachlng the screen-fllm comblnation.
Followlng exposure the fllms were processed ln the 3M
Trlmatlc XP507 processor uslng 3M XAD/2 developer replen-lsher and 3M XAF/2 flxer replenlsher. Sharpness and granular-lty of the processed fllms were determlned by vlsual examln-atlon of ten observers skllled ln maklng lmage comparlsons.
Flgures 6 and 7 are graphs lllustrating the sharpness (SH) and granularlty (GR~ versus the dlfference of sensltlvlty ~S of the double coated sllver hallde element and fluorescent screen palr comblnatlons and that of green sensltlve double coated element comblned wlth green emlttlng fluorescent screen palr taken as reference. In the graphs, the hlgher the numbers of sharpness and granularlty, the better ls the sharpness and granularlty. Sharpness and granularlty of the UV and blue llght sensltlve double coated sllver hallde element comblned wlth UV emlttlng fluorescent screen palr are the best but at the lower level of sensltlvlty, whlle sharpness and granu-larlty of the double coated element and screen palr comblna-tlons of thls lnventlon are better or comparable to that ofgreen sensltlve double coated element comblned wlth green emltting fluorescent screen palr at comparable or hlgher level of sensltlvlty.

Llqht-sensltlve Photothermoqraphlc Fllm GRBLF5 a) Preparatlon of the sllver soap.
To 20 l of water at 80C were added 634.5 g of Humbo acld 9022 (a long chaln fatty carboxyllc acld comprlslng 90% C20 + C22, 5% C18 and 5% other acld), 131 g Humbo acld 9718 (a long chaln fatty carboxyllc acld comprlslng 95% C18 and 5% C16) and 0.44 moles of a 0.08 ~m cublc sllver bromolodlde (6% mole lodlde) emulslon. While stirrlng, were added 89.18 g NaOH dlssolved ln 1.25 l water, then 19 ml of conc. HNO3 dlluted wlth 50 ml water. At 55C were added 364.8 g AgNO3 dlssolved ln 2.5 l water at 55C. The mlxture was heated at 55C for one hour whlle stlrrlng slowly, centrlfugated whlle spray washlng untll 20,000 ohms reslstance of water was obtalned and drled.
b) Preparatlon of the sllver salt homogenate. Sllver soap powder above (12% by welght), toluene (20% by welght) and methylethyl ketone (68% by welght) were mlxed, soaked over-nlght, added wlth 12 g polyvlnylbutyral (ButwarTM B-76) and homogenlzed at 4,000, then 8,000 PSI.
c) Preparatlon of the flrst trlp.
200 g of the sllver soap homogenate above were added wlth 40 g methylethyl ketone and 30 g polyvlnylbutyral (ButwarTM B-76) and mlxed for one-half hour. The mlxture was added wlth 2.2 ml of a 10% HgBr2 ln methanol, stlrred for 5 mlnutes, then added wlth 4 g of NonoxTM W50 Developer of formula HgC~4 /H HO C4Hg - CH -One 50 g part of the dlsperslon above was added wlth 0.6 ml of a solutlon of 0.04 g per 10 ml of the followlng blue sensltlzer ~ C--NCH2COOH(C2H5)3N

A second 50 g part of the dlsperslon above was added wlth 0.6 ml of a solutlon of 0.05 g per 10 ml of the followlng green sensltlzer 1 33 7~ 52 CH2COONa ,,~ H ~ ~C-S

d) Coatlng of the first trlp.
The blue sensltlzed dlsperslon above was coated on a clear 4 mll (2 x 10 4 m) polyethyleneterephthalate support at 5 mlls over the support and drled for 3 mlnutes at 87C. On the opposlte slde of the support the green sensltlzed dlsperslon above was coated at 5 mlls over the support and drled for 3 mlnutes at 87C.
e) Preparatlon of the second trlp.

______________________________________________________________ Component Parts by Welght ______________________________________________________________ A. Methylethyl ketone 78.58 B. Acetone 12.02 C. Methanol 4.gl D. FC-431 (3M Fluorocarbon) 0.04 E. Phthalazlne 0.59 F. 4-Methyl-phthallc acld 0.41 G. Tetrachlorophthallc anhydrlde 0.27 H. Tetrachlorophthallc acld 0.12 I. Cellulose acetate ester 3.06 ______________________________________________________________ 3g A through H were added to a container and mlxed untll sollds were dlssolved. I was then added and mlxed for one hour untll was dissolved.
f) Coatlng of the second trlp.
On the ~lue sensltizlng coatlng prevlously applled, the second trlp was coated over the flrst at 2.25 mlls and dried for 3 mlnutes at 87C. The fllm was turned over and, on the green sensltlzed coatlng prevlously applled, the second trlp was coated at 2.25 mlls and drled for 3 mlnutes at 87C.
g) Evaluatlon of the fllm.
A sample of the flnlshed double-slde coated photothermographlc fllm was exposed wlth a xenon flash sensltometer through a 460 nanometer narrow band fllter at a settlng of 10 3 seconds through a 0-4 contlnuous denslty wedge. The exposed sample was processed for four seconds at 131C ln a roller drlven thermal processor. The sensltometry was recorded as Dmln=0.21 and DmaX=4.22. Another sample was exposed and processed as above but uslng a 560 nanometer narrow band fllter. The sensltometry was recorded as Dmln=0.21 and DmaX=2.19. Clearly there ls no cross-over from the green sensltlzed 39a - 40 - l 3 3 7 8 5 2 layer to the blue sensitizing layer. The blue sensitized layer when coated singly on one side of a polyester sub-strate recorde~ an image as follows : Dmin=0.13 and DmaX=2.56.

Claims

1. A combination of photosensitive elements for use in radiography comprising two separate front and back X-ray fluorescent screens and a silver halide radiographic element comprising a transparent polymeric support base and front and back silver halide emulsion layers each coated on one surface of said support, wherein said front screen is arranged adjacent to said front silver halide layer and said back screen is arranged adjacent to said back silver halide layer, and wherein 1) said front screen comprises a first radiation emitting phosphor and said front silver halide layer comprises silver halide grains sensitive to said first radiation emitted by said front screen, and

2) said back screen comprises a second radiation emitting phosphor and said back silver halide layer comprises silver halide grains sensitive to said second radiation emitted by said back screen, characterized in that a) said first radiation emitted by said front screen is in a wavelength region of non-actinic radiation and said second radiation emitted by said back screen is in a wavelength region of actinic radiation, said front screen comprising a green emitting phosphor having more than 80% of its spectral emission above 480 nm and its maximum of emission in the wavelength range of 530-570 nm, b) said first radiation emitted by said front screen has a wavelength which differs from said second radiation emitted by said back screen by at least 50 nm, c) said front silver halide emulsion layer is substantially insensitive to said second radiation emitted by said back screen, and d) said back silver halide emulsion layer is substantially insensitive to said first radiation emitted by said front screen, the difference in wavelength region of said first and second radiations and the insensitivity of each silver halide layer to radiation emitted by opposite screen being such to reduce crossover exposure of at least 10 percent when compared with a symmetrical combination of a pair of green light emitting fluorescent screens and a double coated green sensitized silver halide radiographic element.

2. The combination of claim 1, wherein a) said front screen comprises a non-actinic light emitting phosphor and said front silver halide layer comprises silver halide grains spectrally sensitized to the radiation emitted by said front screen, and b) said back screen comprises a UV emitting phosphor and said back silver halide layer comprises spectrally insensitized silver halide grains.

3. The combination of claim 1, wherein said front screen comprises a green emitting phosphor represented by the general formula (Ln1-a-b, Tba, Tmb)2O2S

42a wherein Ln is at least one rare earth selected from lanthanium, gadolinium and lutetium, and a and b are num-bers such as to meet the conditions of 0.0005 a 0.09 and 0 b 0.01, respectively, or the general formula (Y1-c-a-b,Lnc,Tba,Tmb)2O2S
wherein Ln is at least one rare earth selected from lan-thanium, gadolinium and lutetium, and a, b and c are num-bers such as to meet the conditions of 0.0005 a 0.09, 0 b 0.01 and 0.65 c 0.95, respectively.

4. The combination of claim 1, wherein said front screen comprises a terbium-activated gadolinium or lan-thanium oxysulphide phosphor having substantially emis-sion peaks at 487 and 545 nm.

5. The combination of claim 1, wherein said front silver halide layer comprises silver halide grains spec-trally sensitized with spectral sensitizing dyes of the cyanine or merocyanine dyes.

6. The combination of claim 1, wherein said front silver halide layer comprises silver halide grains spec-trally sensitized with sensitizing dyes in such a way that it is sensitive for light in wavelength range of 530 - 570 nm.

7. The combination of claim 1, wherein said front silver halide emulsion layer comprises silver halide grains spectrally sensitized with sensitizing dyes repre-sented by the general formula (X-)n1 wherein R10 represents a hydrogen atom or an alkyl group, R6, R7, R8 and R9 each represent a hydrogen atom, a halogen atom, a hydroxy group, an alkoxy group, an amino group, an acylamino group, an acyloxy group, an alkoxycarbonyl group, an alkyl group, an alkoxycarbonyl-amino group or an aryl group, or, together, R6 and R7, and respectively R8 and R9 can be the atoms necessary to complete a benzene ring, R11 and R12 each represent an alkyl group, a hydroxyalkyl group, an acetoxyalkyl group, an alkoxyalkyl group, a carboxyl group containing alkyl group, a sulfo group containing alkyl group, a benzyl group, a phenethyl group or a vinylmethyl group, X? rep-resents an acid anion and n represents 1 or 2.

8. The combination of claim 2, wherein said back screen comprises a UV emitting phosphor having more than 80% of its spectral emission below 410 nm and its maximum of emission in the wavelength range of 300 - 360 nm.

9. The combination of claim 2, wherein said back screen comprises a UV emitting phosphor selected from the class consisting of lead or lanthanum activated barium sulfate phospnors, barium fluorohalide phosphors, lead activated barium silicate phosphors, gadolinium activated yttrium oxide phosphors, barium fluoride phosphors, and alkali metal activated rare earth niobate or tantalate phosphors.

10. The combination of claim 2, wherein said back screen comprises a UV emitting phosphor corresponding to the general formula (Y1-2/3x-1/3y.SrX, Liy) TaO4 wherein x and y are numbers such as to meet the conditions 10-5 X 1 and 10-4 y 0.1.

11. The combination of claim 2, wherein said back silver halide layer comprises chemically sensitized spectrally insensitized silver bromoiodide grains.

12. A method for recording a radiation image comprising the steps of (1) imagewise exposing to X radiations a combination of photosensitive elements comprising two separate front and back X-ray fluorescent screens and a silver halide radiographic element comprising a transparent polymeric support base and front and back silver halide emulsion layers each coated on one surface of said support, wherein said front screen is arranged adjacent to said front silver halide layer and said back screen is arranged adjacent to said back silver halide layer, and wherein 1) said front screen comprises a first radiation emitting phosphor and said front silver halide layer comprises silver halide grains sensitive to said first radiation emitted by said front screen, and 2) said back screen comprises a second radiation emitting phosphor and said back silver halide layer comprises silver halide grains sensitive to said second radiation emitted by said back screen, and (ii) developing said silver halide radiographic element halide, characterized in that a) said front screen comprises a green emitting phosphor having more than 80% of its spectral emission above 480 nm and its maximum of emission in the wavelength range of 530-570 nm, b) said first radiation emitted by said front screen has a wavelength which differs from said second radiation emitted by said back screen by at least 50 nm, c) said front silver halide emulsion layer is substantially insensitive to said second radiation emitted by said back screen, and d) said back silver halide emulsion layer is substantially insensitive to said first radiation emitted by said front screen, the difference in wavelength region of said first and second radiations and the insensitivity of each silver halide layer to radiation emitted by opposite screen being such to reduce crossover exposure of at least 10 percent when compared with a symmetrical combination of a pair of green light emitting fluorescent screens and a double coated green sensitized silver halide radiographic element.

13. The method for recording a radiation image according to claim 12, wherein a) said front screen comprises a non-actinic radiation emitting phosphor and said front silver halide layer comprises silver halide grains spectrally sensitized to the radiation emitted by said front screen, and b) said back screen comprises a UV emitting phosphor and said back silver halide layer comprises spectrally insensitized silver halide grains.

14. The method for recording a radiation image according to claim 12, wherein said front screen comprises a green emitting phosphor represented by the general 46a formula (Ln1-a-b, Tba, Tmb)2O2S

wherein Ln is at least one rare earth selected from lan-thanium, gadolinium and lutetium, and a and b are numbers such as to meet the conditions of 0.0005 a 0.09 and 0 b 0.01, respectively, or the general formula (Y1-c-a-b,Lnc,Tba,Tmb)2O2Tb wherein Ln is at least one rare earth selected from lan-thanium, gadolinium and lutetium, and a, b and c are num-bers such as to meet the conditions of 0.0005 a 0.09, 0 b 0.01 and 0.65 c 0.95, respectively.

15. The method for recording a radiation image according to claim 12, wherein said front screen compris-es a terbium-activated gadolinium or lanthanium oxysul-phide phosphor having substantially emission peaks at 487 and 545 nm.

16. The method for recording a radiation image according to claim 12, wherein said front silver halide layer comprises silver halide grains spectrally sensi-tized with spectral sensitizing dyes of the cyanine or merocyanine dyes.

17. The method for recording a radiation image according to claim 12, wherein said front silver halide layer comprises silver halide grains spectrally sensi-tized with sensitizing dyes in such a way that it is sen-sitive for light in wavelength range of 530 - 570 nm.

18. The method for recording a radiation image according to claim 12, wherein said front silver halide emulsion layer comprises silver halide grains spectrally sensitized with sensitizing dyes represented by the gen-eral formula (X-)n1 wherein R10 represents a hydrogen atom or an alkyl group, R6, R7, R8 and R9 each represent a hydrogen atom, a halogen atom, a hydroxy group, an alkoxy group, an amino group, an acylamino group, an acyloxy group, an alkoxycarbonyl group, an alkyl group, an alkoxycarbonyl-amino group or an aryl group, or, together, R6 and R7, and respectively R8 and R9 can be the atoms necessary to complete a benzene ring, R11 and R12 each represent an alkyl group, a hydroxyalkyl group, an acetoxyalkyl group, an alkoxyalkyl group, a carboxyl group containing alkyl group, a sulfo group containing alkyl group, a benzyl group, a phenethyl group or a vinylmethyl group, X- rep-resents an acid anion and n represents 1 or 2.

19. The method for recording a radiation image according to claim 12, wherein said back screen comprises a UV emitting phosphor having more than 80% of its spec-tral emission below 410 nm and its maximum of emission in the wavelength range of 300 - 360 nm.

20. The method for recording a radiation image according to claim 12, wherein said back screen comprises a UV emitting phosphor selected from the class consisting of lead or lanthanum activated barium sulfate phosphors, barium fluorohalide phosphors, lead activated barium silicate phosphors, gadolinium activated yttrium oxide phosphors, barium fluoride phosphors, and alkali metal activated rare earth niobate or tantalate phosphors.

21. The method for recording a radiation image according to claim 12, wherein said back screen comprises a UV emitting phosphor corresponding to the general formula (Y1-2/3x-1/3y', Srx, Liy) TaO4 wherein x and y are numbers such as to meet the conditions 10-5 c x < 1 and 10 4 c y < 0.1.

22. The method for recording a radiation image according to claim 12, wherein said back silver halide layer comprises chemically sensitized spectrally insensitized silver bromo-iodide grains.

23. A combination of photosensitive elements for use in radiography comprising two separate front and back X-ray fluorescent screens and a silver halide radiographic element comprising a transparent polymeric support base and front and back silver halide emulsion layers each coated on one surface of said support, wherein said front screen is arranged adjac-ent to said front silver halide layer and said back screen is arranged adjacent to said back silver halide layer, and where-in 1) said front screen comprises a first radiation emitting phosphor and said front silver halide layer comprises silver halide grains sensitive to said first radiation emitted by said front screen, and 2) said back screen comprises a second radiation emitting phosphor and said back silver halide layer comprises silver halide grains sensitive to said second radiation emitted by said back screen, characterized in that a) said front screen comprises a green emitting phosphor having more than 80% of its spectral emission above 480 nm and its maximum of emission in the wavelength range of 530 - 570 nm, b) said first radiation emitted by said front screen has a wavelength which differs from said second radiation emitted by said back screen by at least 50 nm, c) said front silver halide emulsion layer is substantially insensitive to said second radiation emitted by said back screen, and d) said back silver halide emulsion layer is substantially insensitive to said first radiation emitted by said front screen, the difference in wavelength region of said first and second radiations and the insensitivity of each sliver halide layer to radiation emitted by opposite screen being such to reduce crossover exposure of at least 10 percent when compared with a symmetrical combination of a pair of green light emitting fluorescent screens and a double coated green sensitized silver halide radiographic element.

24. A combination of photosensitive elements for use in radiography comprising two separate front and back X-ray fluorescent screens and a silver halide radiographic element comprising a transparent polymeric support base and front and back silver halide emulsion layers each coated on one surface of said support, wherein said front screen is arranged adjacent to said front silver halide layer and said back screen is arranged adjacent to said back silver halide layer, and wherein 1) said front screen comprises a first radiation emitting phosphor and said front silver halide layer comprises silver halide grains sensitive to said first radiation omitted by said front screen, and 2) said back screen comprises a second radiation emitting phosphor and said back silver halide layer comprises silver halide grains sensitive to said second radiation omitted by said back screen, characterized in that a) said front screen comprises a green emitting phosphor having more than 80% of its spectral emission above 480 nm and its maximum of emission in the wavelength range of 530 - 570 nm, b) said first radiation emitted by said front screen is in a first wavelength region of the electromagnetic spectrum and said second radiation emitted by said back screen is in a second wavelength region of the electromagnetic spectrum, c) said front silver halide emulsion layer is substantially insensitive to said second radiation emitted by said back screen, and d) said back silver halide emulsion layer is substantially insensitive to said first radiation emitted by said front screen.