US2177701A - Fluorescent screen - Google Patents

Description

Oct. 31, 1939. G. R.- FONDA FLUORESCENT SCREEN Filed Jan. 22, 1955 2 Sheets-Sheet l ma E40 LLl L/FE HUI/MS MMM Inventor: Gortoh R Fonda, by yv/ His A horney.

Oct. 31, 1939., G, R, FONDA 2,177,701

FLUORESCENT SCREEN Filed Jan. 22, 195e 2 sheets-sheet 2 Inventor: @Orton F?. Fonda His Attorney.

Patented oct. 31, 1939 PATENT ori-'ICE z,17i,7o1 FLUoREscEN'r SCREENh Gorton R. Fonda, Schenectady, N. Y., asslgnor to General Electric Company, a corporation of New York Application January 22, 1936, Serial No. 60,276

2 Claims.

'I'he present invention relates to fluorescent Screens of the type used for correcting the spectra of light sources which are deficient in one or more spectral colors and to the combination of such 5 screens and light sources.`

It is an object of my invention to provide a y fluorescent screen having a much longer life than that attainable with such screens as have previously been employed. l Itis a further object of my invention to provide a screen having the above qualities which is also highly eillcient in converting impinging radiant energy into fluorescent light of a complementary color. l I have found that it is possible to attain these ends by employing a fluorescent coating having a thickness in excess of that required to compensate directly impinging radiations but disposed in such a discontinuous fashion that the final blending of compensated and uncompensated light will afford a substantially proper spectral distribution.

The novel features which I consider to be characteristic of my invention will be pointed out with particularity inthe appended claims. The invention itself, however, will best be understood by reference to the following specification taken in connection with the appended drawings, in which Figs. land 2 represent graphically the charactero istics of fluorescent screens which I consider to be significant in understanding my invention; Fig. 3 illustrates partly in section a. light source enclosed in a translucent screen embodying my invention; Fig. 4 shows in elevation the application of my invention to a reflecting screen; Fig.`

85 5 represents a section taken on line 5-5 of Fig. 4'; Fig. 6 illustrates a complete lamp structure involving a possible modification of my invention; while Fig. 7 is a perspective view of a composite film which comprises one aspectof the illy invention.

`It is Well known that the radiations of metal vapor lamps, whether of high or low pressure, tend to accentuate certain portions of the light spectrum at the `expense of other portions. For example, the light generated by the ordinary mercury vapor lamp is very rich in blue and green light, but is almost entirely devoid -of red. In

order to improve the quality of the illumination provided by such sources, it has been proposed to supplement their radiations by fluorescent screens capable of generating complementary colors. Thus, in connection with a mercury lamp, it has been known to use a screen or reflector having thereon a uniform coating of the organic dye known as rhodamine B. It is, however, a

decrease with time.l

(Cl. Z50-80) tice, for example, that the uorescence of a rhodamine screen having a uniform coating of proper thickness will decrease to less than 50 per centl of its initial value in substantially less than 100 hours, thus rendering the screen essentially useless. In accordance with my present invention I have 10 found that both the life and efllciency of fluorescent materials and, particularly, of such materials as are of the class known as organic dyes are functions of the 'thickness of the coating employed. 'I'he nature of this relationship is exu emplified graphically in Figs. 1 and 2, the curves of which are based on the characteristics of rhodamine B dissolved in a 30 per cent solution of cellulose acetate in acetone, the concentration of the composite substance comprising about 0.2 20 gram of rhodamine per liter of solvent. While the characteristics obtained necessarily vary with the materials and concentrations employed, I have found that those illustrated are typically representative of both the organic and inorganic 25 fluorescent substances. Q Referring particularly to curve A of Fig.` l, it will be seen that the percentage of the radiant energy impinging on the fluorescent screen which will be converted into fluorescent energy increases very rapidly with film thickness up to a thickness of about 6 mils, above which value it decreases at a slower rate. Since the percentage of such o mpinging radiant energy which is absorbed (i. e., not transmitted) by the film is essentially constant for the higher values of film thickness, above about 6 mils, the curve A also affords a fairly accurate measure of the efficiency of con version of the screen.

Thus, from the single standpoint of efficiency 40 of conversion, it Will be apparent that when dealing with rhodamine B a coating thickness in the neighborhood of from 2 to 14 mils would be most advantageous. Furthermore, reference .to Fig. 2, which is based on the time required for the flul5 crescentA substance to drop to 50 per cent of its initial effectiveness, shows that the useful life of the screen will be very greatly' enhanced by using as great a thickness as possible. This latter advantage is further enhanced by the fact that 50 during the early period of its use the fluorescent efliciency of ascreen having a relatively great. film thickness appears to increase rather than While I do not wish to be limited to a particular theoretical explanation of 6I this eiIect, I consider that it is due to the fact that the aging of a uorescent nlm seems to be equivalent to a progressive diminution of its thickness. Thus, it is apparently true that after several hundred hours of use, the effective thickness of a film assumed initially to have been 14 mils will be reduced to, say; 8 mils. Referring these figures by way of illustration to curve A of Fig. I it will be seen that this apparent decrease in thickness actually results in an increase in the screen eiliciency, approaching a maximum at an apparent thickness of about 6 mils. On the basis of these facts, therefore, the integrated fluorescent emission during the entire screen life obtained by increasing the iilm thickness is in f excess of that which may be attributed to the increase in liie alone. Otherwise expressed, by increasing the thickness one obtains an improvement in the average intensity of fluorescence proportionately greater than the increase in life which also occurs.

A diiculty encountered in the use of screens having a thickness as great as that shown by my invention to be desirable lies in the fact that such screens will necessarily result in an over-compensation of the light from the light source. Otherwise expressed, a light which is originally predominantly of the color characterizing a given vapor discharge will, upon passage through or reflection from a thickly. coated iiuorescent screen, appear to be predominantly of the color associated with the screen. This eiect is readily explainable by reference to curve B of Fig. l which shows that the percentage of radiant energy absorbed by the fluorescent coating increases very rapidly with increasing thickness up to about 6 mils. I have found, however, that this factor may be overcome while still retaining Athe advantages to be attributed to thick films of iluorescent material by disposing the iilm in a spaced pattern on the screen surface. In Figs. 3 to 6 of the drawings, I have shown Various ways in which this aspect of my invention may be utilized.

Referring particularly to Fig. 3, I have illustrated the use of the invention in connection with the type of mercury lamp described in Bol, Elenbaas and Lemmens Patent No. 2,094,694, issued October 5, 1937, and assigned to the same assignee as the present application. 'I'hese lamps typically -take the form indicatedV at I and comprise a small quartz tube having an internal diameter in the neighborhood of from 2 to 4 millimeters and adapted to sustain the tremendous heat generated by an enclosed mercury vapor; discharge occurring at a pressure in excess of 10 atmospheres,

In the arrangement shown in the drawings, the quartz lamp I is provided with suitable lead-in conductors 2 and 3 by means of which it is suspended from an insulating base of porcelain or other suitable material. This in turn is rigidly connected with an aproned metallic reector which engages with and supportsa bowl-like enpurpose it is possible to use a known organic material such as rhodamine B or a derivative thereof formed by combining an organic group or a metal such as potassium with the acid carboxyl group of the rhodamine B. Such alternative substances are described and claimed, .for example, in my Patents No. 2,149,992 and No. 2,149,993 issued March 7, 1939. CurveD of Fig. 2 is included in the present application as showing the manner in which `ithe life of one of these, namely, the potassium salt of rhodamine B, increases with increasing iilm thickness. f

When employing such organic materials in connection with a mercury quartz lamp, it may be desirable to protect the uorescent screen from the deteriorating effect of ultra-voilet radiations. For this purpose, a cylinder of ordinary glass may be introduced between the lamp and the screen, or the iluorescent coating may be applied to the exterior rather than the interior surface of the bowl 6. In using inorganic substances such as zinc sulphide, on the lother hand, it is practical and even preferable to arrange the fluorescent material for direct exposure to the lamps rays in the mannershown.

If the uorescent medium to be employed is rhodamine B, I prefer to utilize it in the form of a solution thereof in a thirty per cent solution of cellulose acetate in acetone, although other solvents such as polymerized vinyl acetate may be used. I have also found that best results are obtainedwhen the concentration of this solution is equal to or below about 0.2 gram of rhodamine per liter of solvent, (or about 0.007 gram of rhodamine per cubic centimeter of dry coating) although the use of such preferred concentrations is not necessary in the practice of the in vention. As has been previously explained, a considerable increase in the efficiency and life oi' the fluorescent film is realized if the stripes are applied with an optimum thickness, which for rhodamine B of the 0.2 gram per liter concentration indicated above as preferable should be between about 2 and about 14 mils. Equivalent results may be obtained, however, by the use of solutions of less concentration if a proportionately greater iilm thickness is employed. The over-compensation which might be expected to result is neutralized in the present instance by the fact that portions of themercury light are permitted to pass through the uncoated sections of the enclosing screen. These portions, when blended with light modified by passage through the fluorescent film, serve to produce `a resultant illumination of the desired quality.

While I have in this particular instance shown the lm coating as being applied in the form of vertical stripes, it will be understood that this arrangement is by no means essential. Any arrangement which provides a discontinuously coated surface will make it possible to take full advantage of the increased life and, eiliciency attributable to the greater thickness of iilm on the covered portions. It is only required that the relative size of the covered and uncovered areas be such as to approximate a proper spectral distribution in the blended light. For example, with a rhodamine solution having a concentration of about 0.2 gram per 4liter and a lm thickness of 12 mils, I have found it desirable to have the striped and unstriped portions of substantially equal width.

In'Fig. 4, I have shown a positive column mercury lamp of known type comprising an extended glass tube IlI having lead-in conductors II and I2 sealed into the ends thereof. Itv will, of course, be understood .that the tube is provided internally with the suitable discharge electrodes and contains a small quantity` of mercury. In connection with the lamp proper is provided a reflector |.3 of metal or similar light reflecting material having sufcient roughness or diffusing properties to avoid the phenomenon of lighttrapping due to multiple reflection which occurs when coating films are used on a too highly polished surface.

Upon reference to Fig. 5, the reflector will be seen to be of arcuate cross-section. With this combination compensation of the blue mercury light is accomplished by providing 4the reflector I3 with a coating of fluorescent material applied according to my invention inthe form of a plaid of rectangularly intersecting stripes I4 and i5. Here again, as explained in connection with Fig. 3, the thickness of the stripes is greater .than that previously considered desirable for spectral compensation only. Satisfactory results are assured, however, by the fact that the uncoated portions of the reflector throw back enough of the unchanged blue-greenmercury light to result in a substantially balanced spectrum.

In Fig. 6, I have illustrated a modification which shows the application of my invention to a compensating screen adapted both to transmit and to reflect light. This comprises a stand lamp utilizing as a light source a high pressure quartz discharge device Il of the type previously described in connection with Fig. 3. In this case, the quartz tube as well as its lead-in connections are completely enveloped and supported in a transparent glass bulb I8. This light source is entirely enclosed, except at the top, by a bowl I9 of semi-translucent material, such as opal glass, which tends to reflect a portion of theemitted light upwardly where it may be diffused on a large reflecting surface such as a ceiling, but also transmits a portion of the light downwardly so as to produce direct illumination of a desk or other work table. blue-greenness of the mercury light is substantially reduced in both the direct and indirect illumination by providing on the interior surface .its upper lip.

By use -of the means described in the foregoing. I have found it possible to increase the life of fluorescent screens to several hundred hours and under favorable conditions to as much as one thousand hours. lIt is a further aspect of my invention that this improvement may be addi` tionally enhanced if the fluorescent coating is composed of alternate layers of dye material and plain cellulose acetate'or equivalent spacing material. This formation is shown in Fig. 7, which Any predominance of 1v comprises `a perspective view of a small section of lm built up in the manner indicated. The laminationsbearing the indeti numbers 25 preferably comprise a solution of rhodamine in cellulose acetate, while the interposed layers 26 are of pure cellulose acetate.

Since each layer must be applied as a liquid to the layer previously deposited, there will probably be Asome diffusion of the rhodamine into the celluloseacetate so that the lines of division will not be as definite as those indicated. I have found, however, that extremely advantageous results are obtained as long as the lm structure is characterized by recurrent variations in rhodamine concentration through the film crosssection. For example, with films having a total thickness of 12 mils, a laminated structure yields a life of from 1100 to 1600 hours as compared with a life of about 400 hours for a lm of uniform nature. This comparison is all the more surprising since the unlaminated films contain almost twice as much rhodamine as those of the composite construction.

Since the increase in life to be obtained through the laminated film assembly is more marked for thicknesses in excess of that required to complement directly impinging radiations fllms of this type may most suitably be applied to fluorescent screens in the form of spaced patterns such as those previously described. As before the relation between-the film thickness and the spacing of the separate stripes should be such as to result in a proper spectral distribution in the transmitted light.

In connection with all the above-described embodiments of my invention it will be understood that the life of a screen which has become exhausted may be renewed by applying a fresh coating of fluorescent material. It is also contemplated that the screen itself may take the form of removable insert of an inexpensive material-such as a translucent paper or parchment .which will make practical the replacement of a worn out screen with a newly coated unit.

Furthermore, while I have referred to my invention by way of example in terms of particular substance and structure, I consider its principles to be capable of general application in connection with various types of fluorescent screens. I therefore contemplate by the append--l .ed claims to cover all such modifications as fall cient in red comprising a base member and a fluorescent coating applied thereto, said coating comprising layers of a material containing a sub-

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