CA1051084A - Image recording method and apparatus for electrophotographic film - Google Patents

Abstract

A B S T R A C T
A method and apparatus for impressing images on an electrophotographic film of the type whose sensitivity is variable with the level to which it is charged. The method herein particularly provides a step of measuring average light intensity and, simul-taneously charging and measuring the surface potential on the film. The pair of signals generated by such measurement are compared and a comparison signal is generated to initiate the step of exposing the charged film for a fixed or variable duration relating to de-creasing charge level or light intensity.
Termination of the exposure step initiates a signal enabling a toning mechanism to apply toner to the resulting latent image. Toner is applied for a fixed time or a duration dependent upon the charge level.
Termination of toning provides a signal enabling a toner removal arrangement, the termination of the operation thereof generating a signal enabling a fusing mechanism or a transfer and fusing mechanism to make the reproduction permanent. Each operation is sequential, with each step effecting the immediate operation of the following step.

Description

l~S~34 :
This application relates to improvements in a method and apparatus for recording images on electrophotographic film. ~:
. ~ , Canadian Patent Application Serial Number 173,288 filed:
June 5, 1973 is concerned with impression o an image upon an ~.
. ~ .
electrophotographic member having the unique characteristic that its sensitivity varies with the charge level to which it ::
` is raised, and provides a method and apparatus whereby the ~
~ charge level is related to the prevailing light condition of ~-:~ the image to be recorded.
o The improvement described herein is intended to take .
~ further advantage of the aforementioned character.istic.
;j Accordingly, there is provided a method of producing ;, ~
l~ images on a photoconductive film member which comprises the ,¦ steps of charging the member rapidly to a peak voltage, immed- ~:
lately exposing the member to an imaqe and immediately applying:toner to the member; the improvement comprising the ~:
steps of: measuring the light produced by the scene to be l, . recorded, measuring the surface potential of the member, comparing the amount of measured light and the surface charge 20 potential. of.the m~ember, and discontinuing the charging of I the member when the comparison reaches a :, . : :.
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relationship repr~sentative o~ substantially the optimum surface charge for the prevailing light condition of the image be recorded~ .
There is further provided, apparatus which includes means for measuring the light intensity of the image before exposure and deriving a first signal repre-se~tative of said intensity: means for measuring the magni-tude of the charge on the member due to said charging and deriving a second signal representative of said magnituae;

., .
~ lO control means or operating the light passing means to . .
~' effect exposure; means for limiting one of the extent of , exposure due to operation of the light passing means to achieve ,, a predetermined exposure and the operation o~ the charging ~, device to nrovide a peak charge of predetermine~ potentialj, ~, ' . . .
and means for comparing the first and second signals and dis-~,l abling that one of said charging means and light passing ; means which is not limited in response to the relatlonship repre-~ ;
sented by such comparison, when the said one which is not :~
limited reaches a value previously determined as substan-j 20 tially optimum ~or the light intensity measured.
l Further, the invention provides the improvement :ll comprising means for timing the periods of charging, exposure and toning, additional signal-producing means connected with ~l said timing means to produce a signal when the charging ~ 25 period is completed and another signal when the exposure period is completed, said additional sign-producing means being coupled to the charging means, axposure means and toner-applying means, and the latter three means being resp~nsive . ' ' ~, .
~ -3-51~8~L ~
to said additional s.ignals such that the three means are ~ ~ .
set into operation sequentially, completeness of the charging period starting the exposure period and the other signal indicating completion of the exposure period starting the toning period.
~ The pre~erred embodiments of thls in~ention will :. now be describedr by way of example, with reference to the ~ ;
drawings accompanying this specification in which.
: Figure:l is a block diagram of a system from 10- recording images ~n electrophotographic filmr said system ~ ~ embodying the principles of the invention: and ;
.-.( . Figures 2 and 2a are graphs.illustrating in greater detail the ~operation of the system~illustrated in , block diagram fonm in Figure 1 for two different conditions i 15 of ambient light.

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5~ 34 ~s will be evid~nt ~rom the ensuing descrip-tion, bo~h the light intensity of the image and the su1-face potential of the member during the charging thereof are measured slmultaneously. The signals generat-ed by such measurements are compared and the resultant comparison signal utilized to terminate the charging and to initiate the exposure oE the charged electro-photographic ilm. The duration of the exposure can be `i fi~ed or related to the peak charye level. The peak charge level can be fixed and the light intensity varied by a ! ~.
suitable light passing device. Each subsequent step, such as toning, toner removal and fusing is sequential, initiated and terminated by a signal generated upon the termination of the immediately prior step. The length of the afore-said steps may be fixed or may be dependent upon the at-tained charge level, ~hich is related effectively to the ~;
prevailing lIght condition, that is the average light ~ ; intensity of the image to be recorded.
;ii In accordance with the improved technique, the electrophotographic film, or more particularly its photo-conductive layer is cllarged by corona or other means. The light conditions are monitored by a suitable light respon-sive device such as a light meter. The surface charge is measured at the same time and a surface charge is chosen which represents the best film sensitivity for the parti-cular measured liyht condi-tion. The technique preferably ': ' i .
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~L~S~ 14 contemplates that the adjustment will be done automatically, apparatus to accomplish this being provided.
Since the speed of charging is faster than that of conventional photoconductive media, Eor minimum light conditions, for example, the photoconductive coating will normally be charged substantially above saturation, this -l being referred to as shocking the coating. ~Ierein, ! conditions of electrical breakdown are the only physical ~¦ limitations to the charging level of the photoconductive ~ -- 10 coating and the charging time o-E the order of 200 or 300 ~' milliseconds. In this period of time the surface poten~
tial of the coating may rise to 40 or 50 volts which, due to the extremely thin coating, gives extremely high ^Eield strength.
~s soon as the charge level on the surface o~
1 the electrophotographic -Eilm reaches the desired peak ., j . , ~ .
which is controlled by the measurement of light by the light meter, the film is exposed for a fixed time. The ~ exposure is efected immediately aEter the maximum charge `,ll 20 is reached. rrhus exposure occurs at a time when the dark-! .
,l, decay characteristic is dropping rapidly toward the saturation level (under conditions oE minimum light) and ,~ ~
wi~ll probably be completed before the saturation level i5 reached~
;¦ ~ 25 Following completion oE the exposure, the ll toning process is initiated for a period of time which is ~ ~, J~ 5-~ ~
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to at least some extent related to t~e time of expos~re.
For the most part the toning time can be falrly fixed.
; The toner is applied to the surface of the film uniformly and rapidly.
A bias voltage is utilized in close proximity to the film coating to accelerate the particles toward the ilm and to provide even particle distribution which will minimize lateral particle migration, reducing the well-known edge ~l effect in prior art apparatus on relatively dark images.
1~ 10 Finally, after the toning step, if required any excess toner is swept away from the film surface and the remaining toner fused to the surface so that the entire ~;~
process is completed before the surface voltage has dis-,. , ,~:
~,l- sipated substantially.
It is of consequence to mention that -the image resultin~ may be enhanced by varying ~ the length of the toning or the amount of the bias without 3~ ~ ~ bringing up the background. In other words, prior methods and photoconductive media had a continuous background level ., j , , ~1 20 of at least 40 or 50 volts. According to khe invention, .,,I ~ .
,~ since there is no background level to speak o, the back-ground will be affected in no discernible manner if efforts are made to intensify the image by toning for longer than a normal time. -~
When processed ~ the manner described herein, the 1~ image on the film is characterizea by a high degree of ,~' resolution, an almost continuous grey scale extending from ~ -: i i J~, ~ -6-.. . .

~L~5~L~384 `, intense bLack to pu~e white for black and white images and an exceptionally clean background. When the image is projected on a greatly enlarged sca]e for viewing or copying purposes the quality is as good, if not be-tter ~ 5 than photographic. In any event, the subject matter -~ which i5 capable of being recorded by the method and/or i apparatus of invention is no-t limited to copying documents as in the case of prior xerographic and electrofax techniques. As a matter of fact, the methoa disclosed is feasible for use with camera equipment not dissimilar in ~-¦ intended purpose from conventional cameras.
In the description herein, the apparatus of the --~ invention will be set forth in detail as exemplary. In ` ,~.,,-1 . .
Figure 1, the electrophotographic film F which is illus-,~ 15 trated in the drawings is of a construction which includes a transparent plastic substrate 10 which is l~l relatively tough, thin and flexibl~, a photoconductive J~.s ~ ~-31 ~ layer or coating 12 which is preferably cadmium sulfide deposited by R.F. sputtering techniques and an inter-mediate conductive or ohmic layer 14n A pair of re-silisnt grounded contacts 16a and 16b slidably enc~age the opposite eflgea of the conductive layer 14 in a wiping contact to maintain that layer at ground potential. Other methods of assuring that the conductive layer is at ground j 25 potential, at least when it is desired to discharge thephotoconductive surface are feasible~ In order to provlde ~i -7-~3' 1 ' ~: ' . ~ ' :' 1 CD89~
sufficient background to enable unders,tanding of the improvement her"in, reference shall be made to Figure l, I which, to a consiclerahle extent, is quite similar to the I apparatus described in the application of which this is 1 5 additional. The differenc-es will be evidenced, ;
As shown by the first block and the first part ~ of the diagram~ STEP l is stated to be "Discharge Frame ,j :`. ~.
1 by Grounding." The frame of the film F is designated F' ~ -,. ..
``1 and this rectangular area will be carried, across the o various stations o~ the apparatus in a strip film process , ~
~1 ing techni~ue. There is provided a grounded discharge ~ ~
. ~. .
~ head 18 which is intended to remove any electrical ~ , .
~" charge which may be present on the photoconductive laysr J 12. In the case of film members which are to be used over : l .
~15 and over again, as in the event that the toned image is ~,1 to be transferred, there is a possibility of such residual ~
charge remainin~. Mowever, in the use of film which is ~;
; used only once and hence is installed in the apparatus ?
R, ~ new, the handling and exposure to light before use will ~ I
Z0 certainly discharge any static charges which could have accumulatsa by whatever cause.
. 1~l .
Thus, the first most important step of the process or method of the invention is that which is represen,ted by ,~1 the block marked STEP 2 and designated "Charge Erame Rapialy to Peak V,"ltage Determined by Light Conditions~
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~ In accordanc~e with this step, the ~rame F' is advanced ~
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past a charge head 22. This head has a recess 22a in its side ~all over which the frame P' is disposed and I which is coextensive with the area of the film frame F'. A
I corona discharge wire 24 extends across the recess 22a opposite the film F. When a relatively high potential is applied to the wire 24 which is negative with respect - to ground, a corona is generated in the vicinity of the wire 24 and this corona causes the photoconductive coat-` ing 12 defined by the frame F' to become negatLvely charged. Electrons tend to be produced at or below the surface of the coating, holes tending to move toward the ~' conti~uous ohmic layer 14. ;~
The voltage to which the wire 2~ is subjected is of the order of k1lovolts, typically 5000 to 6000 volts.

'''! , 15 In the case of photoconductive surfaces other than that `~l of the electrophotographic ilm utilized herein, the order , ~ of surface potential of the said surfaces is 500 to 600 volts as compared to a surface poten-tial normally less than 50 volts.
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~1 ' 20 The characteristics of the electrophotographic ,~ film F and the charging of the surfa~e thereof will best be explained in connection with Figure 2 which is a graph of the surface voltage of the photoconductive coating 12 i charted against time. Figure 2a is a somewhat similar graph but illustrates the surface voltage for a different ,~ condition of light.
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~S~34 In the case o~ Figure 2, it is assumed that the ;~
lighting intensity is minimum and hence the charging of the photoconductive surface is to proceed to a maximum value~
The thinness of the coating 14, its photoelectric gain and the substantial ratio between dark and light decay ; characteristics result in the great difference in time and voltage quantities mentioned above over the prior art.
In Figure 2, for example, the entire process can be com-pleted never exceeding about 52 volts surface potential and within a space of substantially less than 2 seconds.
Thus t the invention contemplates that the photo-conductive coating 12 is shocked very rapidly to a voltage which is above the saturation voltaye. The charge line 200 is shown to be very steep, rising to about 52 volts in about 300 milliseconds. The peak to which the coating 12 1~' is charged by the wire 24 is here shown at 202. The satura-.
tion voltage for the preferred cadmium sul~ide photoconduckive coating is slightly less than 40 volts and this is illus- ;
: ..
` trated by the broken line 204 in Figure 2~ For other com- ;
:, .
,, 20 pounds this voltage and the others represented in the graph will vary somewhat.
The voltage to which the sur~ace of the coating ;
12 is charged is controlled by the ambient or average light o~ the image or scene to be recorded. This will be ex-plained in connection with the circuitry illustrated but for the moment an explanati~n of the discharge characteristics :

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~)S~ 4 of the photoconducti.ve coating is useful with attention directed to Figure 2.
If from the poin-t 202 the film F remains in darkness (and it is intended that the charging will take place in darkness) then the elec-trons which are sitting at or near the surace will tend to migrate toward the ohmic layer 14 to combine with holes which tend to move in the opposite direction. The discharge, as it is known, will lower the voltage of the surface along the character-istic line 205 at a fairly fast .rate to the saturation `~:
level 210. This is due to the fact that the film surface is in effect "overcharged" and wants to dump charge as fast as it can. Once the saturation level 204 has been reached, the rate of discharge decreases and the curve is flatter as indicated at 208. The curve 206, changing slope at 210 and continuing along the line 208 is known as the :~ dark decay curve. This curve is totally different from .
the dark decay curves of other photoconductive members which fall away at a much greater rate.
If the film is subjected to total brilliant li~ht at t~e time .300 second commencing at the voltage 202, the .l discharge will be practically complete and almost instan-. taneous~ It wi.ll drop along the steep line 212 in a few .'~!
milliseconds to a voltage at the knee 214 which is so close 1, .
, 25 to zero as to be almost immeasurable. The characteristic .,, :, ~:~ discharge thereafter will approach zero asymptotically . . .

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~ 5~ 34 along the line 216. This curve comprises what is known as the light deca~ curve and again it dif~ers radically from that of other photoconductive members, whose Jlght decay curves are not as steep and cannot reach a condition of discharge which is close to zero. In fact, a background charge which persists in those other photoconductive surfaces is of the order o~ 40 volts or move. As is evi-dent in Figure 2, most of the phenomena represented there-in takes place below 40 volts. NOise is another limitation on other photoconductive surfaces such noise being of the same order as the background.
When it is considered that the charge on the surface will cause the adherence o toner particles it can be appreciated that the very flat dark decay character_ istic 206-608 mean~ that it is feasible to achieve in~
: , ,'' tense black areas without overtoning. The light decay characteristic dropping to zero means that it is feasible , :
to achieve practically dead white areas with no speckles or grey background.
The curves 232-234-240, 224-226-230, 217-218-222 represent intermediate discharge curves as where the intensity of light is between total darkness and total brilliant light. The steepness o~ the curves 232~224 and ' 217 indicates that the photoconductive surface , . : . .
~ - 25 utilized herein has enormous gain when subjected to light ; and hence can discharge rapidly~ In every case the :,, -- :
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discharge occurs in a f~w milliseconds time. The sharp- :
~: ness of the knees 234, 226, and 218 indicates that when the light is cut o~f the discharye stops instantaneously.
The flat curves 240, 230 and 222 are nothing more than : 5 portions of the dark decay curve 208 shi~ted from far off the right-hand end of the graph to the left along the lines 236, 228 and 220 respectively. The one that is :
visible is the shifting of the one-half portion of the ~:~
: curve 208 on the right of the point 238 to the left to be a~tached to the bottom end of the curve 234. It will be appreciated that in this discussion it is assumed that ~ the exposure has ta~en place in the period of 30 milliseconds s ~ after charging so that all of the knees 234, 226 and 218 ' '`J occur at the time .330 second. The ~nees 214 being at ::i the bottom of total discharge would occur irrespective J
- of the time of exposure.
The electrical anisotropy of the photoconductive coating utilized herein makes it possible for each incre~
;ment to behave differently in accordance with the intensity of light,~ that is, the number of photons which impinge ~: : against it. Thus, each increment will have a characte.ristic `,l discharge cuxve like one of those described above (for `~l example, 232-234~240~ and there will be as many such curves as increments. The resolution of the electro~
~¦ 25 photographic ~ilm depends upon the independent response : 'i l ~ . .
of the smallest increments which is believed to be limited ~ -, .~ -:I ~J ~13-:.. 1 ~ : .

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only by the size o the crystalline systems produced during ~he deposit of the coating. It ha~ been dete~-mined that the discrimination is so great between such systems that there is no discernible grain in images produced using such film. An important point to be realized from an examination of Figure 2 is that the numher of discharge curves representing the actual phenomena is practically infinite for even the smallest area of film.
According to the invention concerned herein, ~i the voltage to which the photoconductive surface 12 is charged depends upon the amount of am~ient light or the ,;
~l average light of the scene or image to be recorded. The :i .
~`3 reason is that the sensitivity of the ~ilm varies with .......................................................................... .
~,! lS and is dependent upon this voltage. I~he higher the i' ,J surface voltage the greater the sensitivity. Thus, it be-comes feasible to adjust the sensitivity through the use ~ ~
of this phenomenon for different conditions of light. For ~;
low intensity light conditions the sensitivity is increased ~"!,~ 20 and or high intensity light conditions the sensitivity ;~
;' is decreased. Figure 2 graphically shows the conditions ~ when the light intensity is low and Fiyure 2a graphically ,;, .~ .
shows the conditions w~en the light intensity is high. In each case, as will be explained, the time of exposure is ~ixed.

1~ 84 In Figure 2a li~e reference characters designate similar curves and points illustrated in Figure 2 but differing by being primed. Thus, the charging curve 200' `
rises rapidly to the point 202' which, in this case is considerably below the saturation level 204'. This latter is identical to the level 204 of Figure 2. It has been I assumed that the amount of light available for the image : ~, to be recorded is quite substantially greater than it is ;? in the case of Figure 2 and hence there is no need to , j, , charge the photoconductive coating 12 to a voltage of 52.
~1 Instead, it is charged only to a voltage of about 36. At `~ this point, the characteristic dark decay curve 208' commences to fall away slowly, without any rapid initial discharge equivalent to that represented by the curve 206 of Figure 2. The light decay curve commences along the steep discharge 212' and proceeds to the knee 214' slightly abo~e zero and then follows the asymptotic line 216' t~ward total discharge.
The c~arging of the photoconductive coating ~;, .
represented in Figure 2a took only 200 milliseconds in-. ~,, stead of the 300 required under the conditions of Figure 2.
, ,;
This time is controlled by the surface potential 202' which ~¦~ in turn has been chosen as the optimum for the light con-¦ ditions measured by the apparatus.
¦ ~ 25 The e~posure time in this case is again 30 milli- -seconds, this preferably being fixed in the apparatusO In :.: .
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the period of exposure, the diferent increments of the photoconductive coating are again subjected to di~ferent , intensities o light. Each increment will discharge in ;.:
-l aiccordance with the amount of such light, giving a large l 5 number of discharge cha.racteristics typical ones of which - are shown at 224' and 217'. These are similar to the ~................ discharge curves 232, 224, and 217 of Figure 2v The : ~
~' slopes of these curves of Figure 2a are not ~juite as steep ~ ~.
;; as those of Figure 2, although the knees 226', 218, and ~.
~:¦ 10 214' are as abxupt as the knees of Figure 2. The reason ! for the decreased slopes is the fact that the conductive ~ :
1 , .i coating i6 not as sensitive in Figure 2a as it is in "
~ Figure 2. r~he dark discharge curves are substantially ... . .
~ the same, these being 230' and 22~' to the right of the : ~ ~ . .-~i 15 time .23 second. .~ ~
, 1 .
In order to perform the step o measuring the :
ambient light for ascertaining the surface voltage to .;
which the photoconductive coating 12 is to be charged, there is a Iphotocell 32 placed adjacent the film frame. F' be`ing exposed so that its output is proportional to the ~ , .~ amount o~ light which is incident on the film. The photo~
~ .
~ ~i cell will be directed toward the source of the scene or ,3:~ .
:¦ : may even be arranged by suitable optical means to pick up :~
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the light which passes through a ~hosen corner of the ; 25 film, etcO
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It is noted th~t th~ pllotocell 32 o~ a light met~r (or any o~her ligllt measuring apparatus) is shown i~dependent o~ -the projector 56. While it i5 necessary that the photoresponsiv~ device 32 "see" the ambient light of the scene before exposure in order to control ilm sensitivity, it need not be independent of ~he projector 56. It can be in the projector path so that the light of the pro-jected scene is measured, so long as it can respond beforc ~ exposureO It can respond to average incident light, the - lO relation to charge voltage for this condition being worked out by means of a series of tests.
An example of the apparatus has the output of~
~, the photocell 32 inverted so that the signal varies in-., .
versely as the intensity of light. This is a convenience 9in~Q ~he cnd ~e~ul~ -Will b~ d iow~r charging voltaga !
:~ ~or a hrighter light. The charge on the film F is monitor-ed by an electrometer 34 which ie incorporated into the head 22. The electrometer is arranged to d~velop a voltage which is proportional to the surface charge on a non- ;
'f. . ~ ~
~ 20 ~ illuminated portion of the frame, for example, a dark .
corner beyond the image area. Accoraingly, its output i ~ will follow the charging curve 200 or 200'~
As in the apparatus described in the earlier ap-; ~ . . . ~.
~c~ pllcation to w~ich specific reference is made, the output "
signals of the photocell 32 and electrometer 34 are ap-pliea to a dif~erential ampli~ier 36 adjusted to ~ave high , l ~ -17~

1~ :
;'il~ , '`''', .' , ' ' S~L084 gain so that when its two input signals become equal,;~
the output voltage of the ampliier 36 drops rapidly~
A variable resistor 38 which varies the input from the photocell 32 provides an adjustahle reference setting.
The output of the amplifier 36 is applied to a current ariver 38 which is~ in turn, connected to the coil 42a of a relay 42, the other end o which is con-nected by a switch 44 to ground. Coil 42a controls a swi-tch 42b which connects the corona wire 24 in head 22 to a negative voltage supply indlcated by the battery 46 `i Switch 44 is normally open, as is the relay switch 42b ;~
7 Switch 44 is closed when the film frame F' is properly ~ ~ positioned in front of head 22 as indicated in Figure l.
..... . .
The switch closure may be effected manually or automaticallv by way of ~ ~ mechanism which incrementally moves the ilm.
In any event, it is closed for the duration of the charg~
ing operation. ~ `
If the p~otocell 32 senses that -the light inci-, , , jJ ~ dent on the film frame F' is quite intense, then a rela-1 ~ tively low voltage is applied to the amplifier 36. This 'll means that a relatively small charge on the film frame F' will cause an e~ual output from the electrometer 34 and thereby terminatè the charging operation relatively quickly.
In this case, the film will be charged to a relatively low "'I , peak voltage as indicated by point 202' on the curve 200 o~ Fiyure 2a. On the other hand, if the photocell 32 1 , .
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lQS~34 sensec that the incide~t light is not as bright, then a higher voltage is applied from the photocell to the differential amplifier 36, consequently, it will require a greater charge on the film to develop the output from the electrometer 34 that will terminate the charging operatlon. In this event, the film is charged to a hiqher peak voltage as typified by the point 202 on curve 200 in Figure 2.
Desirably, the film is charged to the correct peak voltage as quickly as possible. trhis is accomplished ~. . ..
i by subjec~ing the film to a relatively high voltage which ~
.-~ , .
may be in excess of khe saturation voltage for the film ~ and may even approach the breakdown voltag~ of the film i ~ut wJIl al~avs be below this voltaqe. This is possible ! . .
;! ~ 15 wi-th the present system because immediately after the film is charged, it is exposed to the image being repro-`1: :
j . .
duced, as will~be described presently. Thus, the chaxqe ~ on the film is reduced before actua~ brea~down can occur~
.~, . . . .
As will be deduced from the graphs o Figures 2 ~ 20 and 2a, it is intended that there will be no time elapsing ~`
;l between the moment that the proper surface potential has i`~ been reached and the instant that th~ exposure commences.
This is clear from the fact that in each case the "Expose"
period begins immediately after the "Charge" period. In `3, ~ ~ : 25 ; Figure 2 the exposure of the photoconductive surface 12 to the scene or image to be recorded commences at .300 `''-1 :
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~)5~L084 second and in the case of Figure 2a exposure commences at .200 second. The apparatus used to pracl-ice the in-vention is advantageously constructed to minimize the lapse of time between the end of charging and the commence-; 5 ment of exposure. There are many mechanical techniques ~ which can be used and which enable mov~ment of physical ;~ members at extremely high speed. Note for example, the mechanisms which are used in high speed single reflex ~; cameras to move a mirror. For purposes of explanation it may be said that the film F is moved to a succeeding station from the charging head 22, but movement of the film member is not totally essential. rThe presence of a fine wire such as 24 in an optical train at a location where . it is out o focus will have no e-Efect upon the image i;' 1~ seen by the film. Thus, it is feasible to leave the wire ~ 24 in place at all times and maXe the exposure without ,~,t~ , moving either the film F or thewire support 220 ~ ~ While it is feasible to work`out the apparatus I ~ for practicing the method of the invention in such a manner 2D that the exposure is timed in accordance with the charge on ! the film reaching a certain level, this is not preferred.
`1 It is preferred to establish a time of exposure which is . 1. .
~1 . satisfactory for most of the recording to be executed and '':JI
`~1 use t~at time for all e~posures. Thus in Figures 2 and 2a, although the sensitivity of the ilm is dif~erent in ~3, each case, havi~g been adjusted in accordance with light j -20- -".`~1 ~
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conditions, the time of exposure is 30 milliseconds~
The light and daxk decay curves are shown in these views for both conditions~ Likewise examples of dis-charge curves for increments illuminated somewhere be-.; 5 tween maximum and minimum light are shown in order to ~.
- illustrate the range of voltages and hence the range of grey tones which will be achieved~
~`~ Where there is a low ratio between dark and i light resistivity,`the time of exposure is important to ~ `
¦ 10 the eventual results achieved primarily because of the ~ problems o toning to obtain a good grey scale and even : :j' : . :' `~ simple contrast Thus, assuming that a dark decay curve fall.s of rapidly immediately after charging has been ; completed, ;.t may follow quite closely the light decay , , :
`,~ lS curve so that for a short exposure time it is very diffi-: cult to obtain substantial charge differential on the illuminated and nonilluminated portions of the frame~ In -i! that case, it is advisable to adjust the exposure time to ~.'.j` :'l .
~ give the dark decay curve an opportunity to ~latten out ``1 1 .
I ~1 20 and establish.a greater differential between the charge .. , j , . . .
:¦ on illuminated and nonilluminated areas. ~he charge level of an unilluminated portion for a given type of film ~'! i can be ascertained and this information used in suitable apparatus to control the time of exposure, as will be :~. 25 explainedO This problem however, is minimized and the :~
i, .
~' aforementioned solution is not necessary in the case of . the electrophoto~raphic film w~ich is utilized herein~

. , .' ~ :

~ This can be ascertained from an examination of the graphs :`
of Figures 2 and 2a.
In those two graphs, it can be understood that the illu~ination of the photoconductive surface I2 by j 5 light results in an immediate and precipitous discharge :, :
so that within a period of a millisecond or two there is I a vast difference in charge batween the increments which ~.
are illuminated and those which are not. Accordingly, -~; ~
., : - ' ,':~,there is no naed to extend the exposure to a point where the difference between tha dark and light decays curves increases. As a matter of fact, the discharge curves for increments of intermediate illumination drop at such a rapid rate that too long an exposure can result in such discharge of the surface that the imaqe will :~
deteriorate. ~he high rate of discharge of the photo~
; conductive surface is due to its extremely high electri~
cal gain. A fi]m having a~photoconductive surface with moderate gaîn mlght beneflt~from ~he extenslon of the exposure time to give ~e~ter contrast. This affoxds a good control of density, grey scale, etc. on such film.
i.l:
' ~ Referring again to Figure 1, there is indicated .
a conventional projector 56 which projects the image to be , ; ~ ~ ~ reproduced onto the frame F'. As previously mentioned, ..... .
the apparatus could be constructed in the form of a small camera having a primary lens system for direct viewing of a scene instead of beiny used in copying. A normally J~,, ~ . -22-, 1.: , :
.. 1 : : , ,, 1 ~

~S~i8~
close~ shutter system 58 is positioned between the pro-jector and the filrn E~ to control the duration of the exposure. The shutter is actuated to open as soon as the charging operation is completed by the drop of thP
output voltage o the differential amplifier 360 A
differentiator 62 detects the negative going pulse from the amplifier output and applies a signal to the SET
input of a flip-flop 64~ The output o~ the flip-flop at the I terminal energizes the shutter drive mechanism 63 which can be any type of conventional shutter drive suit-ably modified or adjusted.
The two methods of controlling exposure which ;~, have been men$ioned require apparatus which is readily : $ incorporated into the system. Where shutter 58 is to be ,.......................................................................... .
operated for a preset period of time, the shu~ter mechanism - itself may have an automatic timing device which returns the shutter to its closed condition after the period has . ~ ..
¦ expiredO A simple timing control 65 which is manually ad-,, .
justable is shown connected by the broken line 67 to the .~ .
shutter drive mechanism 63 to signify that it is an alter-~ ~ .
'~ nate to the circuitry shown in solid lines~ In such case, where the flip-flop 64 is used, one can provide for a reset signal to be generated by the shutter drive mechanism ' simultaneously with the end of the exposure period, this .~1 .
signal being transmitted by the line 69 to the rest ter-minal R.

~ .
~ 23 .1 - ;`~:

i :

~S~089~
The second method of control and one which is needed only in the e~ent that a lower gain film is used is somewhat more complex. The charge on the frame F' at an unilluminated portion is monitored during exposure :~
by the electrometer 34. The output signal of the ~. electrometer 34, in addition to being applied to the dif-:.
ferential amplifier 36 i~ also applied to the high gain !
differential amplifier 66~ Thls amplifier 66 also re ceives the output voltage from an adjustable reference ~ .:
voltage source 68. The output signal of the amplifier 66 is applied hy way o~ khe inverter 72 to the RESET input ;.
of the flip-flop 64. The adjustable reference voltage source 68 is arranged to terminate the exposure of the .~ fillil Wil~l t;l~ dLy~ on til~ nolliiiumillated portlon moni.tor-ed by the electrometer reaches a selected value. When .3 ~: ~ : this point is reached, the output voltage of the amplifier 66 drops, resetting the flip-flop 64 ana closing the ~ shutter 580 In this latter.case, the shutter drive mechanism -.iJ ~
:~ 63 is constructed to open the shutter 58 on receiving one . 20 signal rom the I output o the flip-flop 64 and to close I "I
.1 it on receiving a. second signal from the I output of the , :. flip-flop 64.
i The block which is designated STEP 3 in Figure 1 is actually the second most important step of the method and as indicated, it can be exposed for a preset time or in ac-cordance wikh the level of charge to which the photoconductive ~ .

. 1 . .
: surface drops during the e~posure. I'he fixed time step is ~24-.! .
:~ .

~ 1 :
.. ! ..... .....

~L~S1~
preferred and i~ much easier to effect.
As explained in the hereinabove immediatel~ upon completion of the exposing step, toner is applied to the film frame F'. Furthermore~ the toner ~;~
is applied in the presence of a bias field which propels the toner particles toward the film. ~ot only does this speed up the toning process, it also distributes the ¦ toner particles over the charged portions of the frame so as to minimize the edge effect which characterizes l 10 images made by the usual xerographic processes~
, .
~, The commencement of the toning step as indicated by the S~EP 4 block in Figure 1 follows im~ediately after --, exposurec Looking at Figures 2 and 2a, the step begins at .. '.~ ' : ' ; the`times .330 second and .230 second respectively.
,...,~
l 15 Toning can be initiated in any one of several ways. In :' i :
the method where the exposure time is fixed, the output -signal from the shutter drive mechanism 63 signifying the ~' ;`.. 1 , end of the period is applied on the line 69 to reset the flip-flop 64 and also appears at the input to the variable i 20 one-shot multivibrator 76. In the method where the ex-posure time is controlled b~ the level to which the l charge on the photoconductive layer drops, the reset sig-,l ~ nal out of the inverter 72 is applied to the varia~le one-,;l shot multivibrator 760 Any other suitable method can be '".'`,'! 25 used, such as for example, mechanically coupling the shutter 58 itself to a mechanism for toning so that when ;

' ~ the shutter closes it cimultaneously initiates the toning mechanism.

.

. ~,:
s~o~
In the apparatus illustrated the variable one-¦ shot multivibrator 76 is chosen to be of a type which has : ~ , , a variable time constant. The output siynal of the multi- ;
,, vibrator 76 is applied to,the solenoid of a normally , , 5 closed solenoid valve 78 which is connected in a conduit1 or pipe line between a liquid toner supply 82 and a toner dispenser 84 positioned adjacent the film frame F'. Again i .' ' it is appreciated that there will of necessity be rela- ' ' ~ l tive movement between the film F and the toner dispenser '- 1 . " ,:, -84. Suitable mechanisms for accomplishing this must be provided but are within the skill of the artisan familiar with this field. As soon as the one-shot multivibrator, ~, 76 is triggered by the reset signal the valve 78 opens , ~' . '.., ~"', ~ causing the low ~ toner. ~h~ ~;qpens~r ~, ?.n ?.perture `~' ¦~ 15 84a which is dimensioned to be coextensive with the frame ~-, ~ F'~when the latter is properly positioned. The liquld toner , will bathe the~entire frame F'.
An electrode 92 ext~ends around the edge of the ~j ~ aperture 84a. This electrode 92 is connected by way of 'i" 20 the relay switch 94 to one terminal o~ a voltage source !i .
,~: symbolized by a battery 96 whose second terminal is con--¦" ~ ~ ~ nected to ground. The output signal of the one-shot multivibrator 76 iS also applied to the relay coil 94a con-,,,l trolling the switch 94 so that when the one-shot 76 is i - 7 ~ `
~ 25 triggered the relay is closedO This applies a strong, l : ~
~ , ;~ positive potential to the electrode 92 whichhelps to 7~ ~ 26~
" .., ,i :~ ~
.:~ ' : , ~ -,~ . ~ :
, . ~ :

~:. :i,,, , :, . ... . . . .
- . . : . :

~)S~ 34 propel toner particles toward the film to o~tain more I uniform distribution of toner, especially in areas whi~h ! are heavil~ charged, as for example, because not having I been strongly illuminated.

¦ 5 Toner particles will adhere to tho~e portions of the frame F' which were not illuminated during the ~ exposure step and in vaxying degree to those areas that ! were illuminated. The amount of toner which adheres is ¦~ proportional to the charge of the area or increment. I~
.
desired the bias can be varied inversely with the output o~ the electrometer 34 so that a hiyher bias voltage is applied when the amount of light and hence the surface charge ~ is lower. Genexally, however, a fixed bias of from SO
7 ~ to 100 volts d.c. is ef~ective to accomplish the even lS and complete distribution o~ toner. Toning can also be done by dry toner using the same general method.
! . . ~ .
~ n Figures 2 and 2a it will be noted that the ., ; toning time is dif~erent. The charge on the surface of a photoconductor will affect toning. Higher voltages o~
char~e require less toning time. This is an alternate to greater bias. Thus, assuming that the bias is fixed, for the conditions o~ illumination represented by the graph o~ Figure 2a the light is greater but the charge is lower than in Figure 2. Operating at lower voltage will re-;~ . .
quire somewhat greater toning time, hence the toning time in Figure~2a is shown to be for a period o~ O770 second 'I

: 1 ~
,~ .
1 :~ .

l084 instead o~.670 second in Figure Z where the charge ~ ~ voltages are much higher than in the illuminating con- ~`
, ¦ ditions of Figure 2a.
The time constant setting of the variable one-¦ S shot multivibrator 76 could easily be controlled by the ~'~
maximum level of charge measured by ~he electrometer 34.
i -A line from the electrometer coupled to ~he one~sho~
76 through sui~able control circuitry could provide ~he necessary inormation. Otherwise manual means could be provided to vary the time constant of the one-shot 76, i~ the operator raferring to a suitable meter reading de-rived from the electrometar 34.
The structure shown in Figure l uses a fixed :, . . ... .. . . . - - . .. . .... ..
l ~;~ toning tîme. The toning operation ceases upon the re-:., ;
; ~ 15 se~ting o~ the one-sho~ multivibrator 76, the time in-terval depending upon the time constant setting. As -soon as the one-shot 76 is quiescent, its output voltage drops. This sIgnal or absence of s1gnal disables the valve 78 th~s closing the same. The same signal is ' ~1 .
'11 20 used to init~ate the next step which is shown in Figure l ;
as the b10ck~designated STEP 5 'lImmediately Remove Excess , Toner."
" ~ i ~ : : : ,.
, ~ The signal which is produ~ed when the one-shot ~",'1 76 has completed its cycle is detected by the differentiator ~11 25 102 and applied as an input or triggering signa1 to another ,`il~ variable one-shot multivibrator 104. The time constant ~of j ~
this multivibrator 104 is also capable of being manua1ly adjusted to any desired valu~. The output from the one shot .
_28-,i.~!~., :
: . . : , :

~51~
104 is connected to the sol.enoid valve 108 connected in .
a linc between the vacuum pump 110 and a hooa 112 which is open to the frame Fl. Any method of applying suction to the frame F' could be operated by the signal from the vne-shot 104. ~his action immediately sucks excess toner .
away from the illuminated portions of the ~rame whi~h retain little or no charge and evaporates the toner I solvent so that the latent image.on the rame is reduced o visible form.
i 10 The step which is designated S~EP 5 is not es-sential to the process for all types of systems. Where ~ .
systems utilize self-evaporating solvent or powdex that ¦: ~will not adhere except at charged areas, it is not neces-. .
~' . : sary to provide any complex apparatus or go through any 15. involved steps to eliminate excess toner. The physical .j ! . , i¦ nature of the toner itself and/or the conditions under ~: . w~ich it is used may.obviate the need for.active xemoval.
. ~ . .....
Accordingly, it should be underst~od *hat the e~pression : .-removal of toner includes any such removal whether done de1iberately or where conditions cause the elimination of ! excess toner without anyt~ing being d.one either by the ap-'I
paratus or the operator.
The last step which is illustrated in Figure 1 is t~e block STEP 6 and is designated FUSE TO~ER. In this ,¦ 25 step, the closing of the valve 108 after the variable one-shot 104 has completed its cycle is detected by the dif-. ~ . . . .
.~: ferentiator 120 resulting in a siynal that is applied to a ,~ .
¦ -29~ ~ ~ !
.. :.~. .

; , :
1 `--i : :

lOS1084 third variable one-shot multivibrator 124. Thi~ lat~er ~¦ multivibrator produces a ~signal whose output turns on a heater or heat lamp 126 backed by a reflector 128 con-l centrating the infrared rays on the ilm F. The heat - 5 fuses the toner permanently to the photoconductive sur-face Qf the film F and a~ter the cycle of the one shot -I 124 has been completed, as determined by its time setting, ~ the lamp is turned off and the process of recording the ;~ image is completed. ~-~
;~ 10 In the event that there is no intervening step line STEP 5, the output rom the variable ~ne-shot 104 may , ,,j ' ~:
be connected directly to the lamp 126 without the inter-t~' vening elements 120 and 124. If the removal of excess toner automatically requires a small increment of time after the application of toner, the variable one-shot r.-signal from 104 can be delayed by any suitable electronic ;~ ~ delay means. ~ ~ ;
;~ ~ The process of the invention also contemplates ` ~ that~there may be a transfer step between steps 5 ana 6 ,~ 20 Thus, if the film F is to be used to transfer images to a ~,.. , . . ~ .
paper or other acceptor member such transfer will be ~: effected immediately after toning. This is indicated by the broken llne channel extending from the block STEP 5 to the block STEP 5A, this latter being designated "Transfer~

l~ ~ 25 Transfer may he effected by mechanical pressure or corona transfer means. Thereafter, the bxoken llne indicating ~~`
the~process step se~uence extends to the block STEP 6.
',.., ~
~ 30- ~

`;1~ - ~
. , ~ .

~15~34 Fusing is accomplished on the toner transferxed to the ` acceptor member.
1 In choosing the surfac~ potential which is ¦ represented by a condition of charge of thei photoconductive-l 5 coating 120 the most logical method o~ doing so i5 to ¦ measure the surface directly by a suitable voltmeter, ' ''"`. . ' ' previously referxed to as an electrometer. This would normally take into account any variations which could :,..
~ result from differences in the conditions ihat exist in :: ~
the surrounding vicinity that might change the response ~, of the surfaca to a given corona voltage. In other words, irrespective of the corona voltage which might be higher ,' or lower depending upon humidity, variations in the aging of components and the like a reading of potential at the surface would be a~solute. A simplified device which may ¦ not operate as well could have a control of corona voltage related to the response of the light meter in an effort '?;, ~ ^ to produce a more economical device. Since this is in effect control of the surface charge, the invention here-: 1 : , , in includes this expedient and reference to measurement or Il control of the potential of the surface charge of the ,,, .
coating should be taken as encompassing control by ad-justing the corona voltage.
In other simpli~ied apparatus, it is not im- ?
1 25 practical to have a manual control which sets the constants ;1 ~ of the power supply for the corona voltage over a dial i~ calibrated in readings of the light meter. The operator ~:
~; -31- ~
? :

, .

I then reads the light meter, sets the value of the read-ing on the dial and thereafter initiates the operation of the power supply circuit knowing that the corona voltage will quickly rise to a certain value which he .
- 5 has adjusted for. Another and more sophisticated ar- ~ -rangement which is nevertheless not ully automatic `'.i - 3 would have a circuit whose corona power supply is ' energi~ed by a control that responds to surface poten-tial of the coating. The control includes a manual adjustment that moves over a dial calibrated in light " .,.
' values but represents surface potential. ~he corona power ;~
`"
supply would be energized manually by the operator and rise in voltage until the signals provided by the manual ad-~' !, ¦ ' I ' ,justment device and the surface charge are equal (or have ~ 15 a certain relationship). At that point the charging is :,' ' , stopped. " ,, , ` In still another arrangement alternate to aa-d, justing the surface potential to which the film frame F' ; is charged, it is feasible to construct apparatus in which . ~
~, 20 the charging level is fixed for all conditions, but Al means are provided to vary the amount of light projected '¦ onto the frame. This latter is done by adjusting an ,~1 iris in tha projector 56 and~or varying the speed of the shutter 58. A comparison is readily made by the ,~ 25 photocell 32 and the voltage measuring device 34 which ;¦ measures the surface potential of an increment of the ~ -32-.~.-.1 ,~ .
." .~ .
: - ~

.'. 1. ~,' .
.: :. .. : . : . .. ~. ~. . .

~6~5~4 ~rame F' in darkness, and the resulting information used, manually or automatically to effect the light adjustment.
The comparison values will previously have been deter-mined and been built into the apparatus, using means differing in only minor respects to those previously described hereinabove.

In all of these cases and as well in the specific ¦ structures illustrated and described in detail, there is I in e~fect a measurement of the amount of light and a '1 ' i 10 measurement of the potential of the surface charge, whether ~, directly or indirectly. Furthermore, even though there may ~ ~ be preset conditions which are manually produced by adjust-j men~ or dia;s or conlrols it is within ~he ambit or this ¦~ invention to consider that such measurements are being made l¦ 15 continuously even where preset at some value which is ``l reached after a period o~ time. These meanings are to ~ I be taken by definition.

.,." : '' .~

`1 .

.

!
' . '.i , , .~ ~
.... ~ ~
, ' ~ :
''``'', , . ' '" ' : ' . : . . - . - ~ ... . .. . ... ~ . . . .

Claims (28)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method of producing images on a photo-conductive film member which includes the steps of charging the member rapidly to a peak voltage, immediately exposing the member to an image and immediately applying toner to the member; the improvement comprising the steps of: measuring the light produced by the scene to be recorded, measuring the surface potential of the member, comparing the amount of measured light and the surface charge potential of the member, and discontinuing the charging of the member when the comparison reaches a relation-ship representative of substantially the optimum surface charge for the prevailing light condition of the image to be recorded.

2. The method as defined in claim 1 in which the charging of the member and measuring of the surface charge potential are carried on simultaneously.

3. The method as defined in claim 1 in which the measuring of light, charging of the member and the comparison are carried out substantially simultaneously.

4. The method as defined in any one of claims 1, 2, or 3 in which the peak voltage is a fixed value and the duration of exposure is varied with the intensity of light of the image to be recorded.

5. The method as defined in any one of claims 1, 2 or 3 in which the exposing of the member is continued for a fixed period of time irrespective of the surface charge potential reached by said member during charging.

6. The method as defined in any one of claims 1, 2, or 3 in which the exposing of the member is con-tinued for a period of time during which the level of the surface charge potential at an unilluminated portion drops to a predetermined value providing a difference between light and dark decay characteristics that will produce good toning properties.

7. The method as defined in any one of claims 1, 2 or 3 in which the period of applying toner is fixed irrespective of the surface charge potential reached by said member during charging.

8. The method as defined in any one of claims 1, 2 or 3 in which the period of applying toner is adjusted with relation to the surface charge potential reached by said member during charging for good quality toning, the time of toning varying inversely as the surface charge potential.

9. The method as defined in any one of claims 1, 2 or 3 in which the charging is carried out at such a speed and for such period of time which will raise the surface charge potential of said member above the saturation level of the member and below the electrical breakdown level of the photoconductive member.

10. The method as defined in any one of claims 1, 2 or 3 in which the peak charge value is adjusted to be that which has been predetermined as optimum for good quality imaging at a given intensity of light of the image to be recorded.

11. The method as defined in any one of claims 1, 2 or 3 in which termination of each of said method steps is caused to generate a signal operable to initiate the next succeeding step.

12. The method as defined in any one of claims 1, 2 or 3 in which the exposure step is initiated sub-stantially simultaneously with the discontinuance of the charging.

13. Apparatus for producing images on a photocon-ductive coating of an electrophotographic film member and which includes means for charging the coating to a peak charge, means, including light passing means, for exposing the coating to an image, means for applying toner to the coating and means for varying the magnitude of the charge to provide optimum sensitivity, the improvement comprising, means for monitoring the duration of charging, first signal-producing means connected with said means to produce a first signal when the charging period is completed, said first signal-producing means being coupled to the charging means and exposure means and said exposure means being responsive to said first signal so that completeness of the charging period initiates the exposure period and second signal producing means connected with said exposure means to produce a second signal when exposure is complete, said second signal initiating the toning period.

14. Apparatus as defined in claim 13 and means for measuring the light intensity of the image before exposure and deriving an intensity repre-sentative signal;
means for measuring the magnitude of the charge on the member due to said charging and deriving a charge magnitude representative signal;
control means for operating the light passing means to effect exposure, means for limiting one of the extent of exposure due to operation of the light passing means to achieve a pre-determined exposure and the operation of the charging device to provide a peak charge of predetermined potential, and means for comparing said intensity representative and charge magnitude representative signals and disabling that one of said charging means and light passing means which is not limited in response to the relationship represented by such comparison, when the said one which is not limited reaches a value previously determined as substantially optimum for the light intensity measured.

15. The apparatus according to claim 14 in which the light intensity measuring means comprise a photo-responsive device arranged to receive the projected image before exposure and the charge measuring means comprise a voltage responsive device and in which the comparing means comprise circuitry connected to the devices during the charging period and continuously making the comparison.

16. The apparatus as defined in claim 14 in which the charging means include a corona producing member and a corona power supply, means are provided for connect-ing the power supply to the corona producing member and the controlling means comprise a switch in said connecting means which is closed before and opened when the optimum relationship is reached.

17. The apparatus as defined in claims 13 or 14 in which the first signal-producing means axe rendered operative upon the completion of the duration of charging to produce said first signal, said control means for operating the light passing means being rendered operative by said first signal to commence the exposure of said member and said last-mentioned central means being arranged to complete the exposure in a fixed time irrespective of the time of charging.

18. The apparatus as defined in claim 14 in which the first signal-producing means are rendered operative upon the completion of the duration of charging to produce the first signal to commence the exposure of said member and said monitoring means includes a variable time control circuit connected with said charge magnitude measuring means, said monitoring means being responsive to the surface charge potential reached by said coating at a dark area during exposure to produce an additional signal, the last mentioned circuit being coupled with said light passing means operating control means so that the additional signal will disable the same when the charge on the coating decays to a predetermined level during exposure.

19. The apparatus as defined in claims 13 or 14 in which second signal-producing means are coupled to said toner-applying means and rendered operative upon completion of exposure to produce said second or toning signal, said toner-applying means being rendered operative by said toning signal.

20. The apparatus as defined in claims 13 or 14 in which second signal-producing means are coupled to said toner-applying means and rendered operative upon completion of the time of exposure to produce said second or toning signal, said toner-applying means being rendered operative by said toning signal and having structure to provide for continuation of the toning for a predetermined time period at the end of which said toner-applying means is rendered inoperative.

21. The apparatus as defined in claims 13 or 14 in which second signal-producing means are coupled to said toner-applying means and rendered operative upon completion of the time of exposure to produce said second or toning signal, said toner-applying means being rendered operative by said toning signal and having structure to provide for continuation of the toning for a predetermined time period at the end of which said toner-applying means is rendered inoperative, said structure including time adjustment control means for adjusting the time of toning inversely as the magnitude of the charge reached by charging of the coating.

22. The apparatus as defined in claims 13 or 14 in which means are provided for fusing the toner to the coating after toning.

23. The apparatus as defined in claims 13 or 14 in which said second signal-producing means are coupled to said toner-applying means and rendered operative upon completion of exposure to produce said second toning signal, said toner-applying means being rendered operative by said toning signal and means for fusing the toner to the coating after toning, further signal-producing means being provided and rendered operative upon completion of the toning period to produce a fusing signal, said toner-fusing means including means for rendering the toner-fusing means operative by said fusing signal.

24. The apparatus as defined in claims 13 or 14 in which said second signal-producing means are coupled to said toner-applying means and rendered operative upon completion of the time of exposure to produce said second or toning signal, said toner-applying means being rendered operative by said toning signal and having structure to provide for continuation of the toning for a predetermined time period at the end of which said toner-applying means is rendered inoperative, said structure including time adjustment control means for adjusting the time of toning inversely as the magnitude of the charge reached by charging of the coating, further signal-producing means being provided and rendered operative upon completion of the toning period to produce a fusing signal, said toner-fusing means including means for rendering the toner-fusing means operative by said fusing signal.

25. The apparatus as defined in claims 13 or 14 in which means are provided for fusing the toner to the coating after toning and means are provided also for transferring the toned image to an acceptor medium immediate-ly after toning is completed and before operation of said toner-fusing means.

26. The apparatus as defined in claim 14 in which the comparing and controlling means are additionally constructed to provide for the disabling of the charging means in such relation to the light intensity that the member is charged to a potential which varies inversely as the light intensity for different images.

27. The apparatus as defined in claims 13 or 14 and means arranged to complete the exposure in a fixed time irrespective of the time of charging.

28. The apparatus as defined in claims 13 or 14 which includes the said electrophotographic member and said member comprises an elongate belt-like strip of individual frames capable of being moved relative to said charging, light passing and toner-applying means, said latter three means being disposed at stations along the path of movement of said strip and timing means being connected with means for moving said strip relative to said stations so that each frame is successively brought into operative relation-ship with the respective means during movement of said strip relative to said stations.