US2463985A

US2463985A - Exposure timing mechanism

Info

Publication number: US2463985A
Application number: US8268A
Authority: US
Inventors: Walter L Linde
Original assignee: Pavelle Color Inc
Current assignee: Pavelle Color Inc
Priority date: 1948-02-13
Filing date: 1948-02-13
Publication date: 1949-03-08
Anticipated expiration: 1966-03-08

Description

Patented Mar. 8, 1949 EXPOSURE TIMING MECHANISM Walter L. Linde, Brooklyn, N. Y., assignor to Pavelle Color Incorporated, New York, N. Y., a

corporation of Delaware Application February 13, 1948, Serial No. 8,268

2 Claims.

The present invention relates to timing mecha nisms of projection printers, used to control we exposure period in the photographic reproduction of a transparent picture by passing printing light through the picture to light sensitive paper or film material on which the picture is reproduced. In the operation of a projection printer of the above-mentioned type, the duration of the exposure period required for optimum results in printing a picture with light of standard intensity, depends primarily on three factors; namely, on the character of the light sensitive material and particularly on the sensitivity or speed of the light sensitive emulsion coating of said material, on the average density of the negative or other transparency bearing the picture reproduced, and on differences between the densities of different portions of the picture. Thus, for example, if some portion of the picture,,such as a child's face, is of special interest, the exposure period should be longer when that portion of the picture is relatively dense than when it is relatively transparent, even though the average density of the picture is the same in each case.

A primary object of the present invention is to provide improvements in an automatic timing mechanism for projection printers of the type disclosed in an article by J. Robins and L. E. Varden, printed in the June, 1946, issue of the periodical "Electronics. That timing mechanism includes means for projecting light from the source of printing light through a transparent picture being reproduced to a photocell to thereby produce a photocell current of a magnitude which depends on the average density of the transparent picture and which is used to discharge a timing condenser previously given a regulated charge.

Since the photocell current is a function oi the intensity of the light received by the photocell,

the time required to discharge a condenser charge oLgiven magnitude varies wtih variations in the average density of the picture being reproduced.

In timing mechanisms of the above-mentioned type heretofore used, the corrective changes in the exposure period to compensate for variations in the sensitivity or speed of the photographic ma- 2 prises a plurality of resistor elements permanently connected in series. One of said voltage dividers is connected across a source of constant voltage, and the other voltage divider is connected in shunt to a variable portion of the other voltage divider. In prior timing mechanisms of the above-mentioned type, the compensating variations made in the charges put on the timing condenser, were effected by adjustments of a single voltage divider connected across a source of constant voltage, and the adjustments made included the operative connection in and removal from the voltage divider of a plurality of resistor sections, with a corresponding change in the voltage drop across each resistor section operatively retained than any suggested in .the above-mentioned.

- "Electronics" article, and too high to permit those terial emulsion and for differences between theaverage density, of the transparent picture reproduced and the density of the portion of that picture which is or special interest, are effected by adjusting the condenser charging circuit to thereby vary the charge put on the timing condenser. a The major object of the present invention is to resistors to be wire wound variable resistors.

The improvement gives important practical advantages. In particular, it permits the use of individual resistor elements having resistance values small enough so that each such element may be a wire wound resistor and hence have a stability and life length not obtainable when the resistances of the individual resistors are so high that it is practically necessary that the latter be made of carbon. The invention also permits of a widerrange of variation in the condenser charge and hence in the exposure period than is practically possible with the prior arrangement. Furthermore, the improved arrangement permits compensating adjustments of the condenser charges to be made in a suitable multiplicity of predetermined steps. the relativeoperative efiects of which are not significantly varied by the corrective adjustment made necessary by variations in the emulsion characteristics of the photographic material on which the pictures are printed. a

The various features of novelty which characterize my invention are pointed out with particularity in the claims annexed to and forming a part of this specification. For a better understanding'of the invention, however, its advantages. and specific obiects attained with its use, reference should be had to the accompanying drawing and descriptive matter in which I have illustrated and described a prefererd embodiment of the invention. 7

The one figure of the drawing is a diagram illustrating a preferred embodiment of the present invention.

The apparatus diagrammatically illustrated in the drawings, comprises means for automatically controlling the-duration or the exposure periods 01' a projection printer A. The latter as diagrammatically illustrated, comprises a source oi printing light A' which ordinarily is a tungsten filament lamp, a support A for transparency to be reproduced, an objective lens A, a shutter A and a support A for photographic material on which a transparent picture positioned by the support A may be reproduced by light transmitted through the picture and the lens from the lamp A to the photographic material on the support A when the shutter A is open. The shutter A is opened and closed by shutter operating mechanism B when an energizing circuit for the latter is opened and closed-by the adjustment or a switch member C between open and closed positions.

The energizing circuit closed when the switch member is in its closed position comprises a conductor i connecting a stationary switch contact to a supply conductor L, and a conductor 2 connecting a second stationary switch contact to one terminal of the shutter operating mechanism B. The second terminal of the device B is connected by the conductor 3 to a supply conductor L. The conductors L and L may supply alternating current of ordinary voltage and frequency. For example, 110-115 volts and 60 cycles per second. The stationary switch contacts to which th conductors I and 2 are connected, are bridged by the switch member C when the latter is in its closed position. The switch member C is biased to its closed position as by means or a spring 0' but is connected to a relay armature D which cooperates with the relay winding E to hold the switch member 0 in its open position when said winding is energized. The winding E is normally energized as hereinafter explained, but may be deenergized to initiate an exposure period by the momentary opening of a normally closed switch F. A second switch member G shown as rigidly connected to the switch memher 0 and relay armature D controls the condenser charging circuit, as herein described. A; the switch member 0 is adjusted between its closed and open positions, the switch G is respectively adjusted between its open and closed positions and thereby opens and closes the condenser charging circuit.

As diagrammatically illustrated, a prism H passes some of the light transmitted through the picture on the support A to the cathode of a photocell H and makes the latter conductive. When the deenergization oi the relay winding E causes the switch G to be moved into its open position, the photocell H initiates a current flow through a condenser discharging circuit and thereby effects the removal of all but a predetermined portion of the charge which the timing condenser I had previously acquired in a manner hereinafter described. The condenser discharging circuit comprises a conductor I connecting the anode of the photocell H to one end of a resistor 8, and a resistor I which connects the second end oi. the resistor C to one end of a conductor 8. The other end of the conductor 8 is connected to the terminal or plate i of the condenser I. The second terminal or plate 1 of the condenser I, is connected by a conductor 9 to the cathode oi the photocell H. The resistors 8 and 1 form part oi' a voltage divider, the junction oi the conductor and resistor 8 being connected through a resistor HI and conductor II to the positive supply conductor DC or a direct current distribution system. The negative terminal DC or said distribution system is connected through conductors l2, l3 and i4 and a resistor II to the Junction of the resistor l and conductor I. The voltage across the reliltors l and 1 due to their connection to the D. 0. supply conductors DC and DC, causes a current flow in the condenser discharging circuit when the photocell H is made conductive by the light passing to it and the switch G is open.

The direction 0! the condenser discharging current ilow is irom the cathode of the photocell H to the condenser plate or terminal 1. and from the second condenser terminal or plate 1 through the conductor 8, resistors I and 8 and conductor I in the order stated, to the anode oi the photocell. As hereinafter explained, the charge previously acquired by the condenser I when the switch G is closed, makes the condenser plate or terminal i positive relative to the plate or terminal 1'. The condenser discharging current continues to flow until a predetermined portion of the original condenser charge is eliminated. The portion or the condenser charge thus eliminated is that required to make an electronic valve J conductive. The valve J is a triode, shown as having its control grid connected by a conductor It to the conductor a and hence to the negative terminal or plate i or the condenser I. The cathode of the valve J is connected by a cathode bias resistor ii to the conductor 8 and thereby to the positive terminal or late 1 of the condenser I. The anode of the valve J is connected to the supply conductor DC by a resistor i1 and the conductor II. The charge acquired by the condenser I prior to the opening or the switch (3 makes the control grid 0! the valve J suillciently negative relative to the cathode to make the tube J non-conductive and to keep it non-conductive until the end of the 40 exposure period. As the condenser charge is being reduced by the condenser discharging current flow, the potential or the control grid 01' the valve J progressively increases relative to the potential 01' the valve cathode, until the valve becomes conductive.

As shown, the upper or positive end oi. the resistor 'i is connected to'ground by the ground connection It, and current flow through the cathode resistor II, which occurs when the valve J ismade conductive, increases the potential of the cathode oi the valve J relative to the ground potential, and causes a previously non-conductive valve'K to become conductive, and thereby eil'ects the reenergizatlon oi the relay winding E and the adjustment or the relay armature D to the right. The resultant opening movement to the right 01 the switch C deenergizes the shutter mechanism B and thus terminates the exposure period initiated as a result of the deenergization of the relay winding B. The adjustment oi the switch member D to the right also moves the switch member G into its closed position and thus closes the hereinafter described condenser charging circuit. The valve K is a thyratron having its cathode connected to ground and having its control grid connectedby a conductor II to the cathode of the valve J. As shown, the thyratron K has a screen grid connected by a conductor 20 to a slider contact 2i engaging and adjustable along a potentiometer resistance 22 in shunt to a voltage controller shown as a glow tube L having its cathode connected to the supply conductor DC by the conductor, and having its anode connected to a ground connection 23. In practice, the glow tube L may well'be of such type and form as to maintain a potential diflerence between its anode and cathode of 150 volts, so that the potential or the negative supply conductor DC is 150 volts negative relative to the ground potential. The potential oi the positive supply conductor DC may well be, and is assumed to be 250 volts positiverelative to the ground connection.

'The switch F may well be and is assumed to' be of a self-closing type and is closed except when momentarily opened to initiate an exposure period. The opening of the switch F opencircuits and deenergizes the thyratron K as well' potential of the control grid of the thyratron K relative to the potential of the thyratron cathode to fire the latter and thereby reenergize the relay winding B and almost instantly gives the condenser a charge which makes the valve J not conductive. The resultant decrease in the potential of the thyratron control grid relative to the potential of the cathode of the thyratron does not make the latter non-conductive, as-is well known.

The movement of the switch member G into.

its closed position, effected when the relay winding E is reenergized, closes a charging circuit for the condenser I. The condenser charging circuit, closed by the closure of the switch G, comprises one stationary switch terminal connected to the negative terminal or plate 1" of.

the condenser I, and a cooperating stationary switch contact connected to a conductor 24 and connected to the first mentioned stationary switch contact by the switch member G when the latter is in its closed position. The charging circuit also includes the previously mentioned conductor 8 and resistor l5, and includes voltage divider elements M and N through which the conductor 24 is connected to the resistor l5.

The voltage divider element M comprises a plurality of resistance sections 25-34, connected end to end in series, a separate tap 35 through which each of said resistance sections 25-34 is connected to a corresponding one of a plurality of separate switches 25'-34'. Each of said switches 25'-34' when closed connects the corresponding resistance section to the conductor.

24. In normal operation only one of the switches 25'-34 is closed at the same time. The free end of the resistance section 25 is connected by a conductor 31 and slider contact 38 to-a slide wire resistance 39 included in the circuit of the voltage divider N. The free end of the resistance section 34 of the voltage divider M is connected by a conductor 48 to a pivoted switch arm 4| adapted to engage one or another oi! a circular series of eleven switch; contacts 42. Each of those contacts is connected by a corresponding conductor 43 to one end of one or another of ten series connected resistance sections 45-54. The end of the resistance section 45 not connected to the adjacent resistance section 45, is connected to one end of the resistance 39.

v The other end of that resistance is connected to 6 the anode of the glow tube L and to the corresponding ground connection23. The end of the resistance section 54 not connected to the adjacent resistance section 53 is connected through a resistor 55 to thejunction of the conductors I4 and i3, and through the lager is connected to the cathode or negative terminal of the glow tube'L'.

with the voltage divider elements M and N arranged as described, the resistance sections 38,

45-54 and the resistor 55 of the voltage divider N are connected in series across the 810w tube L, and the resistance sections=25-34 of the voltage divider element M are connected across the portion of the resistance of the voltage divider N, between the point along the resistor 39 engaged by the slider contact 38, and the point at the left end of whichever ofthe resistance sections 45-54 is then connected by the corresponding conductor 43 and switch contact 42 to the switch arm 4i.

In the neutral or mid-position of the switch arm 4|, shown in the drawing, it connects conductor 40 to the conductor 43 which is connected to. the connected ends of the

resistors

49 and 50. With that switch ar'm position, the voltage divider M is connected in shunt to the portion or the voltage divider N including the

resistors

49, 48, 41, 45 and 45 and the portion of the resistor 39 between the slider contact 38 and the resistor 45. As the switch arm 4| is rotated clockwise into engagement with the lower right hand switch contact 42, the

resistance sections

49, 48, 41, 46 and 45 are successively eliminated from the portion of the voltage divider N to which the voltage divider M is in shunt. This raises the potential of the negative plate 11' of the condenser I and thus decreases the charge put on the condenser I and consequently 40 shortens the exposure period. When the switch mm 4| is turned counterclockwise from its midposition into engagement with the lower left hand switch contact 42, the resistors 58, 5|, 52, 53 and 54 are successively added to the portion of the voltage divider N to which the voltage divider M is in shunt and the exposure period is correspondingly lengthened.

The operative effect of the voltage divider N on the charge given the condenser I preparatory to an exposure, depends upon which of the switches 25-34 is closed. In the neutral adjustment of the voltage divider M, the switch 30' is closed and the remaining switches of the voltage divider are open. With the switch 38' closed, the potential given the negative condenser plate 1'. is the ground potential less the voltage drop in the portion of the resistance 39 between the ground connection 23 and the slider contact 33, plus the voltage drop in the

resistor sections

25, 25, 21, 28 and 29, and the portion of the resistance section 30 at the right of its connection to the corresponding tap 35. The actual potential drop occurring in each of the resistor sections 25 to 34 depends upon the position oi. the contact 38 along the slide wire resistance 39 and on the adjustment of the switch arm 4|, and does not depend on which of the switches 25-34 is closed since there is no current flow through the closed switch, except closing the switches 292128, 2?, 26' and 28,

one at a time, is to successively increase the po tential of the condenser plate 2, and to corr spcnolingly decrease the exposure period. Similarly, the effect of successivel closing the switches 32', and one at a time, is to successively decrease the potential of the condenser plate 2" and to correspondingly increase the exposure period. I

In the practical use of the invention, the switches '3 3 are ordinarily adjusted as re= quired to compensate for differences between the average density of each transparent picture re-= produced anol the density of a particular interest portion of the picture, when the difference is of a character to require compensation. need for compensation can be determined with substantial accuracy by a suitably skilled. operator from a visual examination of the picture. The need for such compensation can he deter= mined more easily and more accurately by the use of an automatic coder which provides a measure of the ratio of the density of the special interest portion of the picture to the average density. of the picture, as is explained in the above-mentioned Eicctronics article. In the practical use of the present invention, in the commercial reproduction of transparent pictures, each picture is coded and then passed to the oper= ator of the projection printer by which the picture is to be reproduced, with a code indication as to which of the particular switches re -3c should be closed in reproducing the picture.

The accuracy with which the needed compensation results are obtained in commercial operation, is increased by suitably relating the values of the resistance sections between each adjacent pair of taps so. In practice, the resist ance sections are advantageously arranged so that the efiect of opening one of the switches 225 3 previously closed, and the closure of the adjacent switch at the left of the switch opened, is to increase the exposure period 1.41 1 /2) times. This means that the closure of any one of the switches following the opening of the second switch to the right of the switch closed doubles the exposure period for the same transparency. Similarly, the eflect or closin any one of the switches following the opening of the second switch to the left of the switch closed, is to shorton the exposure period by one-half.

It is not essential, of course, that there should be any particular number of switches associated with the resistance sections of the voltage divider M, or that the resistance should have their resistance values related as just described. Regarolless of the number of switches and the relative values of the associated resistance elements of the voltage divider M, it is practically desirable, however, that the voltage dividers M and N be associated with one another in the condenser charging network in the general manner de= scribed. When the voltage dividers are so associated, the adjustments of the switch arm M needed to compensate for variations in the sen sitivity or speed of the photographic printing material coated with emulsions made in different batches, do not change the relative efiects of the much more frequently made adjustments. of the different switches 25'-34' of the voltage divider element M needed to compensate for the difference between the exposure time actually needed in reproducing a particular transparent picture, and the exposure time for the picture The all

correspondin to and based, directly on the aver age density of saici picture.

In ordinary practice, the various switches may advantageously be actuated by an electromagnetic relay mechanism including selector switch push buttons conveniently located for actuation by the operator, in the general manner disclosed in the prior application of Jacob Rabinowitz, Ser. No. 3594, 163, filed November it,

192 5. In such case, it is ordinarily convenient to I have the switch actuating relay system arranged so that at the conclusion of the printing of one or more pictures from any particular transparency, the switch system is left in, or is automati cally adjusted into the condition shown in the drawing in. which the neutral or mioi=position switch so is closed and all of the other switches ES 6' are openeol.

For the convenience of the operator of the printing machine the switches El -343', or the push buttons through which they are controlled, are conveniently designated by suitable symbols, such as the symbols V, -IV, lll, -11, I, it, +1, +31, +Illl and +IV shown in the drawing as respectively associated in the order stated with the switches 2E, 2?", etc; 8G respectively. When the resistors 25-36 are wire wound resis tors of commercial magnitudes, the tap connec tion 35 to each resistor may he aohusteol along the length of the latter in assembling the unit M so as to properly relate the resistance values oi the sections of the voltage divider hetwecn'thc

different switches

25, 26, etc.

In an embodiment of the invention now in commercial use, the switch arm M is adjusted from time to time as required to correct forvaria tions in the sensitivity of the emulsion coating of the photographic material on which pictures are being printed. The quantity or batch or photo-v graphic material coated with the same emulsion ordinarily put in stock in a commercial printing plant may well be so large that thousands of pictures are printed from said batch. In said commercial embodiments of the invention, the various resistors of the voltage divider N have such resistance values that an adjustment of the switch arm in the clockwise direction from any one contact to the immediately adjacent contact, will increase the exposure period approximately 10%,

and each similar adjustment step or the switch arm 48 will shorten the exposure period approximately 10%. In said embodiment, the resistance values of the various resistors in the voltage dividers M and N are approximately as follows:

Resistance es s l s ys ewws $3 5sss swl By way or further illustration and example, it

is noted that the resistors I and I! may have resistance of 1000 and 25,000 ohms, respectively, and that suitable resistance values for the resistors 5, I0, I! and I1 are 15,000 ohms, 25,000 ohms, /2 megohm and 1 megohm. It is to be noted, however, that while the resistance values hereinbefore stated are suitable, and suitably related, for practical use, none of those resistance values is sharply critical. The capacity of the timing condenser I should be suitablyrelated to the conditions of use, and in particular to the magnitude oi the charging voltages impressed on it, and to the voltage difference between the condenser plates 2 and i at the instant in which the tube J becomes conductive and operates through the thyratron K and relay winding E to close the switch G and thus interrupt the condenser discharging operation. In practice, with the resistance values above stated and with 150 volts across the glow tube L, the capacity of the condenser I may vary between .05 and .006 microfarad, and the maximum voltage impressed on the condenser plates 1 and i may vary from a maximum of .140 volts to a minimum of about 5 or 6 volts. With a condenser having a capacity of .02 microiarad and with an effective charging voltage of 140\volts, the exposure period used in reproducing a transparent picture 01 average density on photogra hic printing material of average sensitivity, will be about 30 seconds. With the same condenser, transparent picture and printing material, the exposure period may be about one-half of a second when the effective charging voltage is It is believed that the operation of the apparatus illustrated and described herein has now been made appar t.

The timing mechanism illustrated and described herein comprises improvements on the invention disclosed and claimed in the abovementioned prior application of Jacob Rabinowitz.

While in accordance with the provisions of the statutes, I have illustrated and described the best form 0! embodiment of my invention now known to me, it will be apparent to those skilled in the art that changes may be made in the form 01 the apparatus disclosed without departing from the spirit of my invention, as set forth in the appended claims, and that in some cases certain features of my invention may be used to advantage without a corresponding use of other features.

Having now described my invention, what I claim as new and desire to secure by Letters Patent is:

1. In a projection printer comprising,mechanism for reproducing a transparent picture by projecting light through the picture to photographic printing material and comprising exposure timing means including a timing condenser having two terminals and means for proportioning each exposure period to the time required to effect a predetermined reduction in the charge previously given said timing condenser by a discharge current which flows through a photocell included in said mechanism and rendered conductive by light projected through said picture, and which is proportional to the average density of said picture, the combination with said mechanism, of means for varying th charge put on the condenser preparatory to an exposure comprising a voltage source, a voltage divider having resistance sections connected in series across said source, a second voltage divider having resistance sections connected in series with one another, switching means operative to connect said second voltage divider in shunt to a variable portion of the first mentioned voltage divider, and means for connecting one terminal of said timing condenser to said second voltage divider at one or another or a plurality of of points differing in potential from one another, and a source of predetermined potential to which the second terminal'of said condenser is connected.

2. A combination as specified in claim 1, in which said switching means comprises a plurality of contacts, a switch arm connected to one end of said second voltage divider and adjustable into diiferent positions to thereby engage the different contacts one at a time, and conductor means connecting the different contacts to said first mentioned voltage divider at respectively different points along the length of the latter.

' WALTER L. LINDE.

No references cited.