Image Illumination Compensation Method and Apparatus

In the layout of an imaging system, the apparent uniformity of the illumination projected onto the image plane is affected by the off-axis ability of the optical lens system to be able to gather light from the illuminated object plane scene. This is to say that, generally, the best light coupling occurs on the optical axis of the lens, while any coupling at any off-axis angle will be less good.

In the usual projection optics imaging system, this off-axis falloff does not matter unless the projection angle is wide. Such is the reason why the effect, while present, can be safely ignored in the usual photographic type application.

On the other hand, when the angle is determined to be wide: that is when the maximum off-axis imaging angle is over about 20 degrees, the off-axis nonuniformity effect does lead to deleterious overall results. Such a nonuniform condition may be further aggravated by an object plane scene projection system wherein the image receptor is particularly sensitive to variation in the average light received from the scene insofar as the desired final result is concerned. Therefore, the lens falloff becomes a problem when the intrinsic latitude of the imaging system, including the image receptor, does not have enough range to produce a satisfactory end result.

In an imaging system, such as encountered in a large, usually flatbed, lithographic platemaker camera or the like, the light source may be caused to traverse the long direction of the original artwork which is situated on the object plane. This is say that the light source appears like an intense line of light which is essentially perpendicular to the artwork prime axis. The light is then induced to move, or scan, along the artwork's prime axis.

What happens is that, as the light moves along and scans the artwork, more light will reach the image plane when the light from the artwork scene is on-axis with the lens and near the artwork center than when it is at either off-axis extremity.

Compensation for such a scanning light arrangement can be made on the axis which is perpendicular to the axis of light travel by way of a suitable double taper, e.g., butterfly shaped, mask critically located in the optical path and effective to produce the necessary compensation for lens falloff in the one direction which is essentially perpendicular to the light source travel axis relative to the object plane scene. In the other direction, along the light travel path axis, such compensation of the illumination is best made by causing the overall light reaching the artwork scene object plane to vary in accord with its relationship to the lens central axis. This desired effect is met by controllably dimming the light source by some predetermined amount when it is on-axis with the lens, while allowing it to be proportionately brighter the further displaced it is on either side of this central optical axis.

It is therefore a purpose of this invention to cooperate with a lithographic, or flatbed, photographic process camera so as to vary the average light reaching the original artwork from a moving exposure light source and thereby effectively compensate the average light collected from the artwork bearing object plane and passing through the projection lens system and reaching the image plane for the usual losses produced by inequality of optical efficiency for light rays having various off-axis extensions relative to the projection optics axis.

It is another purpose of this invention to variously effect a compensation of off-axis light rays passing through an imaging optical system to have a greater initial value, as produced by a light source, which will offset the off-axis performance degradation of a lens system and thereby produce a virtually constant illumination of the image plane for any angular displacement between the projection optics, and the object plane and the image plane.

Still another purpose is to teach how a traveling exposure light source may be variously modulated in a controlled way such that the on-axis light rays reaching a lens system are initially less bright than those arriving off-axis, producing ready compensation for the off-axis optical efficiency falloff for the lens system thereby producing a substantially constant light value on the image plane axis which is coincident with the light source travel axis.

Yet another purpose for the instant instruction is to yield an electric control system which may be used conjointly with a traveling exposure lamp to produce a variation in the exposure lamp output which deviates in proportion with the reflection position of the lamp relative to an optical system axis; further wherein the variation is produced in true accord with a preestablished electrical contour which principally coincides with the particular optical system's off-axis effeciency relative to its on-axis efficiency.

Still another purpose is to provide an exposure light source which varies in illumination value as its effective position is moved over the lens field so as to produce a substantially constant image plane illumination at least in the direction of source position movement.

Lastly, a purpose is to provide an exposure light controller which varies in average illuminative value with time and in accord with the optical character of the position of a lens system field, and further having the overall average light value being adjustable with the least effect on the overall ratio of the controller variations relative to off-axis position.

FIG. 1 Depicts the relationship between the object plane, the moving lamp, the imaging optics, the image plane and the moving mask juxtaposed near the image plane.

FIG. 2 Plot showing the illumination off-axis light ray coupling fall-off of a typical imaging optics.

FIG. 3 Plot showing step-like compensation of light source output to offset fall-off imaging optics light ray coupling efficiency.

FIG. 4 Elemental schematic for lamp control obtained through the action of a boost-buck transformer with the primary switching combination controlled by signals derived from a memory.

FIG. 5 Pulse train signal combinations for the switch control elements depicted in FIG. 4.

FIG. 6 Elemental schematic for lamp control obtained through the action of a rate multiplier whereby the average number of power pulses is varied to change the lamp intensity from signals provided by a memory synchronized with the lamp mover.

FIG. 7 Partial connection diagram for rate multiplier and memory interconnections.

FIG. 8 Pulse train signal combinations for various rate multiplier controlled lamp power pulse outputs for the circuit of FIG. 6.

The illumination and projection of an object plane image 10 onto an image plane 30 is shown in FIG. 1. The essential elements include a source of line illumination 40-1, 40-2 which is provided to move 51 from left to right across the object plane by a lamp mover 50, e.g. a motor drive or the like. Imaging optics 20 is provided which collects the illustrative ray 21 from location aa on the object plane 10 and projects it to a position ab on the image plane 30. A moving shutter slit 60 provides an aperture for the ray 21', while blocking any stray rays. The shutter 60 is synchronized 53 to be driven right to left 71 by the shutter mover 70, also a motor drive element. What is provided is a means which efficiently translates the viewed object scene onto the image plane as a line-by-line scan operation, albeit the line transitions appear continuous. With the usual projection optics, the on-axis ray 22' reaching the image plane bb is substantially brighter than any of the off-axis ray 21', 23' typified by positions ab, cb. Therefore, the average light energy produced by the illumination sources should be maximum for the positions near the edge of the object plane scene aa, ca and substantially less for the on-axis position ba.

FIG. 2 depicts the light coupling efficiency of a typical projection imaging optics system. As the curve teaches, when the relative image position bb is on-axis with the optics, a maximum projected image intensity is produced, while off-axis positions ab, cb may only produce about half the effective light energy.

FIG. 3 shows how the illumination level, e.g. effective average brightness, of the light source (or lamp output) may be varied from a maximum value at the edges of the object plane scene aa, ca to a minimum near the on-axis position ba. It is possible to reduce the average effective light energy by about half which serves to correct the equivalent increase in on-axis efficiency shown to occur in FIG. 2.

The embodiment of FIG. 4 provides the control effect of the lamp load 160 necessary to practice the invention. The lamp 160 receives a.c. line power 190 on line 192-1 directly to the lamp, and on line 192-2 through transformer winding 152 and continued on line 154 to the other lamp connection. The winding 152 serves to ADD or SUBTRACT effective line potential from the source 190 value to effectively dim or intensify the lamp. Control is achieved by receiving a position signal 100, which is usually a train of pulses which start at zero at one end of the lamp mover travel, and increase count as the lamp moves. This pulse train advances the counter 110 producing a binary output 116 which addresses a memory 120. Alternatively, the lamp mover synchronization signal 100 may be from a mechanical position to an electrical signal encoder in the form of a signal train of binary bytes which can couple directly into the memory address 116. The memory 120 has a preset program 122 stored therein predetermined to be substantially representative of the imaging optics coupling efficiency over the field of view. The memory acts as a decoder, producing an electrical signal sequence function, which addresses 126 the several switch drivers 130-1 through 130-6. The switch drivers serve to produce trigger pulses for thyristor switches 140-1 through 140-6 by lines 132-1 through 132-6. The trigger pulses are also produced to synchronously change at the power line waveform zero crossover point to reduce transients, and electrical strain on the transformer 150. This zero crossover control is produced by the zero crossover synchronization signal source 170, which couples 172 with the switch drivers. Each switch 140-1 through 140-6 couples with a predetermined tap position 142-1 through 142-6 on the boost/buck transformer 150. In combination with FIG. 5, what is shown to occur is that as the mover clock pulses advance, and the ENABLE is active, switch driver SDA is first ON, then driver SDB, followed by driver SDC, SDD, SDE and SDF, whereupon the sequence reverses descending through SDF, SDE, SDD, SDC, SDB and back to SDA as the lamp mover scan completes. The predetermined arrangement is such that when the ba position in the SDF pulse is reached, the imaging optics are exactly on axis. Therefore, when SDA is ON, the maximum BOOST connection 142-1 is ON, whereas when SDF is ON, the maximum BUCK connection 142-6 is ON.

The arrangement of FIG. 6 is preferable for pulsed Xenon, or other discharge type high intensity lamps. These types of lamps produce a "pulse" of light energy for each power line half cycle. The arrangement is also useful with incandescent type lamps. The lamp 260 couples with the a.c. power line source 290 through lines 292-1 and 292-2, with the latter coupled through a thymistor switch (triac or the like) 270 and line 262. The lamp mover synchronization clock signal 200 couples with a counter 210 which, when enabled 202, produces a continuum of binary bytes 216 which collectively act to address a memory 220. The memory has a preset program entered which is predetermined to best describe the illumination compensation necessary for the imaging arrangement, including the optics, which is subject to adjustment. As the counter ascends count, a train of weighted binary bytes 226 are produced which act as DATA inputs to a rate multiplier 230. As is well understood (ref: R.C.A. Handbook "COS/MOS Integrated Circuits" 55D-203C, pages 413-416) the rate multiplier produces a unique train of individual pulses at the output 232, the number of which are, during any predetermined number of input pulses, determined by the binary word connected to the data (or control) input 226 thereto. For example, if during a predetermined period, a binary word "one" is coupled thereunto, only one pulse will output during the period. Alternatively a binary work "nine" produces nine pulses at the output during the same period. Therefore the action is that, under instruction from the memory, the number of pulses on line 232 coupled with the zero crossover synchronization function 270 will vary. The synchronizer 270 also received clock pulses 242 from the line synchronization clock element 240 which enables all switching action changes to occur when the energy in the circuit is about zero. The trigger circuit 250 receives the synchronized control pulses 272, serving to trigger, or turn on, the switch 270 through coupling 252.

The preferred embodiment for a rate multiplier embodied controller is taught in FIG. 7. The mover clock pulses 300 advance counter 310 when enabled 310. The resultant data bytes 316 address the memory 320, by way of example a preprogrammed National Semiconductor Corporation type DM7577. The eight bit memory is divided into two separate binary lines 326-1, 326-2, each providing a sequence of thirty-two four-bit work bytes. The byte signals on line 326-1 couple with rate multiplier 340, preferably an RCA type CD 4527. The line clock 330 is a continuun of power line frequency pulses coupled, by an inverter 332 for wave shaping, to the rate multiplier clock line 334. The hookup for the two rate multipliers 340, 350 is usual with the output 342 of the first clocking the second. With the BCD rate multipliers shown, the circuit waveforms are shown in FIG. 8. The line clock, usually 60 or 50 hertz is shown AA. With a memory output of 18, the rate multiplier combination AB will be 6.times.3. This is to say, for nine frames of time, three pulses ABA will output during each of six of the nine frames. In a like way, the combination 35 (7.times.5) produces seven frames each providing five pulses. Also shown is the combination 56 (9.times.6) producing nine frames of six pulses each, and the combination 72 (9.times.8) producing nine frames of eight pulses each.

This irregular pulse sequence, characteristic of this class of control using a rate multiplier, does not materially affect the illumination effect because the repetitive cycle is so rapid relative to the usual imaging system scanning elements that the total light is effectively integrated.