JPH01145167A - Recording apparatus and method for recording using same - Google Patents

Abstract

PURPOSE:To enhance the positional accuracy of a pixel to be recorded and the control accuracy of the quantity of light by mounting a two-dimensional recording head, a light source for irradiating the two-dimensional recording head with light, a recording medium receiving light to form a recording image, an optical system for projecting the image on the two-dimensional recording head on the recording medium, and a shutter for controlling the light passing through said optical system. CONSTITUTION:A recording apparatus is constituted of a light source 11, a two-dimensional recording head 1 and an optical system consisting of lenses 13, 15, a shutter 17 and a color filter 14 and, since recording can be performed in such a state that the positional relation of a photosensitive body 16 is fixed, in exposing a silver salt color photograph to light of the three primary colors three times, the positional reproducibility of an exposed pixel of each color is well and shift is not generated at all. Therefore, as compared with a conventional method using a unidimensional recording head together with a mechanical scanning to convert an electric signal to a recording image, there is a beneficial point that the generation of a problem of color shift or a problem of a moire does not exist and an image extremely excellent in color reproducibility can be formed.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a color recording device that performs recording including halftones according to electrical signals, and a recording method using the device.

[Conventional technology]

Conventionally, this type of recording apparatus has adopted a method of photographing a screen formed on a cathode ray tube, a method of forming an image on a color film using three primary color gas lasers, and the like. However, methods using cathode ray tubes have limitations in miniaturization and economy, and the phosphor on the cathode ray tube is non-uniform, resulting in uneven light emission microscopically, making it difficult to obtain high-resolution recordings. Ta. Another drawback was that the image was distorted around the screen. On the other hand, color recording using a gas laser can produce good images, but because the gas laser is expensive and large, it has been more difficult to downsize and make it more economical than with a cathode ray tube.

[Problems to be solved by the invention] On the other hand, in response to the demand for a compact and economical device for recording including halftones, a one-dimensional recording head using a liquid crystal cell array has been used, but in reality On the recording medium of pixels to
In other words, there is a problem in that positional accuracy on the photoreceptor is insufficient, and linear density unevenness in the sub-scanning direction is noticeable due to long-period density fluctuations and non-uniformity in the amount of light of each pixel. Furthermore, when performing color recording, the positional accuracy of each color of the same pixel on the photoreceptor on which three primary color signals are written in a superimposed manner tends to deteriorate, which causes a problem of color variations due to moiré.

For this reason, there is a problem in that the writing position of each color must be macroscopically viewed with sufficient accuracy, and microscopically viewed with accuracy on the order of 10 micrometers. Furthermore, when using a photoreceptor with low photosensitivity such as silver halide color vapor, it is not possible to keep the liquid crystal open for a long time, and there is a limit to the light intensity of the light source, making recording impossible in reality. There's a problem.

This invention has been developed in consideration of the above points, and has good positional accuracy of recorded pixels, very accurate control of the light amount of each pixel, and is also adaptable to low-sensitivity photoreceptors. □The purpose is to provide a recording device capable of recording data and a recording method using the device.

Means for Solving Problem C] The recording device according to the present invention is a two-dimensional recording device in which a plurality of windows are arranged on a two-dimensional plane and have a function of controlling light transmission according to an electric signal and maintaining that state for a certain period of time. The positional relationship between the head, a light source that irradiates the two-dimensional recording head with light, and the two-dimensional recording head is fixed, a recording medium that receives the light to form a recorded image, and a recording medium that records the image on the two-dimensional recording head. It is equipped with an optical system that projects the image upward, and a shutter that controls the light that passes through the optical system.

Further, the recording method according to the present invention uses the recording device to record two digits of a specific digit of a gradation signal digitized in a binary system.
A light transmission state and a non-transmission state are written in the two-dimensional recording head in accordance with the binary value, the state is held, and a light source is caused to emit a light amount corresponding to the maximum value of that digit in the multi-digit system, thereby producing a recording medium. The same recording medium is exposed to light, and the same exposure operation as the above exposure is repeated as many times as necessary according to a binary value of another digit to perform multi-tone exposure on the same recording medium.

[Effect]

The recording apparatus of the present invention performs recording by exposing a plurality of windows constituting a two-dimensional recording head while keeping them in either a light transmitting state or a light non-transmitting state according to an electric signal.

Further, the recording method according to the present invention uses the above-mentioned recording device, and changes the light transmitting state and non-transmitting state to the two-dimensional recording head in accordance with the binary value of a specific digit of the gradation signal digitized using the binary system. The recording medium is exposed to light with an amount of light corresponding to the multi-decimal value of the maximum value of the digit, and then the same exposure as above is repeated according to the binary value of another digit, and the same exposure is performed on the same recording medium. Recording is performed by performing multi-tone exposure.

(Embodiment) FIG. 1 is a front view showing an example of the appearance of a two-dimensional recording head which is a main part of a recording apparatus of the present invention.

In this figure, 1 is a two-dimensional recording head, 2 is a liquid crystal cell array, 3 is a drive circuit incorporating a large number of drivers for driving the liquid crystal cell array 2, and 4 is a wiring section connecting the liquid crystal cell array 2 and the drive circuit 3. , 5 is a substrate.

Here, liquid crystal cell array? The cells are regularly arranged in two dimensions, and each cell can be independently formed into a light transmitting state and a light non-transmitting state by a driving circuit 3. Also,
The liquid crystal cell array used here can maintain either a light transmitting state or a light non-transmitting state for a certain period of time. Examples of this include ferroelectric liquid crystal cell arrays, super twist liquid crystal cell arrays, and thin film transistors. There are cell arrays attached to each cell.

First, a liquid crystal cell array using ferroelectric liquid crystal will be explained.

FIG. 2 is an example of the upper and lower substrates constituting the liquid crystal cell array 2 in the case of FIG. , the windows 7 arranged in the horizontal direction are electrically connected by a common metal wiring 8. Further, each metal wiring 8 is independently connected to the drive circuit 3 shown in FIG. 1, and can be driven independently.

Further, an insulating light emitting layer 9 is applied between each metal wiring 8 to prevent light from leaking from areas other than the window 7.

FIG. 3 shows the metal wiring 10 on the other substrate 6B of the liquid crystal cell array 2 and the window 7 having a transparent conductive layer.
are lined up, and the multi-windows 7 are in electrical contact with the transparent conductive layer and the metal wiring 10 in the vertical direction.
It is individually connected to the drive circuit 3 shown in the figure and can be driven independently. Here, the metal wiring 10 is used for the purpose of limiting the length of the light transmission window in the vertical direction and for reducing the resistance of the wiring because it has a lower resistance than the transparent conductive layer.

Boards 6A and 6B with the wiring shown in Figures 2 and 3
After alignment treatment, for example, a ferroelectric liquid crystal material is sandwiched between them to form the liquid crystal cell array 2 shown in FIG. Here, in addition to the above, polarizing plates in a crossed Nicol state (omitted for complication of illustration) are arranged on both the upper and lower sides of the liquid crystal cell array 2.

2 in FIG. 1 using the liquid crystal cell array 2 having such a configuration.
A dimensional recording head 1 is manufactured. A method for individually driving these multiple windows 7 is basically the same as the method for driving this type of liquid crystal panel, and there is a method known as a matrix drive method or the like. Thereby, image information can be formed on the two-dimensional recording head 1, this state can be maintained, and furthermore, it is possible to rewrite it as necessary.

So far, we have explained ferroelectric liquid crystal cell arrays, but there are other types of liquid crystal cell arrays that have memory properties, such as thin film transistors, diodes, and capacitors, in which a voltage is continuously applied to the liquid crystal for a certain period of time, and then it is refreshed again. There is an active matrix type liquid crystal cell array that applies signals for This allows a liquid crystal cell array that originally does not have memory properties to become a liquid crystal cell array that has memory properties due to the functions of transistors and the like.

Liquid crystal cell arrays having such properties are used in liquid crystal televisions and the like, and other two-dimensional recording heads can be manufactured using this liquid crystal cell array manufacturing technology.

An outline of a recording apparatus using these two-dimensional recording heads 1 will be explained below.

FIG. 4 is a schematic cross-sectional view showing one embodiment of the recording apparatus of the present invention. The light emitted from the light source 11 is made into parallel light by the lens 13, and after passing through the diffuser plate 12 and the color filter 14,
It reaches the two-dimensional recording head 1. The transmission of light is controlled by the two-dimensional recording head 1 according to image information, the optical image is projected onto a photoreceptor 16 which is a recording medium by a lens 15, and the photoreceptor 16 is exposed according to the image information. Here, before the light that has passed through the two-dimensional recording head 1 reaches the photoreceptor 16, the amount of transmitted light can be arbitrarily controlled by a shutter 17 that can be opened and closed by an electric signal provided.

Here, the shutter 17, the diffuser plate 129, the color filter 1
Position 4 can be placed at any position on the optical path. Further, the lens 13 is not necessarily required.

The features of this recording device and the recording method using it will be explained below.

This recording apparatus includes a light source 11, a two-dimensional recording head 1, lenses 13, 15 . Since it is possible to record while the positional relationship between the optical system consisting of the shutter 17 and the color filter 14, etc. and the photoreceptor 16 is fixed, each color to be exposed is The pixel position reproducibility is good and no deviation occurs at all. Therefore, compared to the conventional method that uses a one-dimensional recording head and mechanical scanning to convert electrical signals into recorded images, this has the advantage of not causing color shift or moiré problems, and has extremely high color reproducibility. The first feature is that it can form excellent images.

Next, a recording method for recording halftones using this two-dimensional recording head 1 will be described.

FIGS. 5A to 5D are views showing a part of the liquid crystal cell array 2 of the two-dimensional recording head 1 shown in FIG. 1, respectively. This figure shows eight liquid crystal cells (CI to 08), and a method for performing eight gradation exposure using these cells will be explained. In Figure 5 (A), the cells with diagonal lines (CI, C3, C5,
C7) is in a closed state, and cells C2, C4, and C without diagonal lines
6°C8 is the open state. These states are formed by driving operations and are maintained until they are rewritten by the next new image signal. In the state shown in FIG. 5A, the photoreceptor 16 is exposed to light in units of 1° using the shutter 17 shown in FIG.

Next, each cell (C1 to C8) of the same liquid crystal cell array 2 is put into the state shown in FIG. Make multiple exposures.

Next, each cell (C1 to C8) of the same liquid crystal cell array 2 is put into the state shown in FIG.
7, the same photoreceptor 16 is exposed to a light amount of "4". In this way, each cell (C1 to C8
) The amount of light that has passed through C1 is directed from C1 to 08, and the amount of light is "0" in order.
", "1", °'2", "3", "4", '5", '6
The amount of light is in units of ",7". This value is shown mathematically in the cell of FIG. 5(D).

To explain this principle in more detail, each cell from C1 to 08 is sequentially assigned a binary value from "0" to "7", and at the stage of FIG. 5(A), the binary value assigned to each window 7 is 1
The light transmission state of the window 7 is controlled in accordance with the values of "0" and 1 in the digits.While maintaining the light transmission state of the window 7 formed by this operation, the maximum value of the binary value is , for example, the value converted to a decimal value (for example, the first digit is "1", the second digit is "2.3")
The shutter 17 controls the amount of light proportional to 4'' (the fourth digit is 8''), and the photoreceptor 16 is exposed. Here, exposure is carried out after maintaining the light transmission state of the window 7 of each digit.The reason for the variation in the operation of this type of cell is that the rise and fall for opening and closing of the window 7 are most common. The biggest reason is that it becomes the least when it is maintained. Next, in the step shown in FIG. 5(B), the light transmission state of the window 7 is rewritten corresponding to the value of °'0 and "1" in the second digit of the binary value assigned to the multi-window 7, and the same Exposure corresponding to the second digit is performed. Furthermore, in the same manner, the third digit is exposed at the stage shown in FIG. 5(C) to achieve the desired gradation exposure.

In this explanation, we have explained the exposure of 8 gradations, but in the same way, in the state of the next cell array, the unit of 8, the next "16"
, "32", "64", and 128" units, the exposure level increases, and a large number of gradation exposures can be realized by a small number of repeated exposures. At this time, each exposure is The amount of exposure is accurately controlled using the shutter 17, and a light source that uniformly illuminates the two-dimensional recording head is used, so the exposure precision is extremely high, and the opening and closing of each cell is controlled by the memory state of that cell. It is characterized by the fact that variations in the rise and fall of opening/closing operations between cells due to the use of a cell do not pose a problem, and stable gradation exposure can be achieved.

Furthermore, since various types of light sources can be used as the light source 11, one with a strong light intensity can be used, and it is also possible to increase the amount of light by increasing the exposure time. Therefore, it has the advantage that it can be easily applied to photosensitive materials with lower sensitivity than silver salt photographic films such as silver salt color paper. Furthermore, even if the exposure time is increased, the increase in the recording time is relatively small because the number of exposures is small.

By the way, since the photosensitive characteristics of the recording medium are not linear in the units of the light amount mentioned here, the exposure amount at each stage does not need to be an integral multiple of the initial light amount, but must be a value that takes the photoreceptor 16 into consideration. be. Further, in the explanation so far, a method has been described in which multiple exposure is performed from a small exposure amount to a large exposure amount, but it is also possible to reverse this order or change the order appropriately. So far, we have simply described multi-tone exposure, but if we use a photoreceptor 16 that can be exposed to three primary color lights in sequence, like a silver halide color photograph, the color filter 14 shown in FIG. Color recording becomes possible by sequentially converting the color of the color filter 14 according to the color of the image information. In this case, when the two-dimensional recording head 1 is used, the position of the multi-window 7, the optical system such as the lens 15, and the position of the photoreceptor 16 do not need to be moved at all when exposing colors in an overlapping manner. For this reason, color shift due to the shift in the exposure position that occurs when exposing each color, which is a problem with the conventional method of repeatedly exposing light of three primary colors while mechanically scanning with a one-dimensional recording head, is caused by this two-dimensional recording head. The recording head 1 has the advantage of not causing any problems.

On the other hand, in conventional one-dimensional recording heads, in order to eliminate the moiré phenomenon caused by color misregistration, the recording position of each pixel is
Each primary color had to be placed at an angle where moiré would not be noticeable. This method of making each pixel have a different slope for each color is difficult because the window position is fixed in a one-dimensional recording head, so the selectable angles are limited, and complicated window opening/closing control is required. There was a problem that it would not be possible to record without it. For this reason, color printing using a one-dimensional printing head was considered to be economically disadvantageous and to limit the image quality. The recording apparatus of the present invention has a feature in which this problem does not occur at all.

In the explanation so far, the square window 7 of the two-dimensional recording head 1 has been explained, but FIG. 6 shows another window shape.

FIG. 6 is an enlarged view of a part of another example of the liquid crystal cell array 2, which consists of a large window 21 and a small window 22 for controlling light transmission.
By combining these, it is possible to achieve halftone recording of multiple gradations.

Next, a recording method for recording halftones using this two-dimensional recording head 1 will be explained.

FIG. 7 is a partial diagram showing a state in which recording is performed using the two-dimensional recording head 1 having the pattern shown in FIG.
This shows the state recorded by.

Here, the large and small pixels 18 and 19 are assumed to be one set of unit pixels 20.

In this case, the large pixel 18 and the small pixel 19 have the same vertical width, but the large pixel 18 has the same horizontal width as the small pixel 19.
For example, it is set to 8 times (large pixel area + small pixel area = 9 x small pixel area). However, in order to make the area ratios in the drawings easier to see, the values are shown differently from the actual values.

Furthermore, while changing the opening/closing state of the multi-windows 21 and 22,
As described above, by using the method of controlling the amount of light irradiated onto the photoreceptor 16, it is possible to record in eight levels, for example, from the density of an unrecorded recording medium to the maximum density.

Next, gradation recording using these will be explained in detail.

Figure 8 shows a large pixel 18 and a small pixel 19 in the unit pixel 20.
In order to make it easier to understand the state of exposure, the recorded density is shown in terms of the area of the black part (the meshed part), and nine types are shown here. Further, the number shown at the upper right of the unit pixel 20 is a number indicating how many times the area of the small pixel 19 is exposed. FIG. 8(A) shows a state in which the small pixel 19 in the unit pixel 20 is exposed in units of "1". FIG. 8(B) shows a state in which the small pixel 19 is exposed to a density of "2", and FIG. 8(C) shows a state in which the small pixel 19 is exposed to a density of 3"
The state is exposed to a density of units of . In this way, the density is increased one after another until the maximum density of "7" shown in FIG. 8(D) is reached. On the other hand, FIG. 8(E) shows small pixel 1
The ninth density shown in FIG. 8(D) is shown in a state in which a large pixel 18 having an area eight times that of 9 is exposed to a density of "1". Furthermore, FIGS. 8(F), (G), and (H) show the state in which the small pixel 19 and the large pixel 18 are exposed to light at appropriate densities. ”
It corresponds to unit exposure. Further, FIG. 8(I) shows a state in which both pixels 18 and 19 are exposed to maximum density.

If the method of combining the large pixel 18 and the small pixel 19 is used in this way, the light amount control is performed in 8 steps with 3 small exposures as a unit of exposure, and the small pixel 19 provides 8 gradations and the large pixel 18 Since 8 gradations can be combined, 64 gradations can be obtained for the unit pixel 20, and the ability to express gradation can be greatly improved.

In addition, although the explanation so far has been based on a matrix drive type two-dimensional recording head 1, a two-dimensional recording head in which drive circuits such as the thin film transistors described above are attached to the multi-windows 7 and directly controls the opening and closing of the windows 7 is not applicable. Since the window 7 of the liquid crystal cell array 2 of the present invention can be opened and closed at high speed, the rewriting speed can be increased.

By repeatedly driving a super-twist type liquid crystal cell array with good contrast and maintaining that state, it can be similarly used as a recording head.

Next, a specific example will be explained.

[Specific Example 1] A liquid crystal cell array 2 having the configuration shown in FIGS. 2 and 3 was created as follows.

An indium oxide film having a surface resistance of 50Ω was provided on the substrate 6A, and patterned by photolithography. The metal wiring 8.10 was formed by electroless nickel plating. In addition, the window 7 has a size of 200 x 160μ angle, and
It was made into an ooxtoo window. The alignment treatment was performed by oblique vapor deposition of silicon oxide, and the rubbing directions of both substrates 6A and 6B were parallel to each other. A 2 μm glass fiber spacer is used between both substrates 6A and 6B, and a ferroelectric liquid crystal is placed in the gap formed by the spacer to form the liquid crystal cell array 2.
And so. Further, the polarizing plates were placed in two places, one above the top plate and one below the bottom plate, so that the planes of polarization were perpendicular to each other starting from the top. An independent drive circuit 3 was connected to each metal wiring 8 and 10. The rising and falling speeds of this window 7 were about 1 millisecond each when driven at 20 V, and the contrast of the formed memory states was 120:1.

This two-dimensional recording head 1 was incorporated into the recording apparatus shown in FIG.
A gradation record of gradations was obtained. For color recording, replace the color filter 14 according to the signals of the three primary colors, repeat the same operation, and transfer the three primary colors to instant film (FP-1).
00, manufactured by Fuji Film Co., Ltd.).

The resulting recorded image has good density uniformity in each pixel, and a record with no color shift is obtained, and 64 gradations are obtained for each of the three primary color light sources, resulting in a clear color image. It was done. Moreover, no moiré development accompanying recording of the three primary colors appeared in the color image.

(Specific Example 2) A two-dimensional recording head 1 having the window configuration shown in FIG.
The size of the small window 22 was 200×20 μd square, and the number of windows was 100×100. This 2
The dimensional recording head 1 is incorporated into the recording apparatus shown in FIG.
Exposure from °1" to unit "°4" is performed by rewriting the two-dimensional recording head 1 three times, and the large window 21 and the small window 2 are
Using the combination of 2, gradation exposure of 64 gradations was performed around 20 unit pixels. Even in color recording in this case, by replacing the color filter 14, gradation exposure of the three primary colors is performed, 64 gradations are recorded for each color, and a clear color image can be obtained. We were able to reduce the exposure time by about half.

(Specific Example 3) A liquid crystal panel (transmissive panel for electronic OHP with 400 x 600 pixels) equipped with a thin film transistor was used as a two-dimensional recording head, and the apparatus shown in Fig. 4 was constructed.The recording method and photoreceptor were the same as those in Specific Example 1. The resulting image was a clear color image with no color shift.Also, the total time required for recording was approximately 3 seconds, and high-speed recording was possible.

(Effects of the Invention) As explained above, the recording device of the present invention is a two-dimensional recording device in which a plurality of windows are arranged on a two-dimensional plane and have a function of controlling light transmission according to an electric signal and maintaining that state for a certain period of time. a head, a light source that irradiates light to the two-dimensional recording head;
A recording medium that has a fixed positional relationship with the dimensional recording head and receives light to form a recorded image, an optical system that projects the image on the two-dimensional recording head onto the recording medium, and a shutter that can be opened and closed according to electrical signals. The shutter can be opened and closed accurately according to electrical signals, and the positional relationship between the two-dimensional recording head and the recording medium is always fixed, allowing extremely high-precision recording without color shift. be able to.

Further, the recording method of the present invention uses the above-mentioned recording device,
A light transmission state and a non-transmission state are written in the two-dimensional recording head in accordance with the binary value of a specific digit of the gradation signal digitized using the base system, and while the state is maintained, the maximum value of the digit is multi-valued. For example, a light source emits a light amount slightly corresponding to the decimal value to expose the recording medium, and then the same exposure operation as the above exposure is repeated as many times as necessary according to the binary value of another digit. Since multi-gradation exposure is performed on the recording medium, the exposure amount of each pixel is highly accurate, and the positional accuracy of the recorded pixels is also good, so there is no color shift and no moiré even in overlapping exposure by color recording. It has the characteristic that no phenomenon occurs. For this reason,
In order to prevent moire, it is not necessary to adjust the arrangement angle of pixels, and information processing such as complicated recording order is not required, which has the advantage of being economical. Furthermore, there is the advantage that relatively low-sensitivity photosensitive materials can be used.

[Brief explanation of the drawing]

FIG. 1 is a front view showing an embodiment of the two-dimensional recording head of the present invention, FIGS. 2 and 3 are configuration diagrams of a substrate showing an embodiment of the liquid crystal cell array of the two-dimensional recording head of the present invention, and FIG. 5 is a schematic cross-sectional view showing an embodiment of the recording device of the present invention, FIG. 5 is a diagram showing the open/closed state of the cell and the amount of transmitted light,
The figure shows an example of the shape of a large window and a small window, FIG. 7 shows a recording state, and FIG. 8 shows a pixel exposure state. In the figure, 1 is a two-dimensional recording head, 2 is a liquid crystal cell array, and 3 is a two-dimensional recording head.
is a drive circuit, 4 is a wiring section, 5, 6A, 6B are a substrate, 7 is a window, 8.10 is a metal wiring, 9 is a leading light layer, 11 is a light source,
12 is a diffuser plate, 13.15 is a lens, 14 is a color filter, 16 is a photoreceptor, 17 is a shutter, 18 is a large pixel, 1
9 is a small pixel, 20 is a unit pixel, 21 is a large window, and 22 is a small window. Figure 1 5: Easy 4 and Figure 2 9゛light ix eyebrow Figure 3 Figure 4 Figure 5 Figure 7 Figure 6 N2 Small V/On Figure 8

Claims (3)

[Claims] (1) A two-dimensional recording head in which a plurality of windows are arranged on a two-dimensional plane and has a function of controlling light transmission according to electrical signals and maintaining that state for a certain period of time, and a light source that irradiates the two-dimensional recording head with light. , a recording medium whose positional relationship with the two-dimensional recording head is fixed and which receives the light to form a recorded image; an optical system which projects the image on the two-dimensional recording head onto the recording medium; and the optical system. A recording device comprising: a shutter that controls light passing through the system. (2) The recording apparatus according to claim (1), wherein the window constituting the two-dimensional recording head is constructed by combining a plurality of large windows and small windows. (3) a two-dimensional recording head in which a plurality of windows are arranged on a two-dimensional plane, which has the function of controlling light transmission according to electrical signals and maintaining that state for a certain period of time; and a light source that irradiates the two-dimensional recording head with light. , a recording medium whose positional relationship with the two-dimensional recording head is fixed and which receives the light to form a recorded image; an optical system which projects the image on the two-dimensional recording head onto the recording medium; and the optical system. Using a recording device equipped with a shutter that controls the light passing through the system, the light transmitting state and non-transmitting state are recorded in the two dimensions according to the binary value of a specific digit of the gradation signal digitized using the binary system. While writing on the recording head and maintaining that state, the shutter controls the amount of light corresponding to the multi-digit value of the maximum value of the digit to expose the recording medium, and further according to the binary value of another digit. A recording method characterized by repeating an exposure operation similar to the exposure a necessary number of times to perform multi-tone exposure on the same recording medium.