JP3044389B2 - Density correction device in image recording device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial application field> The present invention relates to a density correction device in an image recording device,
In particular, the present invention relates to a density correction device for preventing a density of a recorded image from fluctuating due to fluctuations of various factors relating to image recording.

<Conventional technology> Conventionally, as an image recording apparatus, a light beam is modulated based on an image signal, and the modulated light beam is scanned on a photosensitive material as a recording medium, thereby developing the photosensitive material exposed to light. A known optical scanning recording apparatus is known (see JP-A-54-42243 and JP-A-59-83150).

One example of the optical scanning recording apparatus will be described with reference to FIG. After that, the light enters a light deflector 5 such as a rotary polygon mirror. The light beam 2 is transmitted to the light deflector 5
The light is passed through the converging lens 6 and scans the photosensitive material 7 in the direction substantially perpendicular to the arrow Y direction (main scanning). At the same time, the photosensitive material 7 is transported in the direction of the arrow Y by a transport device (not shown) to perform sub-scanning, whereby the photosensitive material 7 is irradiated with the light beam 2 two-dimensionally.

The light beam 2 is modulated based on an image signal output from an image signal output device, and an image carried by the image signal is placed on a photosensitive material 7 that is exposed to the modulated light beam. Recorded as an image.

The image signal is a digital signal. As shown in FIG. 8, the image signal is passed through a signal conversion circuit 9 via an image memory 8 and is converted into a light modulation signal based on a predetermined conversion table. After passing through the latch circuit 10, it is converted into an analog signal by the D / A converter 11. Then, the analog light modulation signal is input to the light modulation circuit 12, and the light modulation circuit 12
12 modulates a laser beam emitted from the semiconductor laser 1 and scans the photosensitive material (film) 7 with the modulated laser beam, whereby an image is recorded as a photographic latent image.

The exposed photosensitive material 7 is then sent to a known developing machine 13, where it undergoes development, fixing, washing, and drying. By this developing process, the photographic latent image is developed, and the image carried by the image signal is recorded on the photosensitive material 7 as a visible image.

In the light modulation circuit 12, the semiconductor laser 1
Is directly modulated, but when a gas laser is used as a light source, an intensity modulator such as an acousto-optic device (AOM) is generally used.

<Problems to be Solved by the Invention> FIG. 9 shows an example of an image signal and a density characteristic recorded on a photosensitive material, that is, an example of a gradation characteristic. As is apparent from this figure, generally, the gradation characteristic is not linear with respect to the image signal, and it is impossible to obtain a good gradation signal as it is.

The reasons why the recording density is not linear with respect to the image signal as described above are as follows.

(1) The characteristics of a recording medium such as a photosensitive material are not linear.

(The relationship between the logarithm of the amount of light and the density is substantially linear.) (2) The characteristics of the intensity modulator such as the acousto-optic device (AOM) are not linear.

(3) When a semiconductor laser is used, the relationship between the current supplied to the semiconductor laser and the amount of emitted light is not linear.

For this reason, a signal conversion circuit 9 as shown in FIG. 8 is usually provided to convert an image signal which is an inverse function of a function indicating the relationship between the image signal and the recording density as shown in FIG. A table is provided in the signal conversion circuit 9 so that the input image signal is converted based on the conversion table so that the total characteristic (image signal versus recording density) approaches the linear characteristic as much as possible. .

However, even if the same image recording apparatus / developing machine is used and a certain type of photosensitive material is used, differences in the machine, differences in adjustment conditions, differences in environmental conditions, changes in exposure light sources over time, The recording density may change due to lot switching or the like. For example, in the recording of a medical image particularly requiring a high gradation,
If the above-mentioned problem occurs, there is a problem that the diagnostic performance of the image is impaired.

For this reason, conventionally, there is a configuration in which the conversion characteristics (conversion table) in the signal conversion circuit are changed in accordance with the above-described condition change (Japanese Patent Laid-Open No. 58-190950).
And JP-A-59-83150.

For example, according to the conversion table setting method disclosed in Japanese Patent Application Laid-Open No. 58-190950, image recording and development are performed without using a conversion table based on a known image signal,
A method of measuring the density of a recorded and developed image, obtaining a characteristic function of the recording density for the image signal based on a plurality of sets of the image signal and the obtained image density, and setting a conversion table as an inverse function of the function. Interpolating discrete data consisting of the relationship between the image signal and the recording density,
A conversion table for converting an input image signal into a light amount control signal (light modulation signal) by performing a smoothing process is completely newly set.

As described above, data of discrete conversion characteristics composed of density data obtained by actually performing image recording and development based on a known image signal is continuously processed by performing interpolation calculation and smoothing processing. When a new conversion table is set as data, if the number of sample data is small, there is a problem that an interpolation error for desired characteristics between sample data becomes large and a desired conversion characteristic cannot be obtained with high accuracy. This is because the desired conversion characteristic is generally a curve as shown in FIG. 10, and in a region corresponding to the image signal region R in FIG. 10, the change rate (first derivative) of the curve is locally extremely large. Is due.

On the other hand, according to the method disclosed in JP-A-59-83150, three reference recording densities are set, and a reference signal to be the reference recording density in the reference gradation characteristics is obtained. The image is actually recorded and developed, and the density of the obtained recorded image is measured. Then, based on the obtained density data, the sensitivity (gradation curve level) and the gradation (gradation curve slope) in the reference gradation characteristics are obtained.
The reference gradation characteristic is corrected so as to satisfy the above characteristic.

However, according to the above-mentioned method, when the density characteristics change in a small range, it is possible to sufficiently cope by correcting the reference gradation curve. Since there are few parameters indicating the situation, it is difficult to accurately correct the characteristics corresponding to this, and the reference gradation curve must satisfy the characteristics to some extent. There is a problem that it cannot be used as a reference gradation characteristic and is complicated.

The present invention has been made in view of the above-described problems, and enables a conversion table of an image signal to be updated and set with high accuracy even when the number of sample data is small and the characteristics change significantly. It is an object of the present invention to provide a density correction device in an image recording apparatus capable of obtaining a conversion table corresponding to a condition change by using an arbitrary table as a reference conversion table.

<Means for Solving the Problems> Therefore, in the present invention, as shown in FIG. 1, there is provided signal conversion means for converting an input image signal based on a predetermined conversion table, and the image converted by this signal conversion means is provided. In an image recording apparatus configured to scan a light modulated by a light modulating means on a photographic photosensitive material based on a signal and record a gradation image, a plurality of reference input image signals are converted based on a reference conversion table. Density data detecting means for obtaining density data of a recording image actually obtained corresponding to each of the plurality of reference input image signals, and input image signals and the input image signals recorded on the photographic material. A desired density characteristic table indicating a correlation with a desired print image density is provided in advance, and an input corresponding to each of the plurality of density data detected by the density data detecting means is provided. And concentration correspondence signal setting means for obtaining an image signal from the desired density characteristic table,
On two-dimensional coordinates of the input image signal axis and the converted image signal axis in the reference conversion table, the input image signal obtained from the desired density characteristic table by the density corresponding signal setting means and the corresponding reference input A coordinate table to be corrected on the reference conversion table which is the minimum distance to each of the correction coordinate points determined by the signal obtained by converting the image signal by the reference conversion table, and a conversion table displacement amount calculation for obtaining the minimum distance thereof Means; interpolation calculating means for performing an interpolation calculation of discrete data of the minimum distance to the corrected coordinate point on the reference conversion table obtained by the conversion table displacement calculating means; and corrected coordinates interpolated by the interpolation calculating means. The reference conversion table is corrected based on continuous minimum distance data for a point, and the corrected conversion table is used as the signal conversion table. Was configured from the density correction device contains a conversion table correction setting means for setting a predetermined conversion table in section.

<Operation> According to this configuration, since a plurality of reference input image signals are respectively converted and image-recorded based on the reference conversion table, the reference conversion table is recorded on a photographic material for a predetermined input image signal. If the desired recording image density relationship is satisfied, the density of the image recorded by converting the reference input image signal should match the desired recording image density.

Here, for example, the reference input image signal A is converted based on the reference conversion table to obtain the signal B, and the recording density C obtained based on the signal B does not satisfy the desired characteristics. Assuming that the input image signal for obtaining the density C is D, the reference conversion table is corrected and set so that the input image signal D is converted into the signal B by the conversion table.
It can be seen that the conversion table that satisfies the desired characteristics can be corrected and set.

Here, when the point on the characteristic curve of the reference conversion table is corrected by displacing the point on the characteristic curve of the reference conversion table by a predetermined amount in the normal direction on the two-dimensional coordinates including the input image signal axis and the converted image signal axis in the conversion table. It has been confirmed that the corrected coordinate point (corrected coordinate point) should be a coordinate point composed of the input image signal D and the converted image signal B. Corrected coordinate points (D,
If the coordinate point (corrected coordinate point) on the reference conversion table that is the minimum distance to B) and the minimum distance are known, it is desirable to displace which point on the conversion table in the normal direction and how much. It can be seen that the characteristic point can be corrected.

Therefore, by actually recording an image based on each of the plurality of reference input image signals, the coordinate points (D, B)
As described above, a plurality of corrected coordinate points (points through which the corrected conversion table should pass) matching the desired characteristics are obtained, and each corrected coordinate point (D, B) is determined on the reference conversion table as a coordinate point to be corrected. And a displacement distance (minimum distance between the transformation coordinate point (D, B) and the reference transformation table) for moving the coordinate point to be corrected in the normal direction and displacing it to the correction coordinate point (D, B). Ask for.

Further, if discrete data of the displacement distance (minimum distance) with respect to the coordinate point to be corrected on the reference conversion table obtained as described above is interpolated into virtually continuous data, By displacing each point by a predetermined distance in the normal direction, a conversion table suitable for desired characteristics can be obtained.

Here, even if the reference conversion table has a linear characteristic, the displacement distance data function generally obtained as described above does not have a portion where the rate of change of the function becomes extremely large, and therefore the data of the minimum distance Number, in other words,
Even when the number of reference input image signals is small, it is possible to perform a highly accurate interpolation operation using a known interpolation operation as compared with the case where the interpolation connecting the corrected coordinate points (D, B) is performed. , The reference conversion table can be accurately corrected and set.

<Examples> Examples of the present invention will be described below.

In FIG. 2 showing one embodiment, an image signal is input to an image recording apparatus as a digital signal,
This digital image signal is temporarily stored in the image memory 21.

The input image signals stored in the image memory 21 are sequentially input to a signal conversion circuit 22 as signal conversion means, and are converted into optical modulation signals in the signal conversion circuit 22 according to a predetermined conversion table. The predetermined conversion table is a conversion table for converting an input image signal into a light modulation signal in order to obtain a desired characteristic of a recorded image with respect to the input image signal.

The optical modulation signal obtained by conversion by the signal conversion circuit 22 is converted into an analog signal by the D / A converter 24 after passing through the latch circuit 23.

The analog signal is input to a light modulation circuit 25 as light modulation means, and the light modulation circuit 25 modulates a light beam emitted from a light source such as a semiconductor laser (not shown). When a semiconductor laser is used as a light source, the semiconductor laser is directly modulated and driven by an optical modulation circuit 25. When a gas laser is used as a light source, an intensity modulator such as an acousto-optic device (AOM) is used. Used as modulation means.

The modulated light beam scans on a film (photosensitive material) 26 as a recording medium via an optical device including a collimator lens, a mirror, an optical deflector, a focusing lens and the like and a scanning device including a transport device. The image is recorded on the film 26 as a photographic latent image.

The exposed film 26 is then sent to a known developing machine 27, where it undergoes development, fixing, washing, and drying. By this development process, the photographic latent image is developed, and the image carried by the image signal is recorded on the film 26 as a visible image.

The developing device 27 may be provided integrally with the image recording device, and the film 26 exposed by the image recording device may be automatically sent to the developing device 27 and developed. The apparatus and the developing device 27 may be separate components, and the photosensitive film 26 may be manually conveyed and set in the developing device 27.

Here, in the present invention, in addition to the above configuration, the signal conversion circuit 22
The following configuration is provided in order to correct the signal conversion table in the above in order to cope with a change in the recording density characteristic due to an environmental change or a temporal change.

That is, instead of a normal input image signal, a reference image signal generation circuit 28 for examining characteristics of a recording density actually obtained for the input image signal is provided. In response to a command from the conversion table correction circuit 29, a plurality of test pattern reference input image signals Xi (i = 0,
1, 2, 3,... N) are output. More specifically, a signal is generated so that image recording corresponding to each of the plurality of types of reference input image signals Xi is performed in a predetermined size, for example, in a grid shape.

The conversion table correction circuit 29 is for correcting the conversion table in the signal conversion circuit 22 based on, for example, an external trigger signal or the like to characteristics suitable for the current recording conditions. Controls the switch SW to forcibly input the reference input image signal Xi from the reference image signal generation circuit 28 to the signal conversion circuit 22 instead of the image signal recorded in the image memory 21.

At this time, in the signal conversion circuit 22 and thereafter, an image based on the reference input image signal Xi is recorded on the film 26 by performing the same processing as during normal image recording. When the film 27 is developed, an image recorded based on the reference input image signal Xi is visualized. Here, a density measuring device 30 is provided, and the image recording (test pattern) based on the reference input image signal Xi is provided. ) Can be measured for each of the plurality of reference input image signals Xi.

In the case where the image recording apparatus and the developing device 27 are provided integrally and the development is performed automatically, the reference input image signal Xi is supplied by a command from the conversion table correction circuit 29.
The density measurement by the density measuring device 30 can be automatically performed only when the recording based on is performed (test pattern recording).

When the developing device 27 is provided separately from the image recording apparatus and the development is performed arbitrarily, the density measurement of the test pattern may be performed by a manual operation.

The measurement result by the concentration measuring device 30 is input to the conversion table correction circuit 29 online or by manual keyboard operation or the like.

The conversion table correction circuit 29 updates the conversion table in the signal conversion circuit 22 on the basis of the density characteristic as a result of the actual image recording based on the reference input image signal Xi as described above. The reference input image signal may be set in advance, or may be arbitrarily set by a keyboard operation or the like.

In this embodiment, the function as the density data detecting means is realized by a density measuring device 30, a reference image signal generating circuit 28, and a conversion table correction circuit 29. Further, the conversion table correction circuit 29 It also serves as setting means, conversion table displacement calculating means, interpolation calculating means, and conversion table correction setting means.

Next, the correction setting of the conversion table performed by the above configuration will be described in detail with reference to the flowchart of FIG. 3 and the diagrams of FIGS.

In the flowchart of FIG. 3, first, in S1, the reference input image signal Xi is input from the reference image signal generation circuit 28 to the signal conversion circuit 22, and in S2, the reference input image signal Xi is optically modulated based on the reference conversion table. Convert to a signal.

In the present embodiment, the conversion table according to the change in the recording condition is updated and set by correcting the existing conversion table, and the reference conversion table uses conversion at the time of actual image recording by the signal conversion circuit 22. The conversion table may be a conversion table that simply sets the relationship between the input image signal and the light modulation signal linearly, or a conversion table that can substantially satisfy the desired characteristics of the recording density for the image signal in advance. May be recorded and held, and these may be used. Of course, the conversion table updated last time may be corrected again.

In addition, the purpose of the conversion table correction is, for example, the purpose of correcting a slight variation caused by environmental conditions or the like while using the same image recording device, the developing machine, and the film, or the correction when the type of the machine or the film is changed. Any one of the plurality of reference conversion tables may be selected according to the above.

In step S3, the light modulation is performed based on the light modulation signal obtained by converting the reference input image signal Xi based on the reference conversion table, and the test pattern corresponding to the reference input image signal Xi is scanned by scanning the film 26. The photographic latent image is recorded.

Then, in S4, the recording result is visualized by developing with the developing device 27, and the recording image density di ′ obtained by the development is obtained.
In the next S5, the density measuring device 30 is used for each of the plurality of reference input image signals Xi.
To measure.

The processing after S6 is performed by an arithmetic unit (not shown) incorporated in the conversion table correction circuit 29.

In S6, a desired density characteristic table (see FIG. 4) showing desired characteristics g (X) of the recording density recorded on the film 26 (photosensitive material) for the image signal set and stored in advance is referred to. The density di 'actually obtained as a result of image recording based on the reference input image signal Xi is determined by the desired characteristic g (X) from which image signal Xi' (= g ^-1 (d
i ')))

That is, when the reference conversion table satisfies the desired characteristic g (X) as shown in FIG. 4, when recording based on the reference input image signal Xi, di is obtained as the recording density. But actually the concentration di
However, since the density di 'was obtained, it is understood that the reference conversion table does not satisfy the desired characteristic g (X) with high accuracy.

Therefore, an image signal Xi '(a desired input image signal corresponding to the density data) to be corresponded to the density di' actually obtained based on the reference input image signal Xi is obtained from the desired characteristic g (X). At present, the reference input image signal Xi
Is converted by the reference conversion table into
It has been confirmed that the optical modulation signal f (X
It is known that the density di 'can be obtained by recording according to i).

Therefore, to satisfy the desired characteristic g (X), the density di '
To correspond to the input image signal Xi ′, the image signal X
The optical modulation signal obtained by converting i ′ with the conversion table is f
(Xi) should be good.

Here, as shown in FIG. 5, where the horizontal axis represents the input image signal and the vertical axis represents the light modulation signal (converted image signal), the characteristic curve f (X) indicates the reference conversion table as shown in FIG. When a conversion table that matches the desired characteristic is corrected and set by moving the above predetermined point in the normal direction, the coordinate points (Xi ′, f (Xi ′) )) (Corrected coordinate point) |
ri |, the coordinate point to be corrected (Xi ″,
f (Xi ″)) may be displaced by Δri in the normal direction.

Therefore, in S7, as described above, the reference input image signal Xi
Modulated signal f (Xi) obtained by converting
The density d obtained when the image was actually recorded based on
The correction coordinate point (Xi ', f (Xi)) determined from i' and the image signal Xi 'corresponding to this density is obtained for each reference input image signal Xi, while the correction coordinate point (Xi' , f (Xi))
The corrected coordinate point (Xi ″, f (Xi ″)) on the reference conversion table to be displaced to the corrected coordinate point (Xi ′, f
(Xi)) as a coordinate point on the reference conversion table at the minimum distance from the coordinate point to be corrected (Xi ″, f
(Xi ″)), data of the displacement distance Δri is obtained.

In the present embodiment, when the correction coordinate point (Xi ′, f (Xi)) exists above the characteristic curve f (X) of the reference conversion table, the sign of Δri is set to + and to the bottom. Δ
A distinction was made by setting the sign of ri to-.

By the above processing, at least the number of corrected coordinate points corresponding to the number of the reference input image signals Xi will be obtained,
The characteristic curve connecting the corrected coordinate points is a characteristic curve that matches the desired characteristic. In the present embodiment, instead of setting a conversion table by directly performing an interpolation operation on the corrected coordinate points, the following is performed. To obtain a new conversion table.

That is, in the next S8, a plurality of coordinate points to be corrected (Xi ″, f (X
i ″)), the discrete data of the displacement distance Δri obtained for each of the input image signals Xi ″ is interpolated to obtain the displacement distance data for each input image signal X as virtually continuous data. To be able to

Δri is obtained by converting the reference conversion table to a desired gradation characteristic g.
Data for correcting to a characteristic corresponding to (X),
In particular, when the reference conversion table satisfies a substantially desired gradation characteristic g (X), the absolute value of the reference conversion table becomes a sufficiently small level (see FIG. 6). Even if the interpolation is performed based on, the interpolation accuracy does not significantly decrease, the value to be moved by each point of the reference conversion table can be accurately estimated between the sample data, and the required characteristic can be largely determined from the reference conversion table. Is changed and the absolute value of Δri is increased, the rate of change of Δri with respect to the image signal level is not significantly increased.
It is possible to cope with large fluctuations in required characteristics while maintaining sufficient interpolation accuracy.

In the present embodiment, since the amount by which the reference conversion table (see FIG. 5) is moved in the image signal direction is obtained, the reference conversion table may be set to linear characteristics.
It may have non-linear characteristics. However, when the characteristics are set to be non-linear, it is desirable to use a reference conversion table particularly suitable for substantially desired gradation characteristics.

In S9, the displacement distance Δr with respect to the input image signal X is
By displacing each of the corrected coordinate points on the reference conversion table based on the continuous data of
A conversion table that satisfies the desired characteristics of the recording density with respect to is set.

Specifically, a point (X) on the reference conversion table f (X)
i ″, f (Xi ″)), a distance Δr (Xi ″) on a straight line (normal direction) orthogonal to the characteristic curve of the reference conversion table
The corrected conversion table F (X) can be obtained by plotting the points (X, F (X)) separated by only a distance.

As described above, if the conversion table used in the signal conversion circuit 22 is corrected and set based on the relationship between the input image signal and the actually obtained recording density, the recording medium may be changed due to a change in environmental temperature or a change with time. Even if the exposure characteristic or the light amount characteristic of the exposure light source changes, or the development characteristic of the recording medium changes due to the development temperature or the development speed, etc., it is possible to stably obtain a desired recording density with respect to the image signal. In recording medical images (radiation images) that require high gradation,
Diagnostic performance can be improved.

In the interpolation operation of Δri, spline interpolation (see “Spline Function and Its Application”, Ichida, Yoshimoto, published by Education Publishing 1979, etc.), linear interpolation, a combination of linear interpolation and smoothing, n-order function approximation, etc. May be used.

Also, the reference input image signal Xi and the corresponding density di
Is about 2 to 50 points, preferably 4 to
About 30 is good. In the configuration of the present invention, when the density data detecting means is configured to include a density measurement operation by manual operation, the number of data of the Xi and di is preferably a relatively small number of about 2 to 10 points. In addition, it is particularly preferable to use a higher-order interpolation operation such as spline interpolation or an n-th order function approximation (n ≧ 3) as the interpolation operation. However, when the density data detecting means comprises an automatic density measuring device, That's not true.

Further, in this embodiment, as shown in FIG. 4, the desired gradation characteristic of the recording density with respect to the image signal is a linear characteristic. However, the desired characteristic may not be linear depending on individual requirements. The tonal characteristic is not limited to the linear characteristic.

Further, in the present embodiment, the conversion table alone according to the present invention can be used to correct all of the fluctuation of the exposure light source, the fluctuation of the exposure characteristic of the film, the fluctuation of the development characteristic, etc. A conversion table for correcting fluctuations in the output characteristics of the light source and a conversion table correction setting means dedicated to the conversion table are provided independently, and conversion for correcting the output characteristics of the exposure light source based on data obtained by photoelectric conversion of the scanning light. The table may be set while being appropriately corrected, and at the time of image recording, the conversion table for light source characteristic correction and the conversion table set based on the recorded image density according to the present invention may be combined and used.

Further, a new setting of the conversion table obtained by correcting the reference conversion table as described above may be automatically performed, for example, every time the image recording apparatus is turned on, or may be arbitrarily performed by providing a trigger switch. You may make it.

<Effects of the Invention> As described above, according to the present invention, in correcting the conversion table of the image signal set to satisfy the desired characteristic of the recording density on the photographic photosensitive material with respect to the image signal in accordance with the change in the condition. Even if the number of sample data for examining the state of the density change is small, and even if the characteristics greatly change, the conversion table of the image signal can be updated and set with high accuracy. Since linear characteristics may be used for simplicity, the conversion table for stabilizing and equalizing the relationship between the image signal and the recording density on the photographic material can be updated easily and accurately. There is.

[Brief description of the drawings]

FIG. 1 is a block diagram showing the configuration of the present invention, FIG. 2 is a system block diagram showing one embodiment of the present invention, and FIG. 3 is a flowchart for explaining how to set up the update of the conversion table in the embodiment. FIGS. 4 to 6 are diagrams for explaining how the conversion table is updated in the embodiment, respectively. FIGS. 7 and 8 are system schematic diagrams each showing an example of a conventional image recording apparatus. FIG. 9 is a diagram showing an example of a recording density characteristic for an image signal, and FIG. 10 is a diagram showing an example of a conversion table of an image signal for obtaining a desired gradation characteristic. 22 ... Signal conversion circuit, 23 ... Latch circuit, 24 ... D / A converter, 25 ... Light modulation circuit, 26 ... Film, 27 ... Developer, 28 ... Reference image signal generation circuit, 29 …… Conversion table correction circuit, 30 …… Density measuring instrument

Claims (1)

(57) [Claims] A signal conversion means for converting an input image signal based on a predetermined conversion table; and a light modulated by a light modulation means based on the image signal converted by the signal conversion means on a photographic material. In an image recording apparatus configured to record a gradation image by scanning, a plurality of reference input image signals are converted based on a reference conversion table and input to the light modulation unit, respectively, and the plurality of reference input image signals A density data detecting means for obtaining density data of a recorded image actually obtained corresponding to each of them; and a desired density characteristic table showing a correlation between an input image signal and a desired density of a recorded image recorded on the photographic material. A density pair which is provided in advance and obtains an input image signal corresponding to each of the plurality of density data detected by the density data detecting means from the desired density characteristic table. Signal setting means, on two-dimensional coordinates consisting of an input image signal axis and a converted image signal axis in the reference conversion table, an input image signal obtained from the desired density characteristic table by density corresponding signal setting means, A coordinate point to be corrected on the reference conversion table and a minimum distance thereof that are the minimum distance to each correction coordinate point determined by a signal obtained by converting the corresponding reference input image signal by the reference conversion table are obtained. Conversion table displacement calculating means; interpolation calculating means for performing interpolation calculation on discrete data of a minimum distance to a coordinate point to be corrected on a reference conversion table obtained by the conversion table displacement calculating means; And correcting the reference conversion table based on the continuous minimum distance data for the corrected coordinate point. And a conversion table correction setting unit for setting a conversion table as a predetermined conversion table in the signal conversion unit.