JPH11298677A - Image contrast emphasis method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To physically detect printing irregularities when the printing irregularities are fetched as images by a camera or the like by selecting and using a light source provided with the peak of spectral intensity in a wavelength area where the change rate of the light transmissivity of a colored film becomes maximum. SOLUTION: For a liquid crystal color filter by a printing method, on a glass substrate to which a black matrix for normal light shielding is vapor-deposited, the three kinds of the inks of R(red), G(green) and B(blue) are printed and the colored film is formed. The variation in the film thickness of the colored film brings the fluctuation of a transmitted light quantity and is perceived as gradation irregularities (printing irregularities). When printed matter is fetched as an image by an image fetching device, a light source provided with the peak of the spectral intensity in which the change rate of the light transmissivity of respective colorants becomes maximum. by the film thickness of the printed respective colored films is selected and used. The change of the transmitted light quantity by the film thickness change is largest in the wavelength light of transmissivity 37% and it is preferable to select and use the light source for which the transmissivity of the light transmitted through the respective colored films is about 37%.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image contrast enhancement method capable of physically detecting print unevenness when the print unevenness is captured as an image by an image capturing device such as a camera.

[0002]

2. Description of the Related Art Since a person can perceive and recognize a very small difference in density as unevenness, one of the factors determining print quality is detection of print unevenness. Conventionally,
In the inspection of print unevenness, it is difficult to identify all the print unevenness by physical measurement, and it has depended on a visual inspection by a skilled inspector. Further, for stable imaging of the printing unevenness by the camera, it is necessary to enhance contrast before taking in the image. However, as many quality inspections have been automated, there is no known method of enhancing the contrast before capturing images.
This is a situation in which the inspection of subtle print unevenness has not been automated.

Among them, some research efforts have been made, and when performing a defect inspection of a color filter having a red, green, and blue colored layer formed on a transparent substrate by printing or other means, a spectrum peak is used as an inspection light source. Value is about 59
An inspection method using at least one of 0 nm and about 510 nm has been proposed (JP-A-6-9463).
No. 8).

As described in the above publication, generally, most of the transmission spectrum of a color filter has three peaks (red (R) and green (G) of a color filter near a peak value of a three-wavelength fluorescent tube. ), Blue (B)), and the peak is designed to have a sharp peak with a small half-value width as much as possible to improve color purity.
Therefore, three spectral valleys are set at 510 nm and 59
It is often located near 0 nm.

Therefore, by using a light source for inspection whose spectral peak value is near 510 nm or 590 nm, which is a valley of the spectrum of the color filter, the transmittance of the pixel normal portion of the color filter is lowered, and It is described that the transmittance of a portion having a defect or unevenness in which the colored layer is thin becomes high and the contrast is enhanced, so that it is possible to detect a defect or an unevenness which is difficult to detect with a conventional inspection light source.

[0006]

However, according to the method described in the above publication, an inspection light source whose spectral peak value substantially coincides with the valley of the spectrum of the color filter must be found. , The light source is not always applicable to inspection for print unevenness.

[0007] Therefore, an object of the present invention is to provide an image capturing apparatus, such as a camera, in which image unevenness is captured as an image not only for a color filter for liquid crystal but also for a wide range of printed materials such as general printed materials and injection molded products. Another object of the present invention is to provide a method for enhancing image contrast to such an extent that printing unevenness can be physically detected.

[0008]

According to the present invention, when a printed material is captured as an image by an image capturing device, the rate of change in light transmittance of each colored film due to a change in the thickness of each colored film due to printing is maximized. A light source having a peak of spectral intensity (spectral intensity) in a wavelength region is selectively used.

Further, the present invention is characterized in that a light source having a light transmittance of the colored film of about 32 to 42%, preferably about 37% is selected and used.

[0010] The printed matter in the present invention means not only a general printed matter but also red or red on a transparent substrate by printing or other means.
It includes a wide range of printed materials such as a color filter having green and blue colored films formed thereon and an injection molded product of a thin film capable of transmitting light. Furthermore, a light source having a spectral intensity peak in a certain wavelength range includes not only various light sources having a spectrum having a peak in a specific wavelength range, but also a filter between a light source having a plateau-like intensity distribution and an object. And a light source that forms a spectrum having a peak in a specific wavelength range by interposing a light source.

According to the first aspect of the present invention, a light source that emits light having a peak in spectral intensity in a wavelength region where the rate of change in light transmittance of the film according to the thickness of each colored film is maximized is selected. By using this, the transmittance can be largely changed according to a minute change in the film thickness. Since the brightness distribution of the printing unevenness is significantly different from that of the normal part and an image with enhanced contrast is formed, the printing unevenness is significantly easily detected.

Moreover, even if the wavelength range in which the rate of change of the light transmittance due to the color and thickness of the colored film changes is the largest, the light source having a spectral intensity peak in that wavelength range can be selected. Is always performed under optimal conditions, and therefore can be automated without relying on visual inspection.

According to the second aspect of the present invention, in addition to the operation and effect of the first aspect, it is theoretically possible that the rate of change in transmittance becomes maximum when the transmittance of transmitted light is 37%. Has been confirmed. Therefore, the relationship between the film thickness and the wavelength at which the transmittance is about 32 to 42%, preferably about 37% can be checked in advance with respect to the ink used for printing, and thereby, the specification of the printed matter can be adjusted. The wavelength range where the change in transmittance is maximum corresponding to the ink and the film thickness corresponding thereto is found. If you select a light source that has a spectral intensity peak in that wavelength range,
The contrast enhancement can be made larger.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described with reference to the drawings. As described above, in order to automate the inspection of the print unevenness, which is one of the determinations of the quality of the printed matter, stable imaging by the camera of the print unevenness, in particular, the enhancement of the contrast before capturing the image is performed.

As shown in FIG. 1, a liquid crystal color filter formed by a printing method generally has R (red), G (green) and B on a glass substrate on which a black matrix for light shielding is deposited.
Three kinds of inks (blue) are printed, and a colored film is formed. This variation in the thickness of the colored film causes a variation in the amount of transmitted light, which is perceived as shading unevenness (print unevenness).

The relationship between the colored film and the amount of transmitted light can be expressed by the following equation.

[0017]

T = I / Io = exp (-at) where T is the light transmittance, I is the transmitted light amount, Io is the incident light amount, t is the film thickness, and a is the coefficient determined by the ink and wavelength. Represent.

According to this equation, if the film thickness becomes α times, the transmittance T changes to exp {at (1−α)} times. For example,
It is assumed that the transmittance decreases to 64% when the film thickness changes twice in wavelength light having a transmittance of 80% at a certain film thickness. This means that the original transmittance is 80% = 0.8 = exp
(−at), and the rate of change is exp (−at), so that after the film thickness changes, it is 0.8 × 0.8, which is 64%. On the other hand, for light having a similar thickness and a transmittance of 40% (that is, a light having a larger a), the transmittance similarly decreases to 0.4 × 0.4 to 16%.

The former light intensity reduction is 16%, while the latter light intensity reduction is enlarged to 24%. That is, even with the same change in film thickness, a difference in the amount of transmitted light occurs due to a difference in light transmittance. It is understood that when light having a wavelength that widens the light amount difference is selected, it is possible to enhance the contrast of printing unevenness, which is a slight difference in film thickness.

That is, when the printed matter is captured as an image by the image capturing device, the spectral intensity peak (in the wavelength region where the change rate of the light transmittance of each colored film due to the thickness of each printed colored film is maximized). It is preferable to select and use a light source having the above-mentioned spectral peak value). Further, the change in the transmittance T depending on the film thickness t is given by the equation

[0021]

∂T / ∂t = −a · exp (−at) The variation of the transmittance with respect to the coefficient a becomes maximum when the following equation becomes zero.

[0022]

３ / ∂a (∂T / ∂t) = (at−1) exp
(-At)

Therefore, when at = 1, the change in transmittance becomes largest. The transmittance at this time is

[0024]

## EQU4 ## T = 1 / e ≒ 0.37

As for the change in the amount of transmitted light due to the change in film thickness, wavelength light having a transmittance of 37% is the largest, and a light source having a transmittance of light of about 37% passing through each colored film is selected and used. It turns out to be preferable.

FIG. 2 shows an example of a typical spectral transmittance at a reference film thickness of each ink used in the liquid crystal color filter. From the above theory, R (red), G (green)
It can be seen that the low pressure sodium lamp (589 nm) having the maximum change in transmittance and having a peak near 590 nm, which corresponds to 38%, is suitable as the inspection light source for the ink (see FIG. 3).

As shown in FIG. 3, with respect to B (blue) ink, the transmittance is 380 nm, 5 corresponding to 38%.
It can be seen that a light source having a peak of the spectral intensity near 10 nm is suitable.

Further, when inspecting the printing unevenness of a general printed matter or an injection molded article having specifications different in coloring means and film thickness,
Since the wavelength range in which the change rate of the light transmittance of the colored film is maximum is different, the transmittance of the light source having the peak of the spectral intensity applied to the liquid crystal color filter is not always 37%.

In this case, if the relationship between the film thickness and the wavelength at which the transmittance is about 37% has been investigated in advance for the ink used for printing, the ink and the film thickness in accordance with the specifications of the printed matter are obtained. And the wavelength range where the change in transmittance is maximum corresponding to Then, a light source having a peak of the spectral intensity in that wavelength range may be selected. If no light source having such a peak of spectral intensity is found, a peak is formed in the wavelength range by interposing a filter between the light source having a plateau-like intensity distribution and the inspection object. You may.

It is considered that the color of a general printed matter is identified by the mere reflected light from a colored film such as ink. However, the light passes through the colored film, reaches the paper surface, is reflected, and is re-emitted. It is considered that the light passes through the colored film and the transmitted light enters human eyes and is perceived. Therefore, the film thickness through which light is transmitted is twice that of the colored film, and the same treatment as that of the liquid crystal color filter can be performed if a value considering the reflectance of paper is applied as the incident light amount. .

[0031]

Since the present invention is configured as described above,
According to the first aspect, the change in the thickness of each colored film,
By selecting and using a light source having a spectral intensity peak in the wavelength region where the rate of change of the light transmittance of the colored film is maximum, the transmittance greatly changes due to a small change in film thickness, and the contrast is enhanced. An image is formed. Therefore, the change in luminance in a portion having print unevenness is significantly different from that in a normal portion, and detection of print unevenness becomes easy.

Furthermore, even if the wavelength range in which the rate of change in light transmittance due to the color and thickness of the colored film changes is the largest, a light source having a spectral intensity peak in that wavelength range can be detected. Is always performed under optimal conditions, and thus can be automated without relying on visual inspection.

According to the second aspect of the present invention, in addition to the operation and effect of the first aspect, it becomes easier to select a light source having a spectral intensity peak in a preferable wavelength range. This is because it has been theoretically confirmed that the change rate of the transmittance becomes the maximum when the light transmittance becomes 37%. If the relationship between the film thickness and the wavelength at which the transmittance is about 37% has been investigated in advance for the ink used for printing, the transmittance corresponding to the ink and the film thickness according to the specifications of the printed matter The wavelength range in which the change of the maximum is maximum is known, and it is possible to select a light source having a peak of the spectral intensity in the wavelength range.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of a typical liquid crystal color filter.

FIG. 2 is a graph showing an example of a representative spectral transmittance at a reference film thickness of each ink used for the liquid crystal color filter of FIG. 1;

FIG. 3 is a graph showing a spectral energy ratio between a fluorescent lamp and a sodium lamp.

Claims (3)

[Claims] When a printed matter is captured as an image by an image capturing device, a light source having a peak of a spectral intensity near a wavelength region where a change rate of a light transmittance of a printed colored film is maximized is selected and used. Characteristic image contrast enhancement method. 2. The image contrast enhancement method according to claim 1, wherein a light source that emits light having a light transmittance of the colored film of about 37% is selected and used. 3. A change rate of a light transmittance of a colored film with respect to a change in film thickness due to a certain ink is obtained as a function of the ink and a light wavelength, and a peak of the spectral intensity is obtained near a wavelength region where the change rate is maximum. Selecting a light source having the light source, or selecting a light source that emits light having a light transmittance of the colored film of about 37%, and capturing a printed matter printed with the ink using the selected light source as an image. An image contrast enhancement method characterized by the following.