JPS6046774B2 - Imaging device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an imaging device suitable for imaging a subject illuminated with coherent light.

Imaging techniques for objects illuminated with coherent light have not yet been generalized.

Furthermore, it has not been widely used as a product. When you image a subject illuminated with coherent light such as a laser beam using an imaging device that uses normal light (incoherent light),
Interference fringes appear in the image as shown in FIG. These interference fringes are generated by multiple reflections between the incident surface of the face plate and the photoconductive film in the imaging device. One possible means for preventing the occurrence of interference fringes is to use a fiber plate for the face plate. An imaging device using a cal-fiber plate has problems in terms of resolution and interference prevention effect.

Moreover, the above-mentioned means can only be adopted at the manufacturing stage and cannot be applied to products that have already been commercialized. Another way to prevent the occurrence of interference fringes is to prevent the reflected light from the photoconductive film from returning into the imaging device.
It is conceivable to attach a glass block having a thickness of about 0 to 40 TIr1n. This method has the advantage that it can be applied to image pickup tubes that have already been commercialized. Since a large glass block is included in the optical system, there are disadvantages such as deterioration of resolution, increase in aberrations and distortion, and the problem of increasing the size of the entire device. Therefore, this invention can prevent interference fringes that occur during imaging under illumination with coherent light without causing deterioration of imaging or adverse effects on the optical system, and is also fully applicable to products that have already been commercialized. The purpose is to provide an imaging device. The details of the present invention will be explained below with reference to the drawings.

The present inventors investigated how the visibility of interference fringes is related to reflectance and the like. That is, as shown in FIG. 2, the calculation is performed according to a model of interference at point p of light beams A and B (!7) incident in parallel. If A is the amplitude of the light incident on the Si surface (upper surface) of face plate C (7), ω is the angular frequency of the light, t is time, and δo is the initial phase, the light flux incident on the S1 surface is expressed as.

Next, the light beam A is reflected at point Q on the interface between face plate C and photoconductive film D (surface S2), and after being reflected again at surface S1, the light that enters point P is IA'', which is parallel to light beam A. Let IB'' be the light that directly reaches point P out of the light flux B.

Here, r is the amplitude reflectance on the Si surface, R is the amplitude reflectance on the S2 surface α is the amplitude transmittance of face plate C δ1 is the phase difference of light between S1 plane and S2 plane It is.

Note that the generally used "゜reflectance" indicates the reflectance with respect to the intensity of light, and has the relationship (amplitude reflectance) 2 = (intensity reflectance of light). Therefore, light 1 at point P is It is expressed as

On the other hand, since an image pickup tube senses the intensity of light rather than its amplitude, it is sufficient to take the root mean square of equation (4).

In other words, it becomes.

Here, in order to normalize the amplitude of light, Nα2 = 1, If we set α2rR=A, 2δ1=δ, then becomes.

On the other hand, the visibility ■ of the interference fringes is given by, so from equation (6), considering a x 1,
■≧? becomes.

Therefore, let us consider a case where a general image pickup tube (Sb2S3) is illuminated with a 4880A (7) Ar laser beam using a device as shown in FIG.

In Figure 3, 1 is an Ar laser, 2 is a beam expander, and 3
4 is an object, 4 is an imaging lens, 5 is an image pickup tube, and 6 is a monitor image receiver. In this case, the face plate of the image pickup tube 5 (
The light amplitude transmittance of the transparent glass plate is almost 100%.
Therefore, α=1, the reflectance of the face plate is about 4%, so r=0.04=0.2, and the reflectance of the photoconductive film at 4880A is about 8%, so R=0.283. Therefore, the visibility of the observed interference fringes is extremely high at 11.3%. As mentioned above, what is the visibility of the interference fringes? Since it can be expressed as

Therefore, the present inventors proposed two methods for attenuating the luminous flux reflected by the photoconductive film surface: firstly, applying an antireflection film to the incident surface of the face plate, and secondly, using an ND (neutral density) filter or a colored glass filter. We decided to adopt two methods: to use a plate made of a light attenuating material such as the face plate, or to provide this plate on the face plate.

FIG. 4 shows one embodiment of the present invention, in which 11 is an antireflection film, 12 is an ND filter, 13 is an adhesive layer, 14 is a face plate, and 15 is a photoconductive film.

If the anti-reflection film 11 has about 3 to 5 layers, its reflectance will be about 0.5 to 1%. For example, if the reflectance is 0.5%, the amplitude reflectance is expressed by its square root, and is approximately 0.071. On the other hand, photoconductive film 1
The amplitude reflectance at 5 is 0.283, so ■=0.0
4, or approximately 4%. 5. If the reflectance is 1%.
It is 66%, which is quite noticeable. Here, efforts to reduce the reflectance of the incident surface are limited to around 0.5%, and the effect is reduced by the square root, so it is not expected that this antireflection film 11 alone will cause such a significant reduction. Next, the ND filter 12
For example, if one with a transmittance of 50% is used, the amount of incident light reaching the photoconductive film 15 is reduced by half.

The reflected light between the photoconductive film 15 and the antireflection film 11 becomes A because it travels back and forth through the ND filter 12. Then,
Comparing on the surface of the photoconductive film 15, the reflected light is A and the amplitude reflectance is ↓. That is, if the ND filter 12 is provided, the visibility of the interference fringes is reduced by the attenuation ratio thereof. Note that since the amount of light reaching the photoconductive film 15 is reduced accordingly, a decrease in sensitivity is inevitable. For this reason, it is desirable to select a light attenuating material that transmits at least 30%. Furthermore, if the ND filter 12 is used with a thickness equal to or greater than that of the face plate 14 (2 to 5 n1/m), it is expected that the visibility of the interference fringes will decrease due to the diffusion of the imaging light. . Since it is almost invisible when the visibility is 2% or less, in actual cases, anti-reflection coating 11
It is sufficient to use a three-layer filter (with a reflectance of 0.7%) as the ND filter 12 and obtain a filter with a transmittance of 40% and a thickness of 3n1/m as the ND filter 12.

When photographs were taken using an image pickup tube under these conditions, no interference fringes could be observed.

The conditions at this time are 1 anti-reflection film... 3 layers 0.8% 2ND filter Transmittance 58% Thickness 3m/m
Refractive index 1.487 (AtD line) 3 adhesive...
・Canada Balsam Refractive index 1.542 (AtD line) The effect of anti-reflection coating is generally considered to be that the visibility of interference fringes is given by the square root of the rate of decrease in reflectance.
It is difficult to reduce visibility to 4% or less.

In order to reduce the visibility to 4% or less, use a light reducing material such as an ND filter to reduce the amount of transmission. Visibility decreases in proportion to the rate. Furthermore, in the imaging system, the effect of thickness can also be expected. As described above, the key to preventing interference is the use of a light-attenuating material such as an ND filter.

When used by 6,000 to 7,000 people, the visibility is 22% before applying interference prevention, 8% after applying antireflection film, and after installing an ND filter with 25% transmittance. It becomes 2%. In this way, even if the device is already commercially available, it can be easily made by simply attaching a light-reducing material.

In addition, the insertion of the light reducing material has almost no effect on resolution, deterioration, aberration, distortion, etc., and does not affect optical or mechanical dimensioning. Of course, a significant improvement in S/N (interference prevention effect) can be achieved. Therefore, it is suitable for high-resolution image pickup tubes. Furthermore, it can be applied regardless of the size of the image pickup tube. Furthermore, it has the advantage that it can be applied to any type of device in which a photoconductive film is deposited on a face plate. Although a relative decrease in sensitivity is unavoidable, in an illumination system that uses laser light or the like, a sufficient amount of light can be obtained, so this does not pose much of a problem.

In other words, if there are interference fringes, the S/N will be severely degraded.
This is because although it cannot be provided as an image, the problem of insufficient light quantity can be solved as a problem of laser power. As explained above, according to the present invention, interference fringes that occur during imaging under coherent light illumination can be prevented without deteriorating resolution or adversely affecting the optical system, and in addition, it can be applied to image pickup tubes that have already been commercialized. We can provide a capable imaging device.

[Brief explanation of the drawing]

FIG. 1 is a diagram for explaining the generation of interference fringes by a conventional device, FIGS. 2 and 3 are diagrams for explaining the interference fringe generation prevention means of the present invention, and FIG. 4 is a diagram for explaining an embodiment of the present invention. It is a sectional view of the main part of an example. 11... Antireflection film, 12... ND filter, 13... Adhesive layer, 14... Face plate, 15... Photoconductive film.

Claims (1)

[Scope of Claims] 1. An imaging device for imaging a subject illuminated with coherent light, characterized in that the face plate is a plate made of a light-attenuating material with an anti-reflection coating applied to the incident surface. 2. An imaging device for imaging a subject illuminated with coherent light, characterized in that a plate made of a light-attenuating material with an anti-reflection coating applied to the incident surface is adhered to the front surface of a face plate.