JPH036511A - Variable density filter for infrared ray - Google Patents

Abstract

PURPOSE:To vary the quantity of light through a photographic optical system which handles infrared rays continuously over a wide range by forming a thin film of antimony which is equal in thickness radially from the center and varies in thickness in its circumferential direction on a germanium substrate. CONSTITUTION:The thin film 1d of antimony(Sb) is formed on the germanium(Ge) substrate 1a by vapor deposition and a reflection preventive film 1c is further formed on zinc sulfide(ZnS) thereupon. Then the thickness of the Sb thin film 1d is uniform in the radial directions of filters 1 and 2 and vary continuously in the circumferential directions. Consequently, the density of the filters to infrared rays can be varied over a wide range by varying the thickness of the Sb thin film 1d formed on the Ge substrate 1a.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a variable density filter used for adjusting the amount of light in a photographic optical system that handles infrared light, such as an infrared camera.

[Prior art] Filters used in the visible light range cannot be used as they are in photographic optical systems that handle infrared light (for example, Inconel, which is commonly used as a material for filters in the visible light range, is (The transmittance changes in the wavelength range). Conventionally, it has been common practice to provide an aperture stop in the photographing optical system and adjust the light amount by changing the size of the aperture stop.

[Problem to be solved by the invention] However, in the conventional method as described above, when a high-temperature object is to be photographed, the amount of infrared light irradiated to the detector becomes too large (the output of the detector is saturation), so the aperture diaphragm had to be made quite small, and the effects of diffraction were unavoidable.

In addition to adjusting the amount of light by changing the size of the aperture diaphragm, methods of inserting an ND filter that does not exhibit spectral selective absorption have also been adopted, but the infrared wavelength region is much smaller than visible light. However, the ND filter has to be replaced depending on the temperature range of the subject to be photographed, which is complicated. In addition, since the density of each ND filter cannot be adjusted, it is also necessary to correct variations in density from filter to filter.

In addition, in the case of an infrared camera that uses a solid-state image sensor, it is possible to adjust the light amount by using a shutter function, but if a CCD (charge-coupled device) is used as the image sensor, the number of elements increases. As the temperature increases, the rate of change in the light level that can be handled becomes smaller, and there is a problem in that the temperature of the element itself inevitably increases for high-temperature targets.

Furthermore, in the conventional scanning type photographing optical system, it is not possible to adjust the amount of light using a shutter function.

The present invention was made in view of the conventional problems as described above, and an object of the present invention is to provide an infrared variable density filter that can continuously change the amount of light over a wide range in a photographing optical system that handles infrared rays. It is something to do.

[Means for Solving the Problems] The variable density filter of the present invention is a pair of filters in which the density is uniform in the radial direction from the center and the density changes in the circumferential direction. 1 inserted into the optical path of the photographing optical system with opposite directions facing each other.
A variable density filter having a pair of filters and a rotation drive system that relatively rotates the pair of filters in opposite directions about an axis that is out of the optical path of the photographing optical system, and in order to achieve the above object. In the infrared rays, the pair of filters is composed of a thin film of antimony (sb) formed on a germanium (Ge) substrate, the thickness being equal in the radial direction from the center and varying in the circumferential direction. variable density filter.

[Function] The state of density change in the filter of the present invention will be schematically explained using FIG. 3.

The filter shown in Fig. 3(a) has 12 regions with different densities (22.5 degrees per region), and the densities change stepwise from light to dark (i.e., the film thickness of sb There are four sections (4 sections) where sb is not deposited between the lowest concentration section and the highest concentration section.
90°)) exists.

In the present invention, two filters (a first filter and a second filter) as shown in FIG.
The effective light beam 4a of the photographing optical system is inserted into the optical path so as to pass through the sb film-formed part of the filter (preferably so that the optical axis passes through an intermediate position between the radial direction and the circumferential direction of the sb film-formed part). .

A and B in FIG. 2(b) show the density changes with respect to the angle in the circumferential direction of the first and second filters arranged in this way (sb non-conforming part O~9G'', S (b film forming portion 90 to 360°), the pair of filters in the present invention have almost equal concentration gradients in the circumferential direction.Before relative rotation of the filters, the lightest section and the lightest section of the two filters corresponds to each other, and the sum of the concentrations of the two filters is a substantially constant concentration over the entire film forming portion.

From this state, move the first filter 45 degrees in the θ direction and the second filter
When the filter is rotated +45@ in the θ direction (Fig. 3 (e)
), the two filters are relatively shifted by 90° (4 divisions), and the sum of the densities of the two filters is becomes the concentration shown by the thick line in FIG. 3(C). At this time, the diameter of the effective light beam 4a of the photographing optical system is shown in Fig. 3 (
Since it falls within the range indicated by the arrow in C), the luminous flux has a uniform concentration in the area (A) (in the area (B) and (8) in the figure, one of the filters has a non-film-formed area, so there is a concentration gradient. ) will be passed through.

Next, when the two filters are each returned from the state shown in Fig. 3(C) by 1/4 rotation of 22.5° per section in the opposite direction (i.e., 1θ1 = 45° - 22,5°/4) is the third
This is the state shown in FIG. 3(d), and if the difference in the sum of the densities is averaged, it can be seen that the density is 172 steps higher than the state shown in FIG. 3(c). In this way, in the present invention, the sum of the densities of the two filters changes continuously in proportion to the rotation angle of the filters. The amount of can be changed continuously.

In addition, in order to facilitate the explanation, FIG. 3 shows a case where the density in the circumferential direction of each filter changes stepwise, but in the present invention, the density in the circumferential direction of the filter changes in stages. It is not necessary to change for each category as shown in the figure. By continuously changing the film thickness of sb in the circumferential direction, a filter in which the concentration changes continuously in the circumferential direction can be manufactured.

Further, the concentration gradient and rotation angle (rotation speed) of the two filters do not necessarily have to be exactly the same. If the density gradients are different, the sum of the densities of the two filters will not be completely constant, causing density unevenness, but if the filter is inserted into the pupil of the photographing optical system or at a position conjugate to the pupil, the image of the density unevenness of the filter will be created. You can avoid being influenced by others.

[Example] A first example of the present invention will be described using FIGS. 1 and 2. In the figure, a pair of filters (first filter 1, second filter
The filter 2) is made of antimony (Ge) by vapor deposition on a germanium (Ge) substrate 1a, as shown in the cross-sectional view of FIG.
sb) 1llild is formed on top of which an anti-reflection film 1c of zinc sulfide (ZnS) (sb
The thin film and anti-reflection coating layers are collectively shown as Ib, 2b)
is formed. The thickness of the sb thin film is uniform in the radial direction of the filter 1.2 and continuously changes in the circumferential direction (the 90° region below the filter 1.2 is the sb thin film).
(non-film-formed area).

Table 1 shows that in the filter with the laminated structure shown in Figure 2,
The transmittance of the filter is shown for each wavelength when the thickness of the antireflection film IC made of ZnS is 1.2 μm and the thickness of the sb film 1d is varied from 0 to 1.8 μm.

As shown in the table, when the Sb thin film 1d is not formed, the transmittance of infrared rays shows a high value of 90 or more, and as the thickness of the Sb thin film 1d increases, the transmittance decreases, and the transmittance decreases when the film thickness is 1.7 or more.
At about 1.8 μm, the transmittance reaches a value close to that of the actual value. In other words, the data shown in Table 1 shows that by changing the thickness of the Sb thin film 1d formed on the Ge substrate la in the range of about 0 to 1.8 μm, the concentration of the filter for infrared rays can be changed over a wide range. This proves that it can be done.

The first and second filters 12 having the structure described above are
The thin film forming surfaces 1b and 2b are arranged facing each other so that the direction of change in the thickness of the thin film 1d in the circumferential direction is opposite, as shown in FIGS. 1(b) and (c). The density of each filter as seen from the second filter 2 side is
The first filter 1 goes from dark to light from the lower left edge to the right, and the second
The filter 2 changes from dark to light counterclockwise from the lower right end. The pair of filters 1.2 can be relatively rotated in opposite directions by a rotation drive means (not shown) about the center of the filter as a rotation axis, and by adjusting the relative rotation angle, the first and second filters can be rotated in opposite directions. The sum of the densities (the amount of infrared rays transmitted) of the second filter 1.2 can be changed continuously.

The variable density filter configured in this manner is inserted into the optical path of the photographing optical system so that the optical axis 4 and the rotation axis 3 do not coincide, but in this embodiment, the optical axis is It is inserted at a predetermined angle with respect to 3.

In other words, changes in transmittance of a filter provided with a metal thin film are essentially due to changes in reflectance, so a phenomenon in which thermal radiation from the detector itself is reflected by the filter and received by the detector (Narcissus phenomenon) occurs. However, if the filter is tilted as in the embodiment shown in FIG. 1 so that the light reflected by the filter is not received by the detector, the influence of Narcissus can be removed.

Next, FIG. 4 is an optical path diagram showing an embodiment in which the Narcissus phenomenon is actively utilized. The photographing optical system of this embodiment includes an objective lens 6 and a relay lens 7 that forms an image formed by the objective lens 6 on the light receiving surface of an infrared detector 9.

The detector 9 has an opening through which the light beam from the relay lens 7 passes, and a cold shield 8 that blocks heat radiation from the surroundings.
The detector 9 and cold shield 8 are cooled to about 80°C to avoid increasing thermal noise in the detector output.

In such a photographing optical system, the aperture stop 5 and the aperture of the cold shield 8 have a conjugate relationship (i.e.,
The exit pupil of the optical system and the opening of the cold shield 8 match)
, variable density filters (first filter 1, second filter 2) having the same structure as explained in FIGS. 1 and 2 are inserted perpendicularly to the optical axis so as to sandwich the aperture stop 5. There is.

In such a photographing optical system, the infrared rays emitted from the exit pupil to the object side are only the radiation of the cooled detector 9 itself, and that radiation is transmitted by the first and second filters 1 and 2 that constitute the variable density filter. It is reflected and returns to a position symmetrical to the optical axis on the light receiving surface of the detector 9. For this reason, there is no thermal radiation that enters the detector from normal temperature areas other than the object to be photographed, and the dynamic range of the detector 9 can be increased by that amount. In other words, when the intensity of the luminous flux from the object to be photographed is so high that the detector is further saturated even when the filter is set to maximum density, the level of the output signal of the detector 9 can be reduced by utilizing the Narcissus phenomenon. .

In addition, in the photographing optical system shown in FIG.
The first and second filters are inserted perpendicularly to the detector 9 so that the radiation from the constantly cooled detector 9 returns to the detector 9, but if necessary, the optical axis 3 of the filter 1.2 If the configuration is such that the angle relative to the filter 1.degree. 2 can be changed, the dynamic range of the detector 9 can be changed by changing the angle of the filter 1.degree. That is, there is a case where the filter 1.2 is set perpendicular to the optical axis and only the radiation of the detector 9 itself is returned to the detector at about 80°, and a case where the filter 1.
．． 2 is tilted so that radiation from a room-temperature object (approximately 300 K) other than the object to be photographed enters the detector 9, 3
The dynamic range of the detector 9 changes by the amount corresponding to 0 (1-80K).

Further, in the above embodiment, the case where the amount of infrared rays is adjusted using only the variable density filter has been described, but if the variable density filter of the present invention and the electronic shutter are used in combination, the dynamic range of the detector can be further increased. .

[Effects of the Invention] As described above, in the present invention, an antimony thin film having a thickness varying in the circumferential direction is formed on a germanium substrate.
By configuring the variable density filter using a pair of filters, the amount of infrared rays transmitted through the filter can be continuously changed over a wide range.

By using the variable density filter of the present invention, there is no need to replace the filter depending on the temperature of the object to be photographed, and even if there are variations in the thickness of the sb thin film for each individual filter, the rotation angle of the two filters can be adjusted. As a result, the density can be adjusted to a desired value, so there is no need to correct variations in density from filter to filter.

Furthermore, since such a variable density filter is a reflective filter, it is possible to further widen the dynamic range of the infrared detector by utilizing the Narcissus phenomenon as necessary.

[Brief explanation of drawings]

FIG. 1(a) is a sectional view showing the configuration of the first embodiment of the present invention,
FIGS. 1(b) and (c) are front views of the filter in the first embodiment, FIG. 2 is a sectional view of the filter in the first embodiment, and FIGS. 3(a), (b), (c), (
d) and (e) are explanatory diagrams schematically showing the principle of light amount adjustment in the present invention, and FIG. 4 is an optical path diagram showing the configuration of a second embodiment of the present invention. [Description of symbols of main parts] 1... First filter 1a... Germanium substrate IC... Anti-reflection film 1d... Antimony thin film 2... Second filter 3... Rotating shaft 4...・Optical axis 5...Aperture diaphragm 8...Cold shield 9...Detector

Claims (4)

[Claims] (1) A pair of filters whose density is uniform in the radial direction from the center and whose density changes in the circumferential direction, and which are used as photographic optics in a state where they face each other so that the direction of the density change in the circumferential direction is opposite. A variable density filter comprising: a pair of filters inserted into the optical path of the system; and a rotation drive system that relatively rotates the pair of filters in opposite directions about an axis that is out of the optical path of the photographing optical system. In the variable infrared rays filter, the pair of filters are filters in which a thin film of antimony is formed on a germanium substrate, the thickness being equal in the radial direction from the center and varying in the circumferential direction. Density filter. (2) The variable density filter for infrared rays according to claim 1, wherein the pair of filters are inserted at the pupil of the photographing optical system or at a position conjugate with the pupil. (3) The variable density filter for infrared rays according to claim 1, wherein the pair of filters are inserted so as to form a predetermined angle with the optical axis of the photographing optical system. (4) the pair of filters are located at a position conjugate with an opening of a cold shield arranged to surround an infrared detector on which an image by the photographing optical system is formed on a light receiving surface;
The infrared variable density filter according to claim 1, wherein the infrared variable density filter is inserted perpendicularly to the optical axis of the photographing optical system.