JP2805954B2 - Infrared imaging device - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an infrared imaging device.

[Prior Art] A conventional device of this type has a structure as shown in FIG. In the figure, the image of the target object is represented by an imaging optical system (lens)
By 205), an image is formed on the light receiving surface of the infrared detector 201.
In general, an infrared photoelectric element has a narrow forbidden band and a dark current easily flows. Therefore, the photoelectric element is surrounded by a cold shield 202 (opening 202a) that shields radiated light from surrounding objects other than the effective light flux from the imaging optical system. , About 80K (cooler 207). In the cold shield 202, a cold filter 206 that shields infrared rays other than the wavelength range to be detected is mounted as necessary, and a vacuum chamber 203 is provided around the cold shield 202 in order to increase cooling efficiency. Surrounded by

In such an infrared imaging apparatus, the cold shield 202 is arranged so as not to block the effective light beam (the amount of light from the target object is reduced if the effective light beam is kicked).

[Problems to be Solved by the Invention] However, the conventional infrared imaging apparatus as described above has the following problems.

First, in order to arrange the cold shield so that the effective light beam from the imaging optical system is not kicked, it is necessary to use the cold shield, especially when the photoelectric elements of the detector are two-dimensionally arranged and the light receiving area is large. However, there is a problem that the cooling load cannot be reduced. Further, as the opening of the cold shield becomes larger, the amount of stray light from surrounding objects increases, the influence of changes in the surrounding temperature increases, and accurate detection of the target object becomes impossible.

Further, when the cold filter is mounted, there is a problem that the cooling load increases as the light receiving area of the cold filter increases.

The present invention has been made in view of such a point,
It is an object of the present invention to provide an infrared imaging device capable of miniaturizing while maintaining high detection accuracy by suppressing the influence of stray light from the surroundings.

[Means for Solving the Problems] An infrared imaging apparatus according to the present invention includes an infrared detector, an imaging optical system that forms an image of a target object on a light receiving surface of the detector,
A cold shield that shields stray light from around the detector.In order to achieve the above object, a light flux from the imaging optical system passes between the imaging optical system and the cold shield. A mirror aperture formed of a concave mirror having an opening of a predetermined size.

As the mirror aperture, specifically, each of the apertures has a size corresponding to the union and intersection of the passing areas of the light flux from the imaging optical system that reaches the periphery of the light receiving surface of the detector at the mirror aperture arrangement position. First and second
It is preferable to provide such mirror apertures, and to appropriately insert and remove these mirror apertures according to the target object and surrounding conditions.

In addition, in order to reduce the size of the cold filter and improve the cooling efficiency, the size of the cold shield opening is set to a size corresponding to the intersection of the passing areas on the cold shield upper surface of the light flux from the imaging optical system. Preferably, a cold filter is attached to the opening of the cold shield.

[Operation] In order to explain the operation of the present invention, first, the detection sensitivity of the infrared imaging device will be described.

The ratio detection sensitivity D ^* of the infrared imaging device is proportional to the reciprocal of equivalent noise power (hereinafter referred to as NEP), and NEP is measured as follows.

First, if there is a light source (state 1: S from light source, B from background)
When there is no light source) (State 2: B 'from the part excluding the light source and B from the background)
For each of the above, the amount of light is measured with an infrared detector.
The signal in this case is S ₁ = S + B when there is a light source, and S ₂ = B ′ + B when there is no light source, and the signal difference between them is ΔS
= S-B '.

If the fluctuations (RMS noise) for the respective signals S ₁ and S ₂ are N ₁ and N ₂ , N ₁ > N ₂ because S ₁ > S ₂ . Here, ΔS is set as small as possible, and ΔS / N ₂ = 1
Is the NEP. The temperature difference corresponding to this light amount difference is the equivalent noise temperature difference (NETD). From the above definition, the sensitivity of the infrared imaging device is better when NEP and NETD are smaller and D ^* is larger.

The NETD measurement, like the NEP, takes an image of a target object (black body) and a background (black body) with a temperature difference ΔT, reduces the signal difference ΔS between the target object and the background, and calculates ΔS and the background signal. This is performed by obtaining a temperature difference ΔT when the fluctuation N ₂ has a relationship of ΔS / N ₂ = 1. These NEP, NE
To set a small [Delta] S in the measurement of the TD, [Delta] S is N ₁ N ₂ Smaller, because the assumption of linearity of sensitivity (the light amount difference and the signal difference is proportional) is maintained.

Next, the relationship between the size of the cold shield opening and D ^* will be described.

First, the case where there is no imaging optical system between the light source (target object) and the detector will be described. Assuming that the angle between the line connecting the open end of the cold shield and the center of the light-receiving surface of the detector to the optical axis is θ, D ^* (θ) = D ^* (2π) / sin θ (D ^* (2π) (The ratio detection sensitivity in the case where no light source is provided), and D ^* is improved by reducing θ regardless of the area of the light source (even if the entire background is the light source).

However, when an imaging optical system is interposed between the target object and the detector to form an image of the target object on the light receiving surface of the detector,
It is different from the above case. In this case, only the radiation from the portion conjugate with the light receiving surface of the detector contributes to the signal difference ΔS, so that the magnitude of the signal difference is proportional to the amount of the effective light beam from the imaging optical system. . As for the noise, assuming that there is no target object, sin θ
Is proportional to

However, if there is an imaging optical system, NETD∽F ² / D ^* is satisfied. Here, F is a so-called F-number representing the brightness of the lens, and F = f / D (f: focal length of the optical system, D:
Lens effective aperture).

That is, the angle between a line connecting the opening end of the cold shield and the center of the light receiving surface of the detector to the optical axis is θ, and the F-number of the lens is
Is defined as θ _j (tan θ _j = D / 2f), and θ is π
/ If 2~θ until _j varying (if not an effective light beam is eclipsed), the NETD∽sin θ / tan ² θ _j.

Further, by narrowing the opening of the cold shield, θ is set to θ _j ~
When it is changed to zero, in addition to the improvement in D ^* due to the reduction in noise, the reduction of the signal by blocking the effective light flux occurs, so that NETD∽sin θ / tan ² θ.

Here, since the cold shield itself is actually fixed, the opening of the cold shield itself has a size that does not block the effective light beam from the imaging optical system, and a predetermined distance is provided between the detector and the imaging optical system. The configuration of inserting and removing a mirror aperture with a large opening has the advantage that radiation from the cold cold shield itself is reflected by the mirror aperture and incident on the detector, thereby reducing the cooling load. Regarding the size of the mirror aperture opening, the following can be said.

That is, the amount of light from the target object incident on the detector is determined by the sum of the passing areas of the light beams from the imaging optical system reaching the periphery of the light receiving surface of the detector at the mirror aperture arrangement position (the size of the mirror aperture opening). (Hereinafter simply referred to as the union). It becomes the highest (if the aperture is further increased, the stray light only increases, so it is not considered here to expand beyond the union). Although the amount of light decreases, stray light decreases and thermal noise due to a change in ambient temperature decreases. Then, if the size of the aperture is reduced to a size corresponding to a product set (hereinafter simply referred to as a product set) of a passage area of the light beam from the imaging optical system that reaches the periphery of the light receiving surface of the detector at the position where the mirror aperture is arranged, the surrounding area is reduced. Stray light from the object will be completely blocked.

At this time, if the mirror aperture can be arranged at the exit pupil position of the imaging optical system, the union and the intersection will match, so it is most desirable in terms of both light quantity and stray light blocking. It is not always possible to arrange the mirror aperture at the exit pupil position in the imaging device of the above. Therefore, providing a plurality of mirror apertures having openings corresponding to unions and intersections, and inserting and removing them according to conditions such as the ambient temperature and the radiation amount of the target object is very practically significant. is there. Furthermore, even when the aperture of the mirror aperture matches the intersection, if the height of the mirror aperture from the light receiving surface of the detector (imaging plane) can be increased, the amount of light can be increased accordingly. It may be better (smaller) than the set.

Table 1 summarizes the characteristics of the imaging device when the mirror aperture is inserted and removed. In the table, comparison between the intersection and the union is performed when the height from the imaging plane is the same.

The aperture of the mirror aperture does not necessarily have to coincide with the intersection or union, but from the viewpoint of the amount of light at the center and the periphery of the detector and the uniformity of NETD (described in the detailed embodiment), It is preferable to match either but not the middle.

FIG. 1 is an optical path diagram of a main part of an infrared imaging device according to a first embodiment of the present invention.

In the figure, an infrared detector 1 is mounted on a cooler 7 and is surrounded by a cold shield 2 having an opening (corresponding to a union) of a size that does not block the effective light beam from the imaging optical system (lens 5). ing. A vacuum chamber 3 having an optical window 3a on the upper surface is arranged around the cold shield 2. In the optical path from the lens 5 to the detector 1 (in this case, between the lens 5 and the window member 3a), a mirror aperture 4a composed of a concave mirror is provided near the center point O of the light receiving surface of the detector 1 (necessarily a spherical surface). (Not necessary).

In such an imaging apparatus, the angle formed by the line connecting the point O and the open end of the cold shield 2 with the optical axis is θ _c ,
Is the angle determined from the F-number of θ _j (tan θ _j = D / 2f, s
f: focal length, D: effective aperture of lens 5), the angle θ between the optical axis and the line connecting the open end of mirror aperture 4a and point 0
(Change the size of aperture 4a opening)
The following can be said.

First, as described above, the relationship between NETD and θ is NETD∽sin θ / tan ² θ _j (π / ^{2 to} θ _j ) NETD∽sin θ / tan ² θ (θ _j to zero). FIG. 2 shows this as a graph.

If the detector 1 is not a point sensor but a multi-element (area sensor), then if the cold shield 2 is arranged so that the effective light beam is not kicked, then θ _c > θ _j , so sin θ _a / tan ² θ _{a ^{= sin θ c / tan 2 θ}} j become theta _a is present in the range of 0 <θ <θ _j.

For this θ _a, θ _{a <θ <As} is apparent in FIG. In the range of theta _j, NETD of elements of the detector 1 center,
It becomes smaller than the case where the cold shield 2 is an aperture (θ = θ _C ), and the temperature resolution is improved. That is, θ _a <θ <θ
By inserting the mirror aperture 4a having an opening of _j , stray light can be reduced and the effect of a change in ambient temperature can be reduced without sacrificing temperature resolution.

Here, FIG. 3 shows the passing area of the light beam reaching the four vertices of the detector 1 (rectangle) at the position where the mirror aperture 4a is arranged. The area surrounded by is the union. Lens 5
When the aperture is made to have a size corresponding to the intersection (hatched portion) when the light beam from the lens is narrowed by the mirror aperture 4a, the detector 1
A uniform light amount is incident on the whole. That is, if the opening of the mirror aperture 4a is made to have the size indicated by the thick arrow in FIG. 1, thermal noise from surrounding objects will not occur without inconvenience such as deterioration of the temperature resolution and sensitivity unevenness between the periphery and the center of the detector. Can be held down.

Further, there is not enough space to arrange the mirror aperture of 4a in FIG. 1, but if it is desired to prioritize reduction of thermal noise over securing of the amount of light from the target object, the height of the mirror aperture from the imaging plane should be increased. It may be lowered and arranged like 4b, 4c in FIG.

Furthermore, in the above description, the case where the mirror aperture opening is matched with the intersection is described. However, it is needless to say that the mirror may be matched with the union. However, there is an advantage that no light amount is lost from the target object.

In the above-described embodiment, the mirror aperture is arranged so that its center is located at the center of the light receiving surface of the detector, but the hemispherical mirror aperture is arranged so that its center coincides with the center of the cold shield opening. If the alignment mark (attached to the mirror side) of the window member provided between the mirror aperture and the cold shield is made conjugate with the light receiving surface of the detector, the reflected light from the mark is reflected by the mirror aperture and detected. Since an image is formed on the light receiving surface of the device, the mirror aperture can be easily positioned.

Next, FIG. 4 is an optical path diagram of a main part of an infrared imaging device according to a second embodiment of the present invention.

In the figure, the infrared detector 101 has an image forming surface of an image forming optical system (for simplicity, a single lens 105 is assumed to be at infinity) so that an image forming surface coincides with a light receiving surface (width is a). And the periphery thereof is surrounded by the cold shield 102. In the opening (circle) of the cold shield 102, cold filters 106a, 106b, and 106c that transmit only infrared light in a predetermined wavelength range (for the sake of explanation, cases where the sizes of the cold filters are different are shown together). Is installed.

Here, NETD is the F-numbe of the lens 105 as described above.
NETD∽F ² / D ^* is given by r (F = f / D, f: focal length, D: effective aperture) and the specific detection sensitivity D ^{* of the} detector 101, and the aperture (in the case of FIG. 4) Cold shield 10
Assuming that the angle between the line connecting the opening end of 2) and the center of the light receiving surface of the detector 101 to the optical axis is θ, D ^* (θ) = D ^* (2π) / sin θ (D ^* (2π): cold shield) (The ratio detection sensitivity when no 102 is provided).

Here, F = 1, f = 50 mm, D = 50 mm, a = 8 mm, and the height of the aperture of the cold shield 102 from the imaging plane is d. If the heights in the figures (the aperture matches the union and the effective light beam is not kicked) and the (the aperture matches the intersection and the effective light beam is kicked) are d = 20 mm, then In this case, 1 / NETD is as follows. The difference between the solid angle at the center and the solid angle at the periphery with respect to the opening of the cold shield 102 is ignored.

(1 / NETD) _a = 0.5 ² /0.5 ≒ 0.42 (1 / NETD) _b = 0.25 ² /0.242 = 0.26, where the temperature resolution is 1.6 times better.

Next, when d has the same level of temperature resolution as in the case of d = 20 mm (the height from the imaging plane is increased to match the cold shield 102 opening with the intersection), d is calculated as follows. Solving θ _c ) ² / sin θ _c = 0.42 gives θ _c {21.5}, d = 28 mm.

That is, NETD is the same in the case of, and the area of the cold filters 106a and 106c at this time is approximately 2: 1, and the amount of heat absorbed is smaller than in the case of
Therefore, if there is enough space to increase the height of the cold shield 102, the configuration described above can reduce the cooling load without lowering the temperature resolution.

At this time, as the opening of the cold shield 102 is narrowed, θ becomes θ _j (tan θ _j ) as shown in FIG.
= D / 2f), only the light beam incident around the detector 101 is kicked and the light beam reaching the center is not blocked.
As shown in FIG. 5 (b), the way of changing the light amount and 1 / NETD differs between the center and the periphery of the detector, and the difference becomes _maximum when θ = θ _j . detector 10 and the theta reduced beyond theta _j
(1) The luminous flux reaching the center is also reduced, and when the aperture coincides with the intersection, there is no border between the periphery and the center. That is, in order to avoid sensitivity unevenness due to the position of the light receiving surface of the detector 101, it is necessary to match the cold shield 102 with the union or intersection.

In addition, when comparing the state with the state (where the cold shield 102 aperture matches the union at a position equal to the height from the image plane and the aperture is not equipped with a cold filter), (1 / NETD _D = 0.5 ² /0.53≒0.47 (1 / NETD) _c = (1 / NETD) _a = 0.42, there is almost no difference, and the temperature resolution is superior to the case of Therefore, depending on the case, it is conceivable to select the state.

[Effects of the Invention] As described above, in the infrared imaging apparatus of the present invention,
Between the imaging optical system and the cold shield, since a mirror aperture composed of a concave mirror having an opening of a predetermined size through which a light beam from the imaging optical system passes is disposed so as to be freely inserted and removed, the target object can be moved away from the target object. The desired characteristics (reduction of thermal noise, securing of light quantity from the target object, temperature resolution) can be improved according to the imaging conditions such as the radiation amount of the light and the ambient temperature.
It is possible to reduce the size of the device and improve the cooling efficiency while maintaining the required characteristics. At this time, if the mirror aperture is made to coincide with the union or intersection of the effective light fluxes, the insertion of the mirror aperture does not cause sensitivity unevenness due to the position of the light receiving surface of the detector.

In addition, when using a cold filter, the cold filter itself is made to match the intersection of the effective luminous flux by attaching the cold filter to this opening,
The size of the cold filter can be reduced without sacrificing the temperature resolution, and the cooling load can be reduced.

[Brief description of the drawings]

FIG. 1 is an optical path diagram for explaining an infrared imaging apparatus according to a first embodiment of the present invention, FIG. 2 is a graph showing a relationship between an aperture angle of a stop and NETD, and FIG. 3 is a mirror aperture in FIG. FIG. 4 is a conceptual diagram showing a light beam passage area at an opening position, FIG. 4 is an optical path diagram for explaining an infrared imaging device according to a second embodiment of the present invention, and FIGS. 6 is a conceptual diagram for explaining sensitivity unevenness due to a surface position, FIG.
The figure is an optical path diagram showing a conventional example. [Description of Signs of Main Parts] 1,101… Infrared detector 2,102… Cold shield 3… Vacuum chamber 4a, 4b, 4c… Mirror aperture 5,105… Lens (imaging optical system) 106a, 106b, 106c… Cold filter 7 ... Cooler

Claims (3)

(57) [Claims] An infrared imaging apparatus comprising: an infrared detector; an imaging optical system that forms an image of a target object on a light receiving surface of the detector; and a cold shield that shields stray light from around the detector. In the above, between the imaging optical system and the cold shield, a mirror aperture consisting of a concave mirror having an opening of a predetermined size through which a light beam from the imaging optical system passes is arranged so that it can be inserted and removed. An infrared imaging device characterized by the following. 2. An aperture having a size corresponding to the union and intersection of passing areas of the light beam from the imaging optical system reaching the periphery of the light receiving surface of the detector at the mirror aperture arrangement position, as the mirror aperture. The first that each has
The infrared imaging device according to claim 1, further comprising a second mirror aperture. 3. An infrared detector, an imaging optical system for forming an image of a target object on a light receiving surface of the detector, a cold shield for blocking stray light from around the detector, and the imaging optical system A cold filter that disperses the light flux from the infrared light imaging device, wherein the cold shield is a set of light passing from the imaging optical system reaching the periphery of the light receiving surface of the detector on a cold shield upper surface. An infrared imaging apparatus having an opening of a corresponding size, wherein the cold filter is mounted in an opening of the cold shield.