JPH0720214B2 - Infrared imaging device signal correction method - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for correcting variation in a video signal due to variation in characteristics of an infrared light receiving element in an infrared imaging device.

(Summary of the Invention) The present invention is an infrared imaging device that performs infrared detection with an infrared detection element including a plurality of infrared reception elements, and a lens system is used to provide an equal amount of incident light to each of the light reception elements. It is possible to electrically correct variations in video signals due to differences in characteristics.

(Prior Art) As an infrared detecting element having a plurality of infrared receiving elements,
There is one in which light receiving elements are arranged one-dimensionally or two-dimensionally. The light receiving elements arranged in these detection elements often have different ratios of increase in output signal with respect to increase in dark current value and light amount. Therefore, when the output signals of a plurality of light receiving elements are read using a CCD, for example, the offsets and gains are different for each light receiving element, and these appear as variations in the image signal at the time of image pickup, degrading the image quality. As a measure against the deterioration of the image quality due to such variations in the characteristics of the light receiving element, the video signal is usually electrically corrected.

An example of this correction method is shown below. First, when the infrared imaging device sees a uniform temperature T ₁ (a temperature close to the target background temperature is desirable), the difference between the output signal of the detection element and the reference value set to an appropriate value is received as the offset correction value. First for each element
Store in memory. Next, a gain is calculated for each light receiving element using the output signal when a uniform temperature T ₂ slightly different from T ₁ is observed and the data of the first memory at the temperature T ₁ previously obtained, A ratio with a gain reference value set to an appropriate value is stored in the second memory as a gain correction value. in this way,
The correction value is stored in the memory in advance, and the offset value is corrected by subtracting the value of the first memory of the light receiving element for each signal corresponding to each light receiving element from the image signal at the time of image pickup,
Gain correction is performed by multiplying the value in the second memory.

Next, a conventionally used method for obtaining an output signal for correction at temperature T ₁ and temperature T ₂ will be described with reference to FIG. In the figure, 1 is a first lens system in which the lens system is simplified, 2
Is an optical axis of the first lens system, 3 is an image plane of the first lens system, a first image plane, 4 is an infrared detecting element having an infrared receiving element of a two-dimensional array, 5 is a receiving surface of the infrared detecting element 4, 12a and 1
2b are hot plates having different temperatures.

In FIG. 3, normal image pickup is performed as follows. The light incident from the first lens system 1 forms an image on the first image plane 3 and an optical image is electrically converted by a detection element 4 having a light receiving surface 5 on this plane, and an image signal or an output signal used for correction Is output as.

In the infrared image pickup device having such a configuration, the output signal for correction is obtained as follows. First, the output signal at the temperature T ₁ used for the offset correction is obtained by using, for example, the first signal by using the heating plate 12a having a uniform temperature T ₁ (usually the ambient temperature of the apparatus).
It is obtained by temporarily closing the front surface of the lens system 1.
Also, for the output signal at the temperature T ₂ used for gain correction, a heating plate 12b of a uniform temperature slightly different from T ₁ is prepared,
It can be obtained by the same operation as in the case of offset correction.

(Problems to be Solved by the Invention) By the way, since the conventional signal variation correction in the infrared imaging apparatus is performed by the above method, it is necessary to correct the background temperature around the target accurately. Nevertheless, the temperature around the device is actually corrected. Therefore, there is a problem that the correction tends to be inaccurate.

Further, in order to obtain a hot plate having a temperature T ₂ used for gain correction, it is necessary to heat or cool the hot plate, which is problematic in terms of operability and immediacy.

Further, in the case of an infrared imaging device mounted on an aircraft or the like, it may be practically possible to arrange the heat plate in front of the first lens system.

The present invention has been made in order to solve the above problems, and provides a signal correction method for an infrared imaging device capable of easily correcting variations in a video signal suitable for a background temperature in a short time. With the goal.

(Means for Solving the Problems) In order to achieve the above-mentioned object, a signal correction method for an infrared imaging device according to the present invention is an infrared detection element including a plurality of infrared reception elements, and a light reception of the detection element. First on the surface
In an infrared imaging device having a first lens system having an image plane, a second lens system on the same optical axis is used instead of the first lens system, and is an image plane of the second lens system. By using the output signal of the detection element obtained by setting the position of the second image plane to a position greatly different from the light receiving surface,
This is to electrically correct the variation of the video signal due to the difference in the characteristics between the light receiving elements.

Furthermore, the variation of the video signal may be corrected by using the output signal of the detection element obtained by using the mechanism for adjusting the amount of light incident on the detection element together with the second lens system.

(Operation) In the present invention, the second lens system (however, the first lens system may be included in part) is used in place of the first lens system, and the position of the image plane is used as the light receiving surface of the infrared detection element. If it is displaced from the position, the image at the light receiving surface position becomes unclear. That is,
The light that enters the second lens system from each direction spreads locally or entirely on the light receiving surface of the detection element. Further, in this case, when there is a difference between the temperature around the device and the background temperature in the external environment, the average amount of light incident on the detection element can be adjusted by changing the position of the image plane. Therefore, by using the output signal of the detection element when this average light amount is appropriately adjusted, it is possible to obtain data used for offset correction and gain correction suitable for the background temperature.

Further, by making the image unclear by using the second lens system as described above, the incident light amount from the outside is made uniform and incident on the detection element, and if a mechanism for adjusting the light amount is added, The amount of light incident on the detection element can be variably adjusted without changing the position of the image plane.

(Example) An example of a signal correction method for an infrared imaging device according to the present invention will be described below with reference to the drawings.

FIG. 1 shows a simplified optical system used in a signal correction method for an infrared imaging device according to the present invention. FIG. 1A shows an optical system in a normal imaging state, 1 is a first lens system in which the lens system is simplified, and 2 is a first lens system 1.
Is an image plane of the first lens system 1, a first image plane, and 4 is an infrared detecting element having a two-dimensional array of infrared receiving elements, the center of which is the optical axis 2. Reference numeral 5 is a light receiving surface of the detecting element 4,
It is located at a position corresponding to the image plane 3. 6a is a ray of light incident on the lower end of the first lens system 1 and directed to the upper end of the detection element 4, 6b is a ray of light incident on the upper end of the first lens system 1 and directed to the upper end of the detection element 4, and 6c is of the first lens system 1. A ray 6d is incident on the center and goes to the upper end of the detection element 4, and 6d is a ray which is incident on the lower end of the first lens system 1 and goes out of the upper end of the detection element 4.

FIGS. 1B to 1C show a case where a second lens system is configured by adding one convex lens in order to correct the signal variation, and 7 is a convex lens on the optical axis 2. , 8
Is a second lens system composed of the first lens system 1 and the convex lens 7.
The lens system 9 is an image plane of the second lens system, which is a second image plane.

Next, a signal correction method for the infrared imaging device according to the present invention will be described with reference to FIG. Here, it is assumed that the external average temperature in the visual field is higher than the ambient temperature of the device. Also, the infrared transmittance of the lens system is ignored.

First, as shown in FIG. 1B, a convex lens 7 is inserted on the same optical axis 2 as that of the first lens system 1 in the first lens system 1 in the normal imaging state shown in FIG. 2nd lens system 8
To form. The position of the convex lens 7 at this time is a point where the light ray 6a intersects the optical axis 2, and the focal length is selected so that the light ray 6b passes through the lower end of the detection element 4 after being refracted by the convex lens 7. By doing so, the position of the second image plane 9 is at a position greatly different from the light receiving surface 5 as shown in FIG.
Then, the light incident on the first lens system 1 from the same direction is
For example, like the light rays 6a to 6c in FIG. 1 (a), all of them have been concentrated at one point on the light receiving surface 5 until now, but by inserting the convex lens 7, the light rays 6a to 6d in FIG. Thus, all the light transmitted through the second lens system 8 is evenly distributed and incident on the light receiving surface 5. Therefore, second from all directions
The light that enters and passes through the lens system 8 enters the light receiving elements of the detection element 4 substantially uniformly, regardless of the difference in the infrared radiation intensity in each direction of the outside world.

In this case, light which has not entered the detection element 4 in the state of FIG.
The total amount of light incident on the detection element 4 is larger than in the case where only the first lens system 1 is used.

Further, as shown in FIG. 1 (c), when the convex lens 7 is moved closer to the first lens system 1, that is, when the second image plane 9 is further shifted to the left side, light rays 6a and 6b are now generated. The light that had been incident on the detection element 4 up to now ceases to enter, and FIG.
In this case, the amount of light incident on the detection element 4 decreases.
Then, when the convex lens 7 comes to an appropriate position, the same amount of light as in the case of only the first lens system 1 becomes incident on the detection element 4 almost uniformly, and as the distance further approaches, the amount of incident light gradually increases. The amount of light becomes extremely small, and the amount of light approaches the ambient temperature of the detection element 4.

Here, although the central portion of the light receiving surface 5 of the detection element 4 which is the first image surface is bright and the peripheral portion is dark, the difference in brightness between the two is very small and can be ignored. Also, description will be made with reference to FIG.

In FIG. 4, the brightness of the central portion of the light receiving surface of the detection element serving as the first image surface is proportional to the amount of light entering the convex lens 7 through the central portion of the first lens system 1. The brightness at this time is A. On the other hand, on the first image surface (light receiving surface of the detection element), h /
For the brightness at positions separated by 2, D ₁ is the diameter of the first lens system. Since the amount of light that is incident as the convex lens the position of the D _1/2, can be approximated as A cos .theta. However, θ = tan ⁻¹ {(D ₁ + h) / (2 × f ₁ )}, and f ₁ is the focus of the first lens system. The numerical values are as follows. Generally, f ₁ = about 50 to 200 mm, and h, which is the diagonal length of the light receiving surface of the detecting element, is usually 10 mm or less and 20 mm at the maximum. Now, if we take f ₁ = 70mm and h = 10mm, co
sθ = 0.92. By the way, for the brightness at a position 5 mm away from the optical axis, it is necessary to consider not only the incident light from the temperature T ₀ of the outside world but also the amount of infrared rays entering from the periphery of the device such as the lens barrel due to a part other than the outside world. In this case, the total infrared ray incident amount considering the incident infrared ray amount in the peripheral portion is proportional to Fa.

_{Fa = F (T 0) cosθ} + F (T 1) (1-cosθ) wherein F (T ₀₎ and F (T ₁₎ is an infrared radiant flux emittance from the peripheral external and equipment, respectively. These are T ₀ = 40
℃, T ₁ = 20 ℃, wavelength range 8 ~ 10μm, F (T ₀ ) =
1.360 × 10 ^-2 W / cm ² , F (T ₁ ) = 1.066 × 10 ^-2 W / cm ² . Therefore, the brightness of the peripheral part at a distance of 5 mm is 0.983 in the central part.
Doubled and becomes 1.7% darker. In general, the brightness of the outside world is set to about 10 times the noise level when capturing an image. Therefore, in this case, even if there is a difference of 10% in brightness between the central portion and the peripheral portion, it is about the noise level, and the influence is very small. In the case of this embodiment, it is 1.7% and can be almost ignored.

Next, the uniformity of brightness on the first image surface (light receiving surface of the detection element) when the lens system is moved will be described. As shown in FIG. 5, for example, when the convex lens 7 is moved left by a ₁ and a lens system having a focal length of f ₁ + a ₂ is placed a ₂ left from the position of the first lens system 1. Equivalent to. In this case, θ is given by the following equation.

θ = tan ⁻¹ [h (D ₁ + h) / {2 × (f ₁ h + a ₁ D ₁ + a
₁ h)}] From this equation, consider the case where the brightness of the first image surface when the convex lens 7 is inserted is made equal to the average brightness of the outside world during normal imaging. This is a condition used for normal offset correction. In this case, from the calculation, the convex lens may be moved leftward by 11.7 mm. The condition and D ₁ = 50
mm is calculated as mm and a ₁ = 11.7 mm, cos θ = 0.978
And the same calculation as before, the peripheral part is 0.99 of the central part.
5 times brighter. In other words, the peripheral area is 0.5% from the central area
Get dark. But this is within the noise level,
The effect can be ignored.

If the difference in brightness is large, the ratio of the magnitude depending on the position is known in advance as shown by the formula, and therefore, it can be electrically corrected by a simple method at the stage of signal processing.

From the above, by adjusting the position of the convex lens 7 having an appropriate focal length, the detecting element 4 can be operated in a normal imaging state.
The light amount close to the average of all the light amounts incident on can be made to enter the respective light receiving elements of the detection element 4 substantially uniformly. Therefore,
By using the output signal of the detection element 4 imaged in this state as the data at the temperature T ₁ used for the offset correction, it is possible to perform the offset correction using the light amount substantially equal to the target background temperature. Further, from this state, the data at the temperature T ₂ used for gain correction is obtained from the output signal imaged by moving the position of the convex lens 7 back and forth by an appropriate distance, and the gain correction is performed using this data and the offset correction data. It can be performed.

More specifically, the difference between the output signal of the detection element 4 at the position of the convex lens 7 corresponding to the temperature T ₁ and the reference value set to an appropriate value is stored in the first memory as an offset correction value for each light receiving element. Next, the gain is calculated for each light receiving element using the output signal at another position of the convex lens 7 corresponding to the temperature T ₂ and the data of the first memory corresponding to the temperature T ₁ obtained previously, A ratio with a gain reference value that is set to an appropriate value is stored in the second memory as a gain correction value, and the first signal of the light receiving element is stored for each signal corresponding to each light receiving element with respect to the video signal at the time of imaging. The value in the memory is subtracted to perform the offset correction, and the value in the second memory is multiplied to perform the gain correction.

After the correction data is acquired, the convex lens 7 is removed and the original first lens system 1 is returned to normal imaging.

As described above, for example, a convex lens is newly added to the lens system of the infrared imaging device at a preset position and the output signal of the infrared detecting element obtained by changing the position of the image plane is used to detect the infrared receiving element. It is possible to correct the variation of the video signal due to the variation of the characteristics of.

In the above embodiment, the case where one convex lens 7 is added to the first lens system 1 as the second lens system 8 is shown, but the same effect can be obtained by using other lens configurations.

Further, in the above embodiment, the case where the convex lens 7 is moved to adjust the amount of light incident on the light receiving element has been described, but the same effect can be obtained by moving other lenses or detection elements included in the lens system.

Next, another embodiment of the present invention will be described with reference to FIG.
FIG. 2 shows a case where a field diaphragm is used as a mechanism for adjusting the amount of light incident on the infrared detection element, and 8 is the first.
The second lens system is composed of the lens system 1 and the convex lens 7, and the position and the focal length are the same as those in FIG. 1 (b). Reference numeral 9 is an image forming surface of the second lens system 8, which is a second image surface, and 10 is a second image surface.
It is a mechanism for adjusting the amount of light provided on the image plane, and has a function of a field diaphragm in this example.

Here, it is assumed that the external average temperature in the visual field is higher than the ambient temperature of the device. Further, the transmittance of the lens system is neglected. Further, assuming that the effective light receiving dimension of one side of the infrared detection element 4 is 2d, and when only the first lens system 1 is used, the first image plane 3
The image that was 2d in 2nd is the second when the second lens system 8
It is assumed that the image surface 9 has 2d '.

Now, also in this embodiment, when the same second lens system 8 as in FIG. 1 (b) is used, the light incident on a certain point of the first lens system 1 as shown in FIG. Regardless of this, the light is focused on one point of the detection element 4 through the optical paths such as the light rays 11a to 11d. When such a lens system is used and a diaphragm having an aperture of 2d 'is used as the light amount adjusting mechanism 10 at the position of the second image plane 9, the average of all the light amounts incident on the detecting element 4 in the normal imaging state is obtained. A certain amount of light can be uniformly incident on the detection element 4. Further, as is clear from FIG. 2, the amount of light incident on the detection element 4 can be changed by adjusting the aperture amount of the diaphragm without changing the position of the convex lens 7.

As described above, by using the second lens system 8 and the light amount adjusting mechanism 10, it is possible to make the light amount corresponding to the average temperature of the outside world and the light amounts before and after it incident on the detection element 4 substantially uniformly. It is possible to obtain an output signal used for offset correction and gain correction suitable for the background temperature. In this case, the position of the convex lens 7 need not be moved.

Although the field diaphragm is used as the diaphragm in the above embodiment, the same effect can be obtained by using an aperture diaphragm or an optical system having a similar function to these diaphragms.

Further, in the above embodiment, the case where the diaphragm is used as the light amount adjusting mechanism 10 is shown. However, as in the case where the second lens system 8 described above is used, for example, the total amount of light incident on the detection element 4 is normally set. The same effect can be obtained by increasing the number according to the image pickup state and inserting the appropriately adjusted light amount adjusting mechanism 10 such as the light attenuating plate or the light diffusing plate at an appropriate position.

Further, in the above embodiment, the case where one type is used as the light quantity adjusting mechanism 10 is shown, but the same effect can be obtained by using a plurality of diaphragms, light attenuating plates, light diffusing and the like in combination.

Further, in the above embodiment, the case where the background temperature of the outside world is higher than the ambient temperature of the device has been described, but the reverse case can be similarly described.

(Effects of the Invention) As described above, according to the present invention, the offset correction suitable for the background temperature can be performed simply by performing a simple operation by using the new lens system configuration or the lens system and the light amount adjusting mechanism together. Also, since the output signal of the detection element used for gain correction can be obtained, the video signal can be corrected more accurately and easily and in a shorter time than the conventional method.

[Brief description of drawings]

FIG. 1 is an explanatory view of an optical system showing an embodiment of the present invention, and FIG.
FIG. 4 is an explanatory diagram of an optical system showing another embodiment of the present invention, FIG. 3 is an explanatory diagram of a conventional signal correction method, and FIG. 4 is a convex lens between the first lens system and the light receiving surface of the detection element. FIG. 5 is an explanatory diagram for explaining the difference in brightness between the central portion and the peripheral portion on the light receiving surface when inserted, and FIG. 5 is an explanatory diagram when the position of the convex lens is moved similarly. 1 ... First lens system, 2 ... Optical axis, 3 ... First image plane, 4
...... Detecting element, 5 ...... Light receiving surface, 8 ...... Second lens system, 9
…… Second image plane, 10 …… Light intensity adjustment mechanism. In the drawings, the same reference numerals indicate the same or corresponding parts.

Claims (2)

[Claims] 1. An infrared detecting element comprising a plurality of infrared receiving elements, and a first image plane at a light receiving surface position of the detecting elements.
In an infrared imaging device including a lens system, a second lens system on the same optical axis is used instead of the first lens system, and a position of a second image plane which is an image forming plane of the second lens system. By using an output signal of the detection element obtained by making the position significantly different from the light receiving surface, electrically correcting the variation of the video signal due to the difference in characteristics between the respective light receiving elements. Signal correction method for infrared imaging device. 2. The signal correction method for an infrared imaging device according to claim 1, further comprising a mechanism for adjusting an amount of light incident on the detection element together with the second lens system.