Method for determining characteristic-curve correction factors of a matrix detector that images in the infrared spectral range, method for controlling a guided missile and guided missile

Abstract

The invention relates to a method for determining characteristic-curve correction factors a matrix detector that images in the infrared spectral range. A good image correction can be obtained by virtue of an area of homogeneous temperature being recorded at two different temperatures by the matrix detector, there being two images with different integration times for each temperature. A signal gradient over the integration time is established for each of the pixels from the four pixel values at the two temperatures in each case and the gain being established from the difference of the signal gradients and characteristic-curve correction factors for the gain being stored.

Claims

1. A method for determining characteristic-curve correction factors of a matrix detector that images in an infrared spectral range, which comprises the steps of: recording an area having a homogeneous temperature at two different temperatures by the matrix detector, there being two images with different integration times for each temperature; determining a signal gradient over an integration time for each pixel from four pixel values at the two different temperatures in each case; determining a gain from a difference of signal gradients by determining a signal offset for each of the pixels from at least two pixel values being assigned to one of the temperatures, the signal offset being subtracted from the pixel values and the gain being thereupon determined; and storing the characteristic-curve correction factors for the gain.

2. The method according to claim 1, wherein the characteristic-curve correction factors for the gain are determined while the matrix detector is in a building and the characteristic-curve correction factors for the signal offset are determined during a subsequent use of a guided missile that contains the matrix detector.

3. The method according to claim 1, which further comprises recording the two images with the different integration times immediately in succession.

4. The method according to claim 1, which further comprises determining a dark current for each of the pixels using at least image points assigned to one of the temperatures and characteristics of detector elements assigned to the pixels are corrected using the characteristic-curve correction factors for the dark current.

5. The method according to claim 1, which further comprises using the difference between the two signal gradients as the gain.

6. The method according to claim 1, wherein the characteristic-curve correction factor for the gain is a mean difference of the signal gradients divided by a pixel-individual difference of the signal gradients.

7. The method according to claim 1, which further comprises determining the signal offset from the pixel values at both recording temperatures and the characteristic-curve correction factor for the signal offset of the pixels is determined as a mean value of the two signal offsets.

8. The method according to claim 1, which further comprises interpolating the characteristic-curve correction factors from the two temperatures for a multiplicity of further, different temperatures.

9. A method for determining characteristic-curve correction factors of a matrix detector that images in an infrared spectral range, which comprises the steps of: recording an area having a homogeneous temperature at two different temperatures by the matrix detector, there being two images with different integration times for each temperature; determining a signal gradient over an integration time for each pixel from four pixel values at the two different temperatures in each case; determining a dark current for each of the pixels using at least image points assigned to one of the temperatures and characteristics of detector elements assigned to the pixels are corrected using the characteristic-curve correction factors for the dark current, and using one of the signal gradients or a summation of both of the signal gradients as the dark current; determining a gain from a difference of signal gradients; and storing the characteristic-curve correction factors for the gain.

10. A method for determining characteristic-curve correction factors of a matrix detector that images in an infrared spectral range, which comprises the steps of: recording an area having a homogeneous temperature at two different temperatures by the matrix detector, there being two images with different integration times for each temperature; determining a signal gradient over an integration time for each pixel from four pixel values at the two different temperatures in each case; determining a gain from a difference of signal gradients; determining a detector temperature of the matrix detector; determining the characteristic-curve correction factors in dependence on the detector temperature; and storing the characteristic-curve correction factors for the gain.

11. A method for controlling a guided missile, which comprises the steps of: determining characteristic-curve correction factors of a matrix detector that images in an infrared spectral range, which includes the sub-steps of: recording an area having a homogeneous temperature at two different temperatures by the matrix detector, there being two images with different integration times for each temperature; determining a signal gradient over an integration time for each pixel from four pixel values at the two different temperatures in each case; determining a gain from a difference of signal gradients by determining a signal offset for each of the pixels from at least two pixel values being assigned to one of the temperatures, the signal offset being subtracted from the pixel values and the gain being thereupon determined; and storing characteristic-curve correction factors for the gain; bringing the guided missile, containing the matrix detector in a seeker head, to a deployment site and launched there; recording surroundings by the matrix detector in two images with different integration times; determining a signal offset for each of the pixels from the two pixel values and the characteristics of the detector elements that are assigned to the pixels are corrected using the characteristic-curve correction factors for signal offset and gain; and controlling the guided missile on a basis of the images of the matrix detector that were corrected.

12. The method according to claim 11, wherein the two images are recorded while cooling the matrix detector, which is implemented to establish a readiness of the matrix detector.

13. A guided missile, comprising: a seeker head containing a matrix detector with detector elements; a controller for controlling a deployment flight; and a data memory containing characteristic-curve correction factors for a gain of said detector elements of said matrix detector, the characteristic-curve correction factors of said matrix detector being determined by the steps of: recording an area having a homogeneous temperature at two different temperatures with said matrix detector, there being two images with different integration times for each temperature; determining a signal gradient over an integration time for each pixel from four pixel values at the two different temperatures in each case; determining the gain from a difference of signal gradients by determining a signal offset for each of the pixels from at least two pixel values being assigned to one of the temperatures, the signal offset being subtracted from the pixel values and the gain being thereupon determined; and storing the characteristic-curve correction factors for the gain.

14. The guided missile according to claim 13, wherein said controller is prepared to determine the characteristic-curve correction factors for a signal offset of said detector elements of said matrix detector on a basis of two scene images with different integration times recorded directly in succession during a deployment.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

(1) FIG. 1 is an illustration of a guided missile with a matrix detector in a seeker head;

(2) FIG. 2 is a flowchart for determining characteristic-curve correction factors;

(3) FIG. 3 is a flowchart for correcting signals of detector elements of a matrix detector;

(4) FIG. 4 is a diagram showing plotting detector element signals against integration time; and

(5) FIG. 5 is a diagram with a computationally changed signal characteristic curve against integration time.

DETAILED DESCRIPTION OF THE INVENTION

(6) Referring now to the figures of the drawings in detail and first, particularly to FIG. 1 thereof, there is shown a guided missile 2 having a seeker head 4, which has an optical unit 6 that images a surrounding scenery in front of the guided missile 2 onto a matrix detector 8. The matrix detector 8 is a matrix detector that is sensitive to the infrared spectral range and it contains m×n detector elements, which are arranged in rows and columns, in hexagonal fashion or in any other way over an area, on which the optical unit 6 images. The matrix detector is of the focal plane array (FPA) type. The signals of the detector elements of the matrix detector 8 are evaluated by a control unit 10, which recognize an object as such from the image contents of the images produced by the matrix detector 8. The object can be an obstacle or a target, with the control unit 10 guiding the flight of the guided missile 2 around the obstacle or towards the target. By way of example, control is implemented by means of deflectable fins 12, which steer the flight of the guided missile 2 and, for example, allow an active body 14 to be brought to the target.

(7) For production reasons, the signals of the individual detector elements of the matrix detector 8 are inhomogeneous in relation to one another in terms of the signal. Therefore, they supply a different signal when exactly the same photon flux strikes. This leads to image aberrations, which can make interpreting the object significantly more difficult, particularly if the latter still is far away from the guided missile 2. For the purposes of correcting such artefacts, characteristic-curve correction factors for each detector element of the matrix detector 8 can be stored in a data memory 16 that is accessible to the control unit 10. The signal of the individual detector elements now can be corrected therewith, and so the image aberrations are reduced.

(8) FIG. 2 shows a flowchart for determining such characteristic-curve correction factors. To this end, an area 20 of homogeneous temperature, for example a blackbody emitter in a building, is imaged on the matrix detector 8 in a first method step 18. In FIG. 1, the area 20 is only illustrated schematically. For imaging purposes, the building needs to contain only the seeker head 4 or only the matrix detector 8 with a suitable optical unit 6 and an evaluation unit.

(9) Subsequently, two images of this area 20 are recorded, with the images being recorded with different integration times. Subsequently, a further area or the same area 20 with a different homogeneous temperature is imaged on the matrix detector 8 and two images with different integration times are recorded again.

(10) A result of this method step as illustrated schematically in FIG. 4. FIG. 4 shows a signal s that is triggered by photon flux Φ striking the detector element, plotted against the integration time t of the recorded image. The four measurement points 22a to 22d which the detector element has output as signals for the four images are illustrated. The first image, which was recorded at a blackbody temperature T.sub.BB1, supplies the two measurement points 22a and 22b. The two further images, which were recorded at a higher blackbody temperature T.sub.BB2, supply the measurement points 22c and 22d. The integration time pairs for the two blackbody temperatures T.sub.BB may be equal; however, this is not mandatory. In FIG. 4, the integration times t of all four images are different from one another.

(11) The signal s is substantially composed from three signal components: the signal offset ω, the signal gain or simply gain γ, and the dark current θ. The signal offset ω is constant over the integration time t, as illustrated in FIG. 4 by the straight line at ω. The gain γ and the dark current θ are dependent on the integration time. While the dark current θ grows linearly with the integration time t, it is independent of the temperature T.sub.BB of the recorded area 20. In FIG. 4, the dark current component of the signal s is illustrated using hatching. Although it is unknown, it is the same for both temperatures T.sub.BB.

(12) In the subsequent method step 24, the gradients σ.sub.1 and σ.sub.2 of the signals s are determined as a function of the integration time t. In FIG. 4, the gradients σ.sub.1,2 are indicated by double-headed arrows and emerge immediately from a straight line that extends through the two measurement points 22a, 22b and 22c, 22d, respectively, for one temperature T.sub.BB. In the case of an ideal extent of the two lines, these intersect at a vanishing integration time, t=0, and mark the signal offset ω of the signal s. In FIG. 4, the latter is illustrated by the horizontal dashed line. The signal offset ω for the detector element is calculated accordingly in method step 26. Since reality deviates from the ideal case illustrated in FIG. 4, it is expedient to choose the mean value of the two signal offsets ω that emerge from the points of intersection of the gradient lines through the measurement points 22 with a line for t=0.

(13) Now, the signal offset ω can be determined for each detector element and a reference value for the signal offset can be determined therefrom. Then, the characteristic-curve correction factors Ω for the signal offset for each detector element emerge from the differences of the individual signal offsets ω from the reference value. For the further calculations, the signal offset ω can be subtracted from the signal s or the measurement points 22. However, this is optional.

(14) The gain γ for the detector element is determined in the next method step 28. The gain γ depends on the blackbody temperature T.sub.BB and emerges from the difference of the two gradients σ, as indicated in FIG. 4 by the double-headed arrow. The gain γ can only be determined for the two blackbody temperatures T.sub.BB, although it can be interpolated or extrapolated for further blackbody temperatures T.sub.BB as characteristic-curve correction factor Γ. The characteristic-curve correction factors Γ for the gain are stored linearly in relation to the blackbody temperatures T.sub.BB, for example, and so the corrections for any other blackbody temperature T.sub.BB can be calculated therefrom. Any other function f can also be chosen in place of the linearization. The problem here is that although the photon flux Φ(T.sub.BB) has a theoretical dependence on the blackbody temperature T.sub.BB, other spectral sensitivities, the nature of the semiconductor quantum heads, details of the ND conversion, etc., are included in the overall detector response, and so there is no simple model for the function f. All that is certain is the monotonic property in T.sub.BB. Although the function could be measured in the laboratory, this is not necessary for determining the characteristic-curve correction factors, since the factors f.sub.1: =f(Φ(T.sub.BB1)) and f.sub.2: =f(Φ(T.sub.BB2)) are unknown but globally the same for all detector elements.

(15) The dark current θ for the individual pixels is determined in a further method step 30 and characteristic-curve correction factors Θ for the dark current can be determined therefrom with knowledge about all values for the dark current θ. In relation to the gradients σ, the dark current θ adopts the role of an offset. It can be subtracted as a gradient constant, and so the following arises: σ.sup.(1)=σ−θ. σ.sup.(1) is the first correction to the gradient σ in this case. Here, the absolute gradient of the dark current θ need not be known and can be set to a value, for example the gradient σ.sub.1 at the lower blackbody temperature T.sub.BB1, or the mean gradient value of the two gradients σ.sub.1 and σ.sub.2. This is shown in exemplary fashion in FIG. 5.

(16) FIG. 5 shows a diagram of a signal s of a detector element of the matrix detector 8, the signal having been modified by calculation. The signal s is plotted against the integration time t and, with its gradient, reproduces the relationship σ.sup.(1)=Γ−θ, where the gradient σ.sub.1 was assumed as dark current θ. Here, γ and θ remain unchanged in relation to the real measurements from FIG. 4. This calculation is based on the theoretical assumption that two images with different integration times t were recorded at a black body temperature T.sub.BB0 of 0K. The photon flux Φ is 0, with the dark current θ being present, in principle, and arising directly from the gradient σ.sub.0, such that the dark current θ is known. The latter can now be subtracted according to the equation σ.sup.(1)=σ−θ, as illustrated in FIG. 5.

(17) Now, the three variables of the signal offset ω, dark current θ and gain γ are known for the corresponding detector element. This procedure can be adopted for a multiplicity of detector elements or for all detector elements. From this, it is possible to determine reference values for the signal offset, dark current and gain, from which the individual characteristic-curve correction factors Ω, Θ and Γ for the signal offset, dark current and gain for each detector element are able to be determined with the individual values.

(18) In order now to arrive at a corrected image with a comparable brightness to the original image, the three variables of signal offset Ω, dark current Θ and gain Γ can be selected in such a way that the corrected signal for each variable is corrected to the mean value of the variable. By way of example, if Θ=σ(T.sub.BB1) is formulated, a corrected image according to FIG. 5 arises: σ.sup.(3)(T.sub.BB1)=Mean(σ(T.sub.BB1)), which is well corrected as a constant. Thus, the individual gain Γ of the detector element is replaced by the mean value. In order now to arrive at the corrected values with the original magnitude corresponding to FIG. 4, albeit with the correction, σ.sub.2=Mean(Δ(T.sub.BB1, T.sub.BB2))+Mean(σ(T.sub.BB1)) applies. Likewise, there can be a linear interpolated and extrapolated correction between the two gradients. Finally, the mean signal offset MeanΩ can be added independently of the integration time.

(19) In respect of specific calculations, the characteristic-curve correction factors can be determined as set forth below. Initially, the offset ω and the gradients σ in relation to the integration time t are calculated from the four measurement values 22 s(t.sub.1/2, T.sub.BB1/2) at two different integration times t.sub.1 and t.sub.2 and two blackbody temperatures T.sub.BB1 and T.sub.BB2:
σ(T.sub.BB1)=(s(t.sub.2,T.sub.BB1)−s(t.sub.1,T.sub.BB1))/(t.sub.2−t.sub.1)
σ(T.sub.BB2)=(s(t.sub.2,T.sub.BB2)−s(t.sub.1,T.sub.BB2))/(t.sub.2−t.sub.1)
ω(T.sub.BB1)=s(t.sub.1,T.sub.BB1)−t.sub.1σ(T.sub.BB1)
ω(T.sub.BB2)=s(t.sub.1,T.sub.BB2)−t.sub.1σ(T.sub.BB2).

(20) According to the model, ω(T.sub.BB1)=Ω(T.sub.BB2) should apply; this is usually satisfied to a good approximation. However, the slightly more improved mean value ω=(ω(T.sub.BB1)+ω(T.sub.BB2))/2 is used for the corrections. Naturally, this does not apply to the gradients σ; they depend on the blackbody temperature T.sub.BB. According to the model, the following applies thereto: σ(T.sub.BB)=γf(Φ(T.sub.BB))+θ.

(21) Now, the individual gain and dark current coefficients γ and θ should be determined therefrom, the coefficients, moreover, being dependent on the detector temperature. By way of example, the matrix detector 8 was cooled to 104 K. Proceeding from the two equations:
σ(T.sub.BB1)=γf(Φ(T.sub.BB1))+θ
σ(T.sub.BB2)=γf(t(T.sub.BB2))+θ,
the difference thereof is calculated to this end:
Δ(T.sub.BB1,T.sub.BB2):=σ(T.sub.BB2)−σ(T.sub.BB1)=γ(f(Φ(T.sub.BB2)−f(Φ(T.sub.BB2)).

(22) The dark current component drops out as a result of forming the difference. The multiplicative correction terms Γ in gain for the detector elements are then provided individually by
Γ=Mean(Δ(T.sub.BB1,T.sub.BB2))/Δ(T.sub.BB1,T.sub.BB2),
Where Mean( . . . ) denotes robust averaging over the entire image and ensures the normalization of the corrections to mean value of 1. The normalization could also be carried out in logarithmic fashion, which takes account of the multiplicative character of the Γ-correction to a slightly better extent.

(23) Calculating the dark current component Θ remains to be done, the latter being independent of the scene temperature but subsequently being included in multiplicative fashion with the integration time into the calculation of the characteristic-curve correction factors. The exact extrapolation to f(Φ=0) or else f=0, which in turn would require knowledge of f, is once again not necessary for the correction since one could just as easily take account of a constant offset f(Φ) the dark current component as well (even if the latter is weighted by γ, it only needs to be constant). Therefore, even negative values of Φ are not problematic from a formal point of view. As a reference point for the correction,
θ=(σ(T.sub.BB1)+σ(T.sub.BB1))/2
is formulated as mean value of the two gradient images. It would also be possible to use each of the two gradient images; however, the aforementioned approach is symmetrical and supplies slightly better results.

(24) The dark current deviations are corrected by Θ=Mean(θ)−θ, as a result of which the mean correction is zero.

(25) The sets of coefficients (“images”) Ω, Γ, Θ are available as a result of the laboratory measurement. Here, the two integration times t and the two detector temperatures T.sub.BB chosen for the laboratory measurement are arbitrary in principle, but should be chosen in such a way that, firstly, they cover the typical dynamic range well and, secondly, do not lie in the non-linear range of the detector characteristic curves.

(26) A further parameter is the detector temperature T.sub.D, which was not considered previously and was formulated as a constant. In order to be able to carry out a characteristic-curve correction at different detector temperatures, particularly during the cool down, measurements would be accordingly recorded at different detector temperatures T.sub.D and corresponding interpolation coefficients would be stored in the missile 2 such that
Ω=Ω(T.sub.D), Γ=Γ(T.sub.D), Θ=Θ(T.sub.D)
are present and are used according to the current detector temperature T.sub.D.

(27) Since the offset image Ω may not be as stable in time as the gain image Γ, the integration-time-independent offsets need not be stored as they can be calculated dynamically from the double images s(t.sub.1), s(t.sub.2), in a manner analogous to the laboratory measurement:
Ω=s(t.sub.1)−t.sub.1(s(t.sub.2)−s(t.sub.1))/(t.sub.2−t.sub.1).

(28) With all three parameter sets being present, the corrected image s.sub.corr when applying the characteristic-curve correction to the initial image s, recorded with an integration time t, emerges as

(29) In this form, it is very clear that the offset correction is composed of the integration-time-independent Ω and the dark current integrated over time tΘ. Addition of the mean value images Mean(Ω)+t Mean(Θ) ensures that the mean value, i.e., Mean(s)=Mean(s.sub.corr), is obtained.

(30) By way of example, the guided missile 2 or any other missile can be controlled on the basis of the characteristic-curve correction factors Ω, Γ, Θ. Such a method is illustrated in FIG. 3 in exemplary fashion. The characteristic-curve correction factors Γ, Θ, and optionally Ω as well, are established in a first method step 32 by virtue of an area 20 that is homogeneous in terms of temperature being imaged onto the matrix detector 8 and the area 20 being recorded at two temperatures, there being two images with different integration times t for each temperature. By way of example, the blackbody temperatures are 25° C. and 65° C. Expediently, this is implemented in a building such that the guided missile 2, the seeker head 4 thereof or only the optical unit 6 with the matrix detector 8 and control unit 10 or an image processing unit, which processes the image data of the matrix detector 8, are in a building. The characteristic-curve correction factors Γ, Θ, and optionally Ω as well, are stored in a data memory 16.

(31) In the subsequent method step 34, the guided missile 2 can be transported to a deployment site, or else it is initially transported to a storage site and, from there, to a deployment site at a later time. When the guided missile 2 is deployed 36, the matrix detector 8 is cooled to its deployment temperature, for example to less than 150 Kelvin.

(32) The characteristic-curve correction factors Ω for the signal offset are established in method step 38 before, during or after the cool down of the matrix detector 8 to its deployment temperature. This is implemented using at least two images that were recorded immediately in succession and from which the gradient σ, as described above, is established. In this case, the two images are not of an area of homogeneous temperature but may originate from the surrounding scene around or in front of the guided missile 2. Since the signal offset is independent of the integration time and the temperature of the recorded scene, substantially any scene is suitable for calculating the characteristic-curve correction factors Ω for the signal offset. It is expedient to assign the latter to the detector temperature T.sub.D and optionally carry out the method for two different detector temperatures T.sub.D in order to be able to determine the signal offset even at the deployment temperature of the matrix detector 8 by extrapolation.

(33) Subsequently, the guided missile 2 is launched and the object scene in front of the seeker head 4 of the guided missile 2 is imaged on the matrix detector 8. In method step 40, the signals s of the individual pixels of the matrix detector 8 are corrected on the basis of the characteristic-curve correction factors Ω, Γ, Θ and the image obtained from these corrected signals s is analysed by the control unit 10 in respect of controlling the guided missile 2 to a target emitting in the infrared range, for example. Here, the characteristic-curve correction factors Ω, Γ, Θ are available in the data memory 16, either as a function depending on the detector temperature T.sub.D or as a multi-dimensional dataset, by means of which the characteristic-curve correction factors Ω, Γ, Θ can be calculated by interpolation to the real detector temperature.

(34) The following is a summary list of reference numerals and the corresponding structure used in the above description of the invention: 2 Guided missile 4 Seeker head 6 Optical unit 8 Matrix detector 10 Control unit 12 Fin 14 Active body 16 Data memory 18 Record images 20 Area 22a-d Measurement points 24 Calculate σ 26 Calculate Ω 28 Calculate Γ 30 Calculate Θ 32 Establish characteristic-curve correction factors 34 Transport to a deployment location 36 Cooling of the matrix detector 38 Determine Ω 40 Correct the signals s Signal t Integration time T Temperature Φ Photon flux γ Gain of a detector element determined from measured values Γ Characteristic-curve correction factors for the gain θ Dark current of a detector element determined from measured values Θ Characteristic-curve correction factors for the dark current ω Signal offset of a detector element determined from measured values Ω Characteristic-curve correction factors for the signal offset σ Gradient