Apparatus and method for removing offset and shading from a video signal

Abstract: An apparatus and method for removing offset and shading from a video signal include a filter (28) that convolves the video signal (12) with a second derivative (Fig. 4). The derivative can include first and second non-zero values centered about a third non-zero value, zero-filling between the first and third values, and zero-filling between the second and third values. The filter (28) can be applied to Modulation Transfer Function measurement of an electro-optical sensor, in which offset and shading are removed from the sensor's video signal prior to Fourier analysis. Filtering and Fourier analysis are performed by a digital signal processor (26), which allows the measurement to be fully automated.

Description: The invention relates in general to electro-optical sensors and in particular to an apparatus and method for processing a video signal from an electro-optical sensor. An electro-optical sensor is a device that converts radiant energy of a particular wavelength or range of wavelengths into an electrical signal. One example of an electro-optical sensor is a handheld TV camera, which converts images of visible light into an electrical signal that can be stored in electronic memory or displayed on a television screen. Another example of an electro-optical sensor is a Forward Looking Infrared sensor ("FLIR"), which converts images of infrared energy (i.e., heat), invisible to the human eye, into an electrical signal that can be displayed to a viewer. Because the FLIR can "see at night," it is widely used in military navigation and targeting systems and more recently in commercial security systems. The capability of the electro-optical sensor to resolve the details of an object is referred to as the "resolution" of the sensor. Increasing the resolution allows the sensor to identify small objects at greater distances. High resolution allows the FLIR to identify an enemy target before that target gets too close. Shading and offset diminish the resolution of the electro-optical sensor and, therefore, must be removed. When the target is small, the sensor's electronics must increase the gain and brightness to a high level in order to see the necessary detail in the target. Although the details of the target become more visible, the background becomes distorted by internal reflections and temperature gradients which result from the increases in gain and brightness. Offset refers to the average level of brightness (temperature) over an entire visible (infrared) sensor, and shading refers to gradients in brightness (temperature) across the sensor. These distortions make it harder to distinguish the target from the background. Shading and offset cause additional problems when the resolution of the sensor is being measured. A standard measurement for sensor resolution is the Modulation Transfer Function ("MTF"). During MTF measurement, a very small target (e.g., a slit target) is presented to the sensor, and data samples of the sensor's video signal are collected. After all of the samples have been collected, shading and offset are manually removed from the samples and Fourier analysis is performed. This procedure requires an operator to manually derive the offset and shading from trends in data, and then subtract the offset and shading from the data. Operator intervention during the MTF test is undesirable. The operator must have experience and training to determine trends in the data. Even then, the determinations are subjective and prone to error. Moreover, operator time is required to determine the trends, preventing the test results from being available in real-time. EP 0565861 discloses an image signal processing apparatus in which shading correction and MTF correction are carried out on an image signal. Signal storage means corresponding to at least one line is provided for the shading correction and MTF correction, and the data which are stored for the processing are limited according to the accuracy which is necessary for the various operations. According to a first aspect of the present invention there is provided apparatus for removing shading and offset from a video signal of an image sensor, the video signal being an electrical signal representing an image, which image includes a target which simulates an optical impulse function, characterised in that the apparatus comprises: means (28) for determining the width of that part of the video signal which represents the optical impulse function; means (28) for defining a function which has first, second and third non-zero values, and which is zero-filled between the first value and the second value and between the second value and the third value, the spacing between the first value and the third value being proportional to the width of that part of the video signal which represents the optical impulse function; and means (28) for convolving the thus defined function with the video signal to remove shading and offset from the video signal. According to a second aspect of the present invention there is provided a method of removing shading and offset from a video signal of an image sensor, the video signal being an electrical signal representing an image, which image includes a target which simulates an optical impulse function, characterised in that the method comprises: determining the width of that part of the video signal which represents the optical impulse function; defining a function which has first, second and third non-zero values, and which is zero-filled between the first value and the second value and between the second value and the third value, the spacing between the first value and the third value being proportional to the width of that part of the video signal which represents the optical impulse function; and convolving the thus defined function with the video signal to remove shading and offset from the video signal. FIG. 1 is a schematic diagram of an electro-optical sensor and apparatus for measuring the resolution of the sensor in accordance with the present invention; FIG. 2 is a flowchart of steps for removing offset and shading from a video signal generated by the sensor, the steps being performed by a filter which forms a part of the apparatus shown in FIG 1; FIG. 3 depicts a waveform for the video signal (in solid lines) and a differential of the waveform (in dashed lines); FIG. 4 is a plot of a Low Frequency Second Derivative utilized by the filter; and FIG. 5 depicts the video signal after convolution with the Low Frequency Second Derivative. Fig. 1 shows an electro-optical sensor 10 under test. The electro-optical sensor 10 converts light of a particular wavelength or range of wavelengths into a video signal 12. As used herein, light means radiant energy having wavelengths within the electromagnetic spectrum. Thus, light includes, but is not limited to, infrared radiation, visible light, ultraviolet radiation and x-rays. A long wave infrared ("LWIR") sensor, for example, is capable of converting light having wavelengths between 8 and 12 microns into a video signal. MTF of the sensor 10 is measured by obtaining samples of the sensor's impulse response in one dimension, removing shading and offset from the video signal 12, and calculating the magnitude (modulus) of the Fourier Transform from the filtered video signal. The sensor impulse is obtained by simulating an optical impulse function input signal, such as a slit target 14, to the sensor 10 and directly sampling the video signal 12. The slit target 14 is generated by a target generator 16, which includes a slit target mask 18 and a source 20 of back irradiation for the mask 18. The slit target mask 18 contains a small rectangular aperture having a width that is proportional to the angular field of view of a single resolution element in the sensor 10. The source 20 can be a nichrome coil for the FLIR or a tungsten lamp and integrating sphere for a TV camera. Target generators 16 in general are well known to those skilled in the art. The slit target 14 is focused onto the sensor 10 by collimating optics 22. Collimation, which is achieved by placing the slit target mask 18 at the focal plane of the collimating optics 22, allows a smaller slit target 14 to be used at practical distances from the sensor 10. Collimating optics 22 are also well known to those skilled in the art. The sensor 10 converts a particular frequency component (e.g., LWIR) of the target 14 into the video signal 12. The video signal 12 can be either an analog signal or a digital signal. If the video signal 12 is an analog signal, it is converted into a digital video signal 12' by an analog-to-digital converter 24. If the video signal is already a digital signal, only a digital interface for the sensor 10 is required. The MTF is derived from the digital video signal 12' by a digital signal processor 26 which includes a filter 28 for removing shading and offset, and a post-processing module 30 for performing Fourier analysis on the filtered digital signal 12''. The filter 28 removes the steady state (dc) and at least first order frequency components from the digital video signal 12'. Shading could be linear, or it could be a higher order function of the internal reflections and temperature gradients within the sensor 10. An exemplary filter 28 is described below in connection with FIGS. 2-5 The post-processing module 30 performs a Discrete Fourier Transform module 30 on the filtered video signal 12'' and calculates the magnitude of the transform. The post-processing module 30 could also normalize the resulting real-valued sequence to unity by scaling the Transform by the maximum value. The digital signal processor 26 can be implemented in a personal computer having an Intel 486 microprocessor and appropriate software for performing the filtering and the Discrete Fourier Transform. In the alternative, the digital signal processor 26 can be implemented in a Finite Impulse Response computer, which would filter the video signal 12' and perform the Discrete Fourier Transform FIG. 2 describes the steps 110-130 performed by the filter 28. In step 110, the width of the video signal 12' is determined. The video signal 12' provided by the sensor 10 in response to the slit target 14 is expected to have a waveform somewhere between a Gaussian and Sync waveform. A fast and simple approach for determining the width of the video signal 12 is to find the peak value of the waveform and determine the width at the 50% point. A more elaborate method of determining the width of the video signal 12' is to differentiate the video signal 12' by performing a low pass differential. Assuming that the video signal 12' has a Gaussian waveform ( G ) of the equation G i = e ( i - N 2 ) 2 σ 2 -1 + OFFSET + SHADING where i is the i th sample, N is the sample size, σ 2 is the standard deviation, the differential of the waveform (Gdif) can be determined by processing the video signal 12' according to the equation Gdif j = Σ derivcoeff m ( G j+m - G j-m ) where derivcoeff m is the m th derivative of Gdif. The width of video signal 12' is defined as the separation between the maxima and minima of the resulting differential. The waveform (in solid lines) and differential thereof (in dashed lines) are shown graphically in FIG. 3. In step 120, a Low Frequency Second Derivative L is derived from the width of the video signal 12'. The Low Frequency Second Derivative L has three non-zero values L0, L1 and L2, where values L0 and L2 are located symmetrically about value L1. As shown in FIG. 4, values L0 and L2 are equal to -0.5, and value L1 is equal to 1.0. However, L0, L1 and L2 can be of any value, provided that L0+L1+L2=0 and L0=L2. Spacing between the values L0, L1 and L2 is proportional to the width of the region of interest, which is equal to the width of the video signal 12' times an application factor. Although the application factor is determined empirically, a factor of 3.2 has been found to work well in several different LWIR sensors. The Derivative L has the value of zero between values L0 and L1. Similarly, the Derivative L is zero-filled between the values L1 and L2. In step 130, the video signal 12' is convolved with the Derivative L to remove shading and offset. The filtered video signal 12" that results is supplied to the post-processing module 30. The post-processing module 30 utilizes only the region of interest of the video signal 12", which is centered about the positive peak. FIG. 5 shows the filtered video signal 12". Prior to filtering of the video signal 12' by the digital signal processor 26, noise might have to be removed from the video signal 12' in order to increase its signal-to-noise ratio. The noise could be removed by the digital signal, processor 26, which would utilize a well known technique such as frame averaging. It is understood that various changes and modifications may be made without departing from the scope of the invention. For example, a bar chart target could be presented to the sensor instead of the slit target (four bars for FLIRS and three bars for TV cameras), with samples of the resulting sinusoidal response being taken across the bars, and MTF being calculated at desired spatial frequencies. Other examples include using an analog signal processor instead of a digital signal processor and analog-to-digital converter; and determining the width of the video signal by multiplying the peak value by a constant. It is also understood that the filter according to the present invention could be applied to other resolution measurements, such as an Optical Transfer Function. Finally, it is understood that use of the filter is not limited to resolution measurements, and that the filter can be used in any situation where it is desirable to remove offset and shading from a video signal. For example, it would be desirable to remove offset and shading from a video signal that was displayed directly on a screen, and it would be extremely desirable to remove offset and shading from a video signal prior processing for target acquisition. Accordingly, the present invention is not limited to the precise embodiment described hereinabove.

Claims: Apparatus for removing shading and offset from a video signal of an image sensor, the video signal being an electrical signal representing an image, which image includes a target which simulates an optical impulse function, characterised in that the apparatus comprises: means (28) for determining the width of that part of the video signal which represents the optical impulse function; means (28) for defining a function which has first, second and third non-zero values, and which is zero-filled between the first value and the second value and between the second value and the third value, the spacing between the first value and the third value being proportional to the width of that part of the video signal which represents the optical impulse function; and means (28) for convolving the thus defined function with the video signal to remove shading and offset from the video signal. Apparatus according to claim 1 wherein the means for determining the width of that part of the video signal which represents the optical impulse function comprises means for obtaining a differential of the video signal, and means for measuring the separation between a maximum and a minimum of the differential. Apparatus according to claim 1 or 2 wherein the means for measuring the width of that part of the video signal which represents the optical impulse function includes means for measuring the width of a Gaussian signal. Apparatus according to any of the preceding claims wherein the first and third values are located symmetrically about the second value. Apparatus according to any of the preceding claims wherein the second value is of the opposite sign to the first and third values. Apparatus according to any of the preceding claims wherein the sum of the first, second and third values is equal to zero. Apparatus according to any of the preceding claims wherein the first value is equal to the third value. Apparatus according to any of the preceding claims further comprising a sensor for detecting radiant energy to produce the video signal. Apparatus according to any of the preceding claims, for use in target acquisition. A method of removing shading and offset from a video signal of an image sensor, the video signal being an electrical signal representing an image, which image includes a target which simulates an optical impulse function, characterised in that the method comprises: determining the width of that part of the video signal which represents the optical impulse function; defining a function which has first, second and third non-zero values, and which is zero-filled between the first value and the second value and between the second value and the third value, the spacing between the first value and the third value being proportional to the width of that part of the video signal which represents the optical impulse function; and convolving the thus defined function with the video signal to remove shading and offset from the video signal. A method according to claim 10 wherein the step of determining the width of that part of the video signal which represents the optical impulse function comprises obtaining a differential of the video signal and measuring the separation between a maximum and a minimum of the differential. A method according to claim 10 or 11 wherein the second value is of the opposite sign to the first and third values. A method according to claim 12 wherein the sum of the first, second and third values is equal to zero and the first value is equal to the third value.

Inventors: ORLANDO HAROLD J, WEIDEMAN DEAN L, ORLANDO, HAROLD J., WEIDEMAN, DEAN L.
Applicants: NORTHROP GRUMMAN CORP, NORTHROP GRUMMAN CORPORATION
Application year: 1996
Main classification: H04N5
IPC codes: H04N5, H04N17