Image analysis – Image transformation or preprocessing – Correlation
Reexamination Certificate
2000-08-14
2004-10-12
Mehta, Bhavesh M. (Department: 2625)
Image analysis
Image transformation or preprocessing
Correlation
C382S115000, C382S124000, C382S209000, C382S219000, C340S005100, C340S005520
Reexamination Certificate
active
06804412
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to an optical correlator, for comparing images. Such devices can be used for optical recognition, for example for fingerprint recognition.
Several designs for optical correlators have been proposed. For example, Binary Phase-Only Matched Filter (BPOMF) based designs have been produced for a variety of applications. Correlation in a BPOMF is obtained by multiplying together the Fourier transform of the reference and input functions (r & s). This product is then Fourier transformed again to give the final correlation of r & s. In order to form the product in an optical system the input is displayed on one spatial light modulator and Fourier transformed with a lens. The reference r is Fourier transformed off-line and the result is converted to suit the type of spatial light modulator. The Fourier transform of s then passes through the spatial light modulator containing the Fourier transform of r giving the product. This is where the weakness of the system lies as the Fourier transform of s must be scaled and aligned with the reference to within one pixel at the spatial light modulator. Hence optical design and alignment of opto-mechanics are critical and very difficult to implement outside the laboratory. Another disadvantage of these systems is that the spatial light modulators (SLMs) used are too slow, difficult and expensive to obtain, or both.
Spatial light modulators based on ferroelectric liquid crystals are very fast and offer a potentially cheap technology for optical systems. However, they are limited by their binary modulation, i.e. by the ability of each cell only to display two states. Joint transform correlators using such devices are known from Guibert et al, “On-board optical transform correlator for road sign recognition”, Optical Engineering, Volume 34 (1995) page 135. This paper describes the use of ferroelectric liquid crystals with an optically addressed spatial light modulator.
However, such a correlator is difficult to construct and there are similar problems in optical design and mechanics as there are with the BPOMF. Also, optically addressed spatial light modulator (OASLM) technology has yet to become reliable and cannot deliver comparable performance to an electrically addressed silicon backplane spatial light modulator.
In a joint transform correlator (JTC), the input and reference images are displayed side-by-side on a display. In a so-called 1/f JTC, as described in J. L. Horner and C. K. Makekau ‘Two-focal-length optical correlator’, Applied Optics 28 (12) 1989, pp 2358-2367, the display is illuminated by collimated laser light and the side-by-side images are Fourier-transformed using a lens to form the joint power spectrum (JPS) as an intermediate image. Then, the intermediate image is non-linearly-processed and Fourier-transformed again, using the same or a different lens. The result gives a measure of the correlation between the input and reference images. In this prior-art JTC, the processing on the JPS was not designed to reduce the zero order light in the correlation plane. This was mostly due to the choice of display technology which restricts the modulation of the light to amplitude only. This device was also slow and could not be used to achieve high-speed correlation.
There is thus a need for an improved optical correlation method and correlator to alleviate these difficulties.
BRIEF SUMMARY OF THE INVENTION
According to the first aspect of the invention there is provided a method of optical correlation including the steps of modulating an input image and a reference image with a phase-encoded chequerboard pattern, displaying the modulated images side-by-side on a spatial light modulator, and performing a joint transform correlation on the displayed image.
The joint transform correlation is preferably performed by obtaining the joint power spectrum (JPS) corresponding to the Fourier transform of the input and reference images, and then obtaining a correlation image corresponding to the Fourier transform of the JPS. The correlation image contains information about the correlation between input and reference images.
The correlation is preferably performed by shining collimated light onto the spatial light modulator, forming an intermediate image of the spatial light modulator through a lens, recording and processing the intermediate image (JPS) and displaying the result on a spatial light modulator, shining collimated light onto the latter spatial light modulator, and recording a resulting correlation image of the spatial light modulator through a lens.
The advantage of carrying out the phase-encoding in a chequerboard pattern is that the collimated light passing straight through adjacent areas of the spatial light modulator, i.e. the zero-order light, destructively interferes. This greatly reduces the central zero-order spot of the image, and so helps reduce the contrast that the camera must record.
It is highly advantageous for the method to be a two-pass method, using only one spatial light modulator (SLM), lens and camera; in other words the SLMs and lenses mentioned are the same in each pass. Such a method comprises the steps of firstly displaying the reference and input images on the spatial light modulator and recording the intermediate image with a camera, secondly processing the intermediate image and thirdly displaying the processed intermediate image on the same spatial light modulator, and finally recording the correlation image with the camera to give an indication of the correlation between the input and reference images.
In alternative embodiments, two separate sets of modulators, lenses and cameras are used: this could operate slightly faster but would be more complex and expensive.
In one arrangement, the spatial light modulator (SLM) is a transmissive SLM, so that the light is transmitted through the SLM, through the lens and is then recorded by a camera located approximately one focal length behind the lens.
An alternative arrangement is to use a reflective spatial light modulator. In this arrangement reflected light is passed in the same way through the lens, reflected by the modulator and recorded by a camera.
Preferably the recorded image corresponds to the Fourier transform of the image displayed on the spatial light modulator. This is achieved by using collimated light and the arrangement of the camera one focal length behind the lens. Carrying out a Fourier transform twice on the side-by-side reference and input images gives a correlation image containing information about the correlation between the images. Of course, the Fourier transform will not be exact, since the camera can only record the intensity of the recorded light, not the phase, and background noise will always be present.
According to a second aspect of the invention there is provided a method of optical correlation for obtaining a correlation image corresponding to the correlation between an input and a reference image, including displaying the input and reference images on a spatial light modulator, and performing a joint transform correlation by shining collimated light onto the spatial light modulator, forming an intermediate image of the spatial light modulator through a lens, recording the intermediate image electronically as a plurality of pixels, binarising the intermediate image by thresholding each pixel using an average value of the surrounding pixels, displaying the binarised intermediate image on a spatial light modulator, shining collimated light onto the spatial light modulator, the aforesaid correlation image being the image through a lens of the intermediate image on the spatial light modulator. The intermediate image corresponds to the joint power spectrum of the reference and input images.
The method of binarising an image using the average value of the surrounding pixels is known, in crude edge detection methods, but has not previously been applied to joint transform correlation. The use of this method greatly enhances the correlation image by suppressing the zero order.
Preferably,
Cambridge Correlators Limited
Hamilton Brook Smith & Reynolds P.C.
Kassa Yosef
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