Thomas A. Isberg

Rotation-invariant image recognition at low light levels

Rotation-invariant filtering at low light levels is investigated. Low-light-level input scenes are cross correlated with a complex circular-harmonic component of a reference image stored in computer memory. Approximate expressions are given for the probability-density function of the correlation signal and for the squared modulus of the correlation signal when the number of detected photoevents is Poisson distributed as well as when the number of detected photoevents is fixed. Experimental measurements of the squared modulus of the correlation signal, obtained by using a two-dimensional, photon-counting detector and position-computing electronics, are found to be invariant with respect to rotation of the input image. In addition, when the reference object is input, its orientation can be determined from the real and imaginary parts of the correlation signal. Good discrimination between the reference image and other test images is observed, and the experimental results are in excellent agreement with theoretical predictions.

Image correlation at low light levels

Low-light-level input images are cross correlated with a classical intensity reference image that is stored in computer memory. Experimental measurements of the correlation signal, obtained using a two-dimensional, photon-counting detector and position-computing electronics, are reported. The experimental results are in good agreement with theoretical predictions.

Application of diffractive optics to laser scan lenses

The control of optical distortion is useful for the design of a variety of optical systems including those used for laser scanning. A lens used for focusing a scanned laser beam onto a flat image field with constant intensity profile must also satisfy the f-(theta) condition, i.e., the image height is proportional to the input field angle itself, so that the scan velocity across the image plane remains constant. The lens needs to be free from coma, astigmatism, and field curvature and must have a prescribed amount of distortion. We describe the design and development of a diffractive f-(theta) lens and present experimental verification of the theoretical predictions.