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Optical mask boosts iris identification

25 Sep 2008

Iris recognition systems could become faster and more accurate thanks to a carefully designed phase mask, which increases the imaging depth of field.

An engineer in the US has increased the imaging volume of an iris recognition system in a bid to make the technology cheaper and easier to use. In the design, which will be prototyped next year, a phase mask is added to the image capturing region to make the system less dependent on eye position (Applied Optics 47 4684).

"The aim of this research is to make iris recognition less sensitive to the location of the eye relative to the position of the camera," Shane Barwick, an engineer at Rocky Mound Engineering, told optics.org. "Since an array of cameras may be required for very large fields of view in some applications, fewer cameras will be needed if each camera has a larger imaging volume. This means that system cost and complexity will be reduced."

Iris recognition is particularly attractive for security applications since the iris is an extremely reliable identifier of an individual. However, obtaining accurate images in a simple manner is the biggest technical hurdle standing in the way of the widespread deployment of these systems.

"Existing cameras use a diffraction-limited lens, which requires the eye of the subject to be located near the best-focus plane of the lens," explained Barwick. "As the eye moves away from this plane, the performance of the system drops off precipitously."

Barwick instead proposes adding a transparent phase mask that alters the phase of the light distribution at the lens pupil of the camera. The mask operates at a centre wavelength of 768 nm and is designed to capture all of the information necessary to identify the iris while simultaneously decreasing defocus sensitivity.

"The larger imaging volume makes it easier for the subject to place their eye at a location where the camera can capture a sufficiently good image for identification," commented Barwick. "An example is checking the identities of passengers at airports as they quickly walk through a gate."

Barwick first calculated the optimum phase function at the pupil. The phase mask was then carefully designed so as to force the lens to express this phase function at the pupil.

"A varying phase delay is realized at the pupil by spatially varying the thickness of the phase mask by an amount determined by the desired phase function and by the mask's index of refraction at the design wavelength," said Barwick.

The ultimate goal for Barwick is to optimize all of the components of the system including source, lens, phase mask, signal processing and production cost metrics. "High-order phase masks can push the envelope of current diamond-turning manufacturing of freeform optics," he concluded. "As is frequently the case, economics limits performance gains."

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