A previously developed apparatus that tracks the orientation of the human eye has been modified to make it less susceptible to error induced by ambient infrared light. The apparatus is a commercial product intended primarily for use as a computer-control interface for a person who is physically unable to use a keyboard.
The unmodified apparatus includes a near-infrared source that illuminates the eye, plus an infrared video camera that monitors the eye. The source and camera optics are combined so that the infrared illumination strikes the eye along the optical axis of the video camera. The total video image comprises a bright image of the pupil plus a very bright specular reflection from the cornea. The total video image is processed to obtain a vector between the center of the pupil and the corneal reflection. This vector constitutes the desired information on the orientation of the eye (the gaze direction).
The unmodified apparatus incorporates several features to minimize the effect of ambient infrared light: The infrared source is a narrow-band light-emitting diode (LED) that operates at high power. A band-pass optical filter is placed in front of the video camera to block as much ambient infrared light as possible while passing the infrared light from the LED. Despite these features, infrared light from the Sun, incandescent lamps, and other sources can decrease the signal-to-noise ratio of the pupil and corneal-reflection images so much as to introduce errors or even cause the apparatus to lose track of the eye. Thus, to ensure reliable operation, the unmodified apparatus must be used indoors, with shades drawn to reduce sunlight, and with no incandescent lamps lit (fluorescent lamps are acceptable).
Accordingly, modifications were made to increase the signal-to-noise ratio in the presence of ambient infrared light. The modifications were (1) replacement of the LED by a different LED of narrower spectral width and of greater power within eye-safe limits and (2) replacement of the band-pass optical filter with another one that is better matched to the new infrared LED. The new infrared LED operates at a power of 32 mW in a wavelength band 40 nm wide at a nominal wavelength of 880 nm. The new optical band-pass filter has a wavelength pass band only 10 nm wide. Although the filter wavelength pass band is only 1/4 as wide as the emission wavelength band of the LED, computer simulations nevertheless showed that a greater signal-to-noise ratio could be achieved with this filter than with a filter of 40-nm bandwidth.
The modifications have increased the signal-to-noise ratio by a factor >20. The modified apparatus operates under ordinary indoor lighting, without need to turn off incandescent lamps.
This work was done by John Morookian and James Lambert of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tspunder the Electronics & Computers category. NPO-20398