Implementation Issues

Figure 3 shows the pan-tilt camera with the illuminators used in the eye tracking system. The pan-tilt camera is a Sony EVI D30, and the illuminators are constituted by two sets of light sources. For convenience, near infra-red (IR) light with wavelength 875nm is used, which is invisible to the human eye. The IR illumination also makes the system insensitive to changes in indoor ambient illumination, i.e., the room lights can be turned on/off without affecting the operation of the system. We slightly modified the original camera optics to adjust it to operate in near IR (by removing its IR block filter) and introducing a pair of extra lenses to increase its optical magnification.
 
 

Figure 3: Camera and IR illumination setting.

 

The light sources LIGHT1 and LIGHT2 in Figure 3 are composed of sets of 7 IR LEDs each. LIGHT2 is composed of two sets of LEDs, symmetrically placed on the left and right sides of the optical axis. Symmetry around the optical axis is desired because it reduces shadow artifacts by producing more uniform illumination, but asymmetrical configurations also perform adequately. LIGHT1 is placed near the camera's optical axis, so it generates the bright pupil image (Figure 2a) when it is on, and LIGHT2 is placed off-axis to generate a dark pupil image (Figure 2b), adjusted for similar brightness in the rest of the scene.

The video signal from the camera is composed of interlaced frames, where one frame can be decomposed into an even field and an odd field. Thus, a field has half the vertical resolution of a frame. Let F_t be an image frame taken at time instant t, with resolution c columns (width) by r rows (height), or $c \times r$. F_t can be de-interlaced into E_t and O_t, where E_t is the even field composed by the even rows of F_t and O_t is the odd field composed by the odd rows of F_t.

When LIGHT1 is synchronized with the even fields and LIGHT2 with the odd fields, i.e., each illuminator stays on for just half the frame period, one interlaced frame will contain both bright and dark pupil images. Figure 4 shows a block diagram of the pupil detection process. Once an interlaced frame F_t is captured, it is de-interlaced and the odd field O_t is subtracted from the even field E_t (dark from the bright pupil images). Thresholding of the difference image then creates a binary image, which is the input of a connected component labeling algorithm. Each connected component (blob) is checked for particular geometric properties, such as size and aspect ratio, and those that satisfy these constraints, are output as pupils. Observe that it is also possible to detect pupils using (O_t, E_t-1), i.e., between frames, thus increasing the detection rate to 60 fields per second.
 
 

Figure 4: Pupil detection block diagram.

 

The only piece of hardware build for the system was a very simple device to keep the even and odd fields synchronized with LIGHT1 and LIGHT2 respectively. Figure 5 shows a block diagram of the light synchronization device. The video signal from the camera is received by a video decoder module that separates the even and odd field signals. The video decoder is a National LM1881 chip, that is mounted on the same board that supports the IR LEDs (see Figure 4). The signal is fed to amplifying buffers that provide power for the IR LEDs.
 
 

 

Figure 5: Synchronization device block diagram.

1999-07-13