DEVELOPMENT OF A NOVEL BLUE FIELD ENTOPSCOPE TO DETERMINE THE LEUCOCYTE CHARACTERISTICS AND THE SIZE OF FOVEAL AVASCULAR ZONE

Vo Van Toi and Jianguo Sun, Tufts University, Electrical Engineering and Computer Science Department, Biomedical Engineering Lab, Medford, MA 02155.

Purpose. To provide a more reliable and accurate psychophysical technique for measuring the speed and number of leucocytes elicited by the blue field phenomenon and the size of the foveal avascular zone elicited by the Purkinje phenomenon, we have developed a new blue field entopscope using a state-of-the-art real-time LCD Spatial Light Modulator (SLM).

Methods. The new device has a 430 nm blue light source backlighting an SLM which is seen by a Maxwellian view eyepiece. An IBM-PC monitors the SLM and displays on it either the look-alike-leucocyte particles or a circle whose characteristics can be adjusted. To determine the speed and number of leucocytes, the particles are displayed. An observer looking through the eyepiece perceives simultaneously both these simulated particles and his/her own elicited leucocytes. By adjusting the particle's speed and number the observer can easily equalize them to those of the perceived leucocytes. Further, a rotating polarizer filter can be placed in front of the eyepiece to modulate sinusoidally the light and this flicker stimulation elicits changes of leucocyte's characteristics. By measuring these changes for various flicker frequencies one can establish the frequency response of these characteristics. To determine the size of the avascular zone a rotating disk having an 1mm diameter eccentric pinhole is used instead of the polarizer. In looking through the eyepiece, the observer perceives part of his/her own retinal capillaries including the foveal avascular zone. A circle is now displayed on the SLM instead of the aforementioned particles. By adjusting the size of the circle the observer can match it with the avascular zone and directly measure the size of this zone.

Results: An SLM with high contrast (200:1), resolution (640X480 pixels) and speed (40ms) was used. Graphic software was developed to produce either the particles or the circle, display them on the SLM and computer monitor, govern the user interface experimental protocol, and store and analyze the results. Special algorithms and advanced programming techniques were developed to make the simulation fast enough to operate even on a 286 series (16 MHz) PC. The device was built and tested on twenty normal vision subjects. The experimental results were reliable and agreed with physiological data. We attribute the reliability to the fact that the new concept of our device allows the simultaneous perception of both particles and leucocytes.

Conclusion: The development of this tool opens doors for new psychophysical retinal vascular studies.