A microchip processing apparatus processes a microchip with at least one main separation channel. The eye in which the blood flow is monitored is subjected to a small pressure, sufficient to equalize the pulsatility index measurements of the internal and external portions of the eye artery. The optical modulator includes a number of surfaces with dielectric mirrors formed thereon to modulate the beam generated by the laser. The pupil position is tracked based on intensity changes of the reflected light beam represented by the intensity signal.