A Low-Cost Laser Pupillometer

An electronic pupillometer especially suitable for experiments with anesthetized (paralyzed) cats is described. This pupillometer uses a low-power laser as a light source. The narrow laser beam is projected on a mirror galvanometer which reflects the beam with a horizontal sweep across the iris 50 times per second. The reflected light is detected by light sensitive diodes. The diode output is low when the spot travels across the pupil and high when the spot is directed at the iris. The duration of this low voltage period is measured and transformed to a voltage. Repetition of this conversion 50 times per second produces a continuous representation of the pupil diameter. This relatively lowcost pupillometer is easy to build and occupies only little space.