timer was opened by the "Start" circuit when the Photo-Stimulator was triggered. The gate was closed by the timer "Stop" circuit when the voltage from the derivative signal exceeded a pre-set level of the "Stop" circuit. The counter, in the meantime, counted the number of cycles that passed from the crystal oscillator through the gate at a 10,000 cycle per second rate.
The "Stop" level of the timer was pre-set so that a derivative signal of -0.30 volts did not stop the timer, while -0.35 volts did stop it. A voltage of -0.35 was equivalent to a pupil contraction velocity of 3.5 mm. per second (manufacturer's specifications).
One of the advantages of using the derivative pupil signal is that the same "Stop" level on the timer can be used for all subjects. Once the diameter of the pupil attains equilibrium (in darkness), its derivative becomes zero and all measurements are taken from this zero point, i.e., all measurements are independent of the initial absolute diameter of the pupil or of the iris color.
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| Figure 6. Presetting of "stop-clock" timing circuit. |
