It is not necessary to record the EOG throughout this period, but testing should begin at least 5 minutes before the light phase to establish a baseline and insure its stability. If there is no clear light peak, then recording should continue for at least 20 minutes to insure that a delayed light peak is not missed. The choice of luminance levels for the stimulus will depend on whether the pupil is dilated. For practical purposes, however, it is difficult to measure pupillary diameter during the test, while the patient is looking into the diffusing sphere, and most commercial stimulus units do not have the capability of finely adjusting the luminance of the sphere. Thus, the Standard recommends separate fixed light intensities for dilated and undilated pupils, as the best compromise between an adjustable light level and the reality of clinical testing.
Overshoot occurs when subjects look beyond the fixation target and then return to a stable position.
Falloff occurs during AC recording, as the amplitude fades from its maximal value. Sharp overshoots can be ignored and the stable amplitude level used for measurement. If there is significant falloff from AC filtering of the signal, we recommend measurement of the leading edge, or at least the initial stable value. The light peak is determined as in the preceding section. The value of the light peak to dark baseline ratio will typically be lower than the Arden ratio.