Precise stimulus for the measurement of visual flicker sensitivity

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Abstract
The stimulus described uses a high‐output, red, light‐emitting diode (LED) as a light source. The luminance of the LED is kept constant by solid state cooling under feedback control. A specially developed 16 °, soft‐edge field target is presented to the observer through Maxwellian view optics with a 1.6 mm artificial pupil. An adjustable, three‐point head restraint holds the observer fixed relative to the stimulus, which itself can be aligned with the observer in three dimensions. These adjustments are resettable to within ±0.5 mm for repeat sessions on one observer. A two‐alternative forced choice paradigm is used in conjunction with a modified staircase method of limits under on‐line computer control. The stimulus is presently being used in a clinical research facility in the investigation of psychoactive drugs effects on flicker sensitivity.