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 twoalternative 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.