Flash stimuli are delivered by a xenon flash unit, while steady background illumination can be delivered by an incandescent source. An IR-sensitive TV camera with zoom lens provides an image of the subject's eye, allowing ready measurement of pupil size, and permitting visualization of the corneal positioning of the DTL electrode. Provided that the pupil size is measured carefully at the time of delivery of flashes, it is not necessary for the subject's pupil to be dilated. Side view of the apparatus. The TV camera at the left of the sphere is used to monitor the eye of the observer at the right, who is positioned at the monocular viewing port.
Infrared illumination is provided by an LED beneath the lens.
The xenon flash unit is mounted near the upper pole, and the incandescent source is located just below this. The sphere is hinged, and may be opened for maintenance. The effective intensity of the xenon flash can be varied over a 10 000-fold range, simply by controlling the flash duration, using electronic inputs to the flash unit to trigger and then quench the arc. Control of the flash intensity in this way alleviates the need to change neutral density filters. Cone-isolated responses are obtained using the red filter, in the presence of a blue background that saturates the rods.