This report describes a second-generation photostimulator with four
primary lights that allows independent control of the stimulation of
the four receptor types in the human eye. The new design uses LEDs
(with light levels controlled by eight drivers that include
voltage-to-frequency converters that provide 1-μs pulses at
frequencies up to 250 kHz), with four center channels being combined by
use of a fiber optic assembly, and likewise for four surround channels.
Four fiber optic bundles are merged into a single bundle whose output
is fed into a spatial homogenizer terminated by a diffuser. An
interference filter is sandwiched between each LED and the fiber optic
bundle. Two camera lenses collimate light from the diffusers, one for
center and one for surround. The center-surround field configuration is
formed by a photometric cube with a mirrored ellipse on the hypotenuse.
A field lens places images of the diffusers in the plane of an
artificial pupil. The fields are highly uniform. Following alignment
and calibration, the center and surround fields are indistinguishable.
An observer calibration procedure, designed to compensate for
prereceptoral filtering, is shown by calculation to correct also for
normal observer receptoral spectral sensitivity variation. With the
instrument calibrated for the individual observer, a peripherally
fixated 200-ms 40% contrast rod center field pulse, highly conspicuous
under dark adaptation, is invisible following light adaptation.