Fieure 1. A device for presenting a homogeneous field of light for light adaptation. A, reflector for illuminating
sides of diffusing surface (folded out from normal position); B, light box serving as light source; C, blower
for ventilating space between light source and diffusing surface; D, micro-switch actuated by forehead; KE, ap-
erture for nose and mouth; F, diffusing surface; G, microswitch actuated by chin; H, adjustable chin rest;
I, adjustable stand.


AM 66-38

A HOMOGENEOUS FIELD FOR LIGHT ADAPTATION

Henry W. Mertens

Approved by Released by
J. Roser Dinix, M.D. P. V. Srecen, M.D.
Curer, Civin Arromepican InstrruTe Feprerat Air Surcron

September 1966

FEDERAL AVIATION AGENCY
Office of Aviation Medicine



Qualified requestors may obtain Aviation Medical Reports from Defense Documentation
Center. The general public may purchase from Clearinghouse for Federal Scientific
and Technical Information, U.S. Dept. of Commerce, Springfield, Va. 22151


A HOMOGENEOUS FIELD FOR LIGHT ADAPTATION

Many experiments in the field of visual per-
ception require long sessions in the dark and
involve stimuli of small retinal size and. low
luminance. Since considerable dark adaptation
is likely to occur under these conditions, periodic
light adaptation may be useful in controlling the
adaptive state of the eye to prevent stray light
from introducing extraneous cues in the experi-
mental display. Also, since details of the light
adaptation field and observation position (illumi-
nated by the light of the adaptation field) pro-
vide stimuli which influence experimental judg-
ments in some cases, a homogeneous adaptation
field may be desirable The light adaptation
device described below and shown in Fig. 1 was
designed for use in presenting to S a homo-
geneous light adaptation field that can be easily
used between experimental observations even
though S is in an otherwise dark observation
position.

The adaptation field of the device is provided
by a transilluminated white plexiglass diffusing
surface (F). In the center of this surface is an
aperture (Z’) consisting of a tapered slot for the
nose and a round hole for the mouth which per-
mits the chin and forehead of S to rest directly
against the diffusing surface. This aperture is
closely fitted to the average S’s? nose and mouth
and, therefore, is not visible when his head is in-
serted properly. The diffusing surface also
curves around the sides of S’s head and extends
enough above and below the eyes so that when
8 is positioned properly, and the diffusing sur-
face is illuminated, he can see only a homo-
geneous field of light. The diffusing surface is

1W. C. Gogel. Size cue to visually perceived distance.
Psychol. Bull., 62: 217-235, 1965.

2 The dimensions of the aperture in the prototype were
determined from measurements of men on the labora-
tory staff. This aperture provided a good fit for over
80 male Ss who subsequently used the apparatus. The
only exception was one § whose nose had an extremely
receding bridge.

sloped away from the frontal plane at a 15 degree
angle to allow more comfortable positioning of
the face. The advantages of this technique are
that it hides S’s nose from view and places every
visible part of the adaptation field well within
the minimum. accommodative distance.

Proper positioning of S’s head so that only a
homogeneous field is seen is insured by two micro-
switches which protrude just above the top of
the nose slot (D) and the bottom of the mouth
aperture (G@). These micro-switches can be ad-
justed in and out for each § to insure that his
chin and forehead are pressed directly against
the diffusing surface before the adaptation light
is turned on. An adjustable chin rest (H) is
used to support his head at the proper height.
When the apparatus is properly fitted to 8,
movement of his head in any direction from the
proper viewing position will immediately open
at least one of the miscro-switches. If S takes
his head out of the viewing position while the
adaptation light is on, therefore, the light source
will be turned off before any source of extraneous
cues (detail of the apparatus or observation posi-
tion) can come into view. Any convenient light
source may be used as long as an even distribu-
tion of light across the diffusing surface is pro-
duced. Additional versatility can be gained by
mounting the device on a stand of adjustable
height and by adding a timer and dimming cir-
cuit to the light source. A mechanism to grad-
ually increase the intensity of the light source
to its maximum value may be desirable for S’s
comfort when light adaptation to a field of high
brightness is required. If a colored homogeneous
field is desired, colored sheets of acetate or any
other filter material can be inserted between the
light source and diffusing surface. The adapta-
tion device should be placed near the observation.
position so that S can conveniently insert his
head for viewing the homogeneous field with
minimum movement.