Kevin Fly Hill: Introduction to Dark Adaptation
© Kevin Fly Hill 1996-2008 - All Rights Reserved
We are privileged to have permission from Kevin Fly Hill of Rose Star Products, originator of ASTROGOGGLES, to reprint in our software programs "Redscope" and "Eyepiece" his technical paper on the process of dark adaptation. The Help file page for that topic is provided below, for your convenience:
PAGE 1 DARK ADAPTATION Information Help File / EYEPIECE: Version 2.00
Copyright (c) 1996 Stephen R. Waldee - All Rights Reserved
=== (A) INTRODUCTION TO DARK ADAPTATION ======================================
This version of the Waldee-Wood Telescope Parameter Program
is optimized for the advanced user who has access to a portable
color laptop computer for use in the field during observing
sessions, or for a color computer installed in an observatory
environment for use at the telescope.
With optional red text on a black background, this version of
the program will not cause a serious deterioration of the "visual
purple" pigment in the dark-adapted retina.
The following detailed information on the dark-adaptation
process was supplied by Kevin Fly Hill of Rose Star Products of
Tyler, Texas, the designer and distributor of a unique amateur
astronomer's viewing accessory, "Astrogoggles"(tm). These
special dark-adaptation goggles will dramatically enhance the
observer's ability to discern faint images in the telescope
eyepiece.
The following sections will provide the information listed:
PAGE
SECTION B: Describes the improvement in astronomical
viewing sensitivity provided by dark adaptation..1-2
SECTION C: Discusses the physiology of the eye and the
effect of light....................................2
SECTION D: Details the photo-chemical reactions that
occur during the process of sight..................2
SECTION E: Explains the technique of dark-adapting the
eye for astronomical viewing.......................3
SECTION F: Outlines the process of using averted vision
for most sensitive discernment of faint images.....4
SECTION G: Explains why red light is recommended for
reading charts or text while observing.............5
SECTION H: Describes the construction and use of
"Astrogoggles(tm)".................................5
SECTION I: Observing Hints for Well-Tuned Retinas...........5-6
SECTION J: for preserving dark-adapted vision; how to order
"Astrogoggles"(tm) from Rose Star Products....... 7
Written by Kevin Fly Hill, and reproduced with his permission
in this program:
=== (B) THE ROLE OF DARK-ADAPTATION IN IMPROVED VIEWING ======================
Improved viewing is the constant struggle of amateur
astronomers. From increased aperture to diligent training
(at least that is what I tell my spouse I'm doing) to monster
eyepieces, we seek out sharpness and more light. High on
everyone's list of viewing criteria is Dark-Adaptation.
PAGE 2
To demonstration this try turning on a white light at a
viewing party. It can cost you your dignity if not your life.
The rude comments about your parentage and origins demonstrate
the vehemence by which the amateur adheres to the right of
finely tuned retinas. Understanding the physiologic process
of dark adaptation and its protection is the key to
maximizing visual sensitivity to far flung photons.
=== (C) THE EYE AND LIGHT ====================================================
Light passes through the cornea and lens of the eye, and is
focused onto the retina at the back of the eye. The retina,
which covers the inside orb of the eyeball, is a collection of
photoreceptors, nerve cells, and pigment epithelium. The photo-
receptors are the points of light reaction and the beginnings
of the of the nerve impulse of sight. The retina contains two
types of photoreceptors: rods (100 million) and cones (6
million.)
When visible light -- the electromagnetic spectrum from
400 to 700 nanometers (nm) -- is absorbed by pigments of the
photoreceptor cells, a photon-chemical reaction initiates a
nerve signal. The signal is amplified and modulated in the
retina and is sent to the brain, where perception occurs.
To insure continued stimulation, the pigment of the photorecep-
tor cells must be constantly renewed. However, the chemical
reaction initiating the nervous impulse must stop simultaneously
so that the nervous impulse will stop after cessation of
the stimulus.
=== (D) PHOTO CHEMISTRY ======================================================
Human photoreceptors contain four lightabsorbing pigments
(opsins), each tightly bound to retinal, the aldehyde of
vitamin A1. Rhodopsin ("visual purple-obsolete") is the
photopigment of rods and has a maximum absorption at about
507 nm (green light) and works best in dim light. The cones
contain the other three photopigments and have a maximum
absorption at 440 nm (short wavelength-sensitive, blue),
535 nm (middle wavelength-sensitive, green), and 570 nm (long
wavelength-sensitive, red.)
Retinol (vitamin A) is utilized in the retina as retinal.
When the pigment absorbs light, the retinal molecule undergoes
a change to all-trans-retinal with a substantial change in
shape. This change of retinal is followed by a series of
chemical reactions ending in a split of the exposed photopigment
to yield free opsin and all-trans-retinal, which begins a nerve
impulse.
For additional photosensitive pigment (rhodopsin) to be
synthesized, the all-trans-retinal must be changed back to
retinal. This occurs either by exposure to light of the
PAGE 3
photoreceptors or by a sequence of chemical reactions to
light of the photoreceptors or by a sequences of chemical
reactions in the retinal pigment epithelium and its
photoreceptors. [ Chemical formulas of retinal and retinol
are included in the illustrated instruction sheet for
"Astrogoggles"(tm) - SRW ]
Of prime importance to visual astronomy is rhodopsin in
the rods, since cone pigments are not activated in dim light.
Notice the lack of color in low light conditions. This is
referred to as scotopic vision. One note of interest: since
rhodopsin is most sensitive to greenish light at 507 nm,
this might account for the apparent brightness of green/blue
planetary nebulae when compared to other objects of similar
magnitude. [Editor's note: see this article about the "Purkinje effect", explaining the shift of human eye color sensitivity to the blue spectral range at dim light levels. -- srw]
=== (E) ADAPTATION ===========================================================
The increase in sensitivity of the eye to detection of light
that occurs in the dark is called "dark-adaptation." The pupil
dilates, and both neural (largely unknown) and biochemical
changes in the retina occur. In darkness, after exposure to
bright light which bleaches the visual photopigments, there
is an initial hundredfold increase in sensitivity following
an exponential time course that reaches a plateau after 5 to 9
minutes. This initial phase is attributed to regeneration of
photo-sensitive pigments in the cones.
Thereafter, there is a 1000 to 100,000 times increase in
sensitivity following a slower exponential time course that
reaches a plateau in 30 to 40 minutes. This second phase is
attributed to regeneration of rhodopsin in the rods. In
addition to rhodopsin regeneration, neural changes increase
sensitivity further. Dark-adaptation is delayed by prolonged
exposure to bright light. Thus, it does take longer to
reach maximum adaptation at night after a day in bright
sunshine. When fully dark-adapted, the retina is about
100,000 times more sensitive to light than when light-adapted.
Exposure of the dark-adapted eye to bright light results
in a marked decrease in sensitivity involving two changes:
(1) a neural process that is completed in about 0.05 second,
and
(2) a slower process, apparently involving the uncoupling of
retinal and opsin in rhodopsin, occurring in about
1 minute.
The neural mechanism occurs regardless of the area of the
retina stimulated, whereas the photo-chemical mechanism
involves only the region of stimulation. In the light-
adapted eye the rhodopsin is bleached, and the pupil is
constricted.
PAGE 4
Further, under bright light exposure, pigment epithelium
cells which lay next to the rods tend to "grow around" the
rods. This may be a mechanism that protects the receptors,
since the "job" of these cells is to dampen out any
reflected light within the eye. This slow process may be
one factor in delayed dark-adaptation response following
extended bright light exposure.
=== (F) AVERTING VISION ======================================================
The dark-adapted retina is most sensitive in the region
2.5 mm around the fovea centralis. The fovea is the area
directly behind (across from) the pupil. When looking directly
at an object, the image is projected at the fovea. For
this reason looking slightly to one side of an object will
project the object on this most sensitive area. Furthermore,
the nasal (medial) periphery of the retina has the highest
concentration of rods.
Theoretically, looking to the left side of a dim object
with your right eye (opposite for the left eye) should give best
results from averting vision. The down-side to this specific
maneuver is that the "blind spot," where the optic nerve
exit the eyeball, is in this medial area. No rods or cones
are in this area and an object focused on this spot will not
be seen -- at all.
=== (G) PERCEIVING RED LIGHT =================================================
The difference in time between the point at which light is
detected by the eye and when color is discriminated is called
the photo-chromatic interval. Studies on this suggest that
the rods give only achromatic -- or colorless -- vision,
and that it is the cones that permit wavelength discrimination.
The photo-chromatic interval for long wavelengths (red light)
is about zero, which means that the intensity required to
detect light and to reach the sensation of color are about
the same. This is because the rods are insensitive to red
light.
Long wavelength cones are used in detecting red light,
and rods are left relatively unstimulated and ready to use
for observing faint objects.
This above discussion explains phenomena with which most
astronomers are familiar:
(1) Red light does little to degrade dark adaptation;
(2) White light will "zap" your night vision; and
(3) Averted vision enhances dim objects.
PAGE 5
=== (H) "ASTROGOGGLES" (tm) =================================================
The concept of "Astrogoggles" -- a light tight red lens
goggle -- was developed for astronomers to enhance and
preserve dark adaptation. By using the "Astrogoggles"
(featuring a red translucent sidewall), all stray white
light is blocked. Glasses may be worn under the goggles.
The red lens allows only long wavelength light to reach
the retina. Consequently, rhodopsin is preserved and
dark adaptation begins, even under external white light
conditions. This concept is not new and has been used
by radiologists and military professionals for years.
The "Astrogoggles" are used to start dark adaptation
early (at dusk) to become fully sensitive to dim objects
at the beginning of your viewing session. They are also
used to maintain night vision when white light must be
encountered. These conditions might include: approaching
auto headlights; opening car doors; Neanderthals with
uncovered flashlights; and returning to the house for
toilet breaks, forgotten charts, phone calls, favorite
beverages, and other real or imagined needs.
=== (I) HINTS FOR WELL-TUNED RETINAS ========================================
The maintenance of well-kept optics, the use of dark
viewing sites, and the protection of dark adaptation are all
within our control. This last requirement for effective
astronomical observing is enhanced by following these
precepts:
(1) Avoid or protect your eyes from bright sunlight on
the days prior to viewing. Dark, good quality sunglasses
are helpful.
(2) Shield your dark-adapted eyes from any "white" light.
(3) Use the lowest possible intensity of red light for
reading charts or finding your way around.
(4) Use shielded red lens glasses such as "Astrogoggles"(tm)
to start the dark adaptation process early and to prevent
rhodopsin bleaching.
(5) Use averted vision to maximize the viewing of dim
objects.
PAGE 6
Specific suggestions for the best use of "Astrogoggles"(tm)
include the following:
(1) NEVER LOOK AT THE SUN DIRECTLY, with or without your
"Astrogoggles." Although ultraviolet light transmission is
less than 1% through the red lens of the "Astrogoggles,"
infrared rays from the sun can still pass through to cause
damage to the eye, and may not be noticed until it is too
late! "Astrogoggles" are therefore NOT for solar observing!
(2) Of course, do not wear "Astrogoggles"(tm) while driving,
as traffic signals and signs will not be recognizable by
color; although if you are riding, you may wear them and
arrive at your viewing site dark-adapted!
(3) Put "Astrogoggles" on at sunset or just before. During
the intervening dusk you will become almost completely dark-
adapted by the arrival of the time of full astronomical darkness.
You CAN wear them all day, but I find the giggling at work
distracting.
(4) Under dim light conditions, added lighting may be
required: this may be white or red light (at star parties
the former can get you kicked.) Don't remove the goggles
or >> ZAP << there goes your dark adaptation!
(5) To note when to remove your goggles, note the time of
sunset and add 45 to 60 minutes. It will be difficult to
tell when full astronomical darkness has arrived unless
you remove your goggles (which again, messes up everything
if it is still too bright!)
(6) You MAY indeed wear conventional eyeglasses under
your "Astrogoggles," as the red translucent sidewalls will
fit comfortably over your spectacles, and air vents will
reduce any tendency of lens-fog.
(7) If perspiration does cause fogging of the lenses,
close your eyes and lift the goggles to let in air.
If heavy fogging persists, close your eyes, remove the
"Astrogoggles" and wipe their lens (and your eyeglass
lenses if in use) with a soft cloth.
(8) Carefully clean the polycarbonate red lens of the
"Astrogoggles" with a running water and a soft cloth.
However, slight scratches on the lens will not degrade
satisfactory use of the goggles.
by KEVIN FLY HILL
Rose Star Products
You may order "Astrogoggles" from:
Orion Telescopes & Binoculars at this link.
Note: we do not benefit in any way for providing this link.
• Phototransduction, Dark Adaptation, and Rhodopsin Regeneration
by Trevor D. Lamb and Edward N. Pugh, Jr: a highly technical paper presenting research on, among other things, the processes of dark adaptation, and the effects of disease and aging.
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Modified Monday 24 October 2005 at 2:13 pm; last edited Tuesday 15 January 2008 at 11:15 am. Copyright © 2006-2008 Stephen R. Waldee - All Rights Reserved. All Trademarks or Copyrights are © or Property of Their Respective Copyright Holders.