There has been some discussion
in various newsgroups (aviation, rec, astro) regarding what color
is best for night vision. In particular, the assertion has
been made that the Air Force is switching to blue-green, and that
red is passe. Here is what the applicable military
specification has to say.
---------------
MIL-1472D General Requirements
5.2.1.2 Display
illumination and light distribution. 5.2.1.2.1 Display illumination.
5.2.1.2.1.1 Normal. When maximum
dark adaptation is not required, low brightness white light (preferably
integral and adjustable as appropriate) shall be used; however, when complete dark adaptation is required,
low luminance [0.07 - 0.35 cd/m(2) (0.02 - 0.01 fL)] red light
(greater than 620 nm) shall be provided.
| Condition of UseLighting Technique
(*)Brightness of Markings
cd/m2 (Ft-L) |
Brightness Adjustment |
| Indicator reading,
dark adaptation necessaryRed flood, indirect or both, with
operator choice0.07-0.35 (0.02-0.1) |
Continuous throught
range |
| Panel monitoring, dark
adaptation necessaryRed edge lighting, red or white flood or both, with operator
choice0.07-3.5 (0.02-1.0) |
Continuous throughout
range |
| Chart reading, dark
adaptation necessaryRed or white flood with operator choice0.35-3.5 (0.1-1.0) |
Continuous throughout
range |
(*) Where detection of
ground vehicles or other protected assets by image intensifier
night vision devices must be minimized, blue-green light (incandescent
filament through a filter which passes only wavelengths shorter
than 600 mm) should be used in lieu of red light.
---------------
The answer is red is best for night vision. Use of blue-green is to
protect night vision goggles, not unaided night vision! Most red filtered incandescent bulbs emit red and
a lot of infra-red and therefore are easily visible in
night vision goggles and would overload night vision goggles in
the cockpit. Night vision goggles are not very sensitive
in the blue-green part of the spectrum because they use blue-green cutoff filters. That's why blue-green is used in the cockpit. The military has split the spectrum
giving blue-green to the unaided vision and yellow through infra-red to night vision goggles. MIL-STD
1472 makes another point -- adjustability -- one
should be able to adjust the brightness to match
the need.Mr. Brian Skiff (We are
crediting Mr. Skiff for the information, we in no way imply
that Mr. Skiff endorses our products) posted some additional background
to sci.astro.amateur in April 1997;
---------------
"... it's
been well-tested under laboratory conditions. See the following
papers, all of which were published 40 years ago in the "Journal
of the Optical Society of America":Hulburt, 1951, "Time
of Dark Adaptation After Stimulation by Various Brightnesses and
Colors, " Journ. Optical Soc. America (JOSA), 41, 402.
Smith et al., 1955, "Effects
of Exposure to Various Red Lights upon Dark Adaptation Measured
by the Method of Constant Stimuli," JOSA 45, 5.
Kinney 1955, "Sensitivity
of the Eye to Spectral Radiation at Scotopic and Mesopic intensity
Levels," JOSA, 45, 507.
Sweeney et al. 1960, "Season
Changes in Scotopic Sensitivity," JOSA 50, 237.The first two papers
show quite clearly that the redder the better for getting
re-dark adapted after exposure to light. The bottom line
of these papers is that red lights are best, but if you can see
that itís red on the paper you look at, then it's too bright.
You see people at star parties with blazing bright flashlights
and light boxes for reading charts, thinking that since they're
red itís okay. But if the red light you're using
shows any more colorfully than funny brown-grey, then it's too
bright."
---------------
And this last point is
a key point -- regardless of color, if its bright enough to dazzle
your eyes, it will affect your night vision -- hence adjustability as per MIL-STD 1472. So
for most of us, the answer is red and adjustable
brightness for preserving night vision.Preserving night vision
is distinct from visual acquity. If all one is doing
is picking up an eyepiece, checking some equipment, walking around,
glancing at a control, any task that doesnít require high
visual acquity, then adjustable
brightness red light is best for preserving night vision.If one wishes to read
fine details that requires visual acquity, that requires using
oneís foveas which are packed with cones (color sensitive
retinal cells),. Then adjustable
brightness white is a is a perfect choice for sharper vision and shows colored
markings on charts correctly.
For civilian
users:
Starlite with 2 red LEDs is the
perfect flashlight for astronomy.
Skylite switchable between 2 red and 2 white LEDs is the perfect for civil aviation.
Moonlite with 2 white LEDS is
the perfect a general purpose outdoors flashlight.
For military
night vision goggle compatibility
Mil-Skylite switchable between 2 green and 2 white LEDs
Mil-Infralite switchable
between 2 green and 2 infra-red LEDs.
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