http://www.skyandtelescope.com.au 37N
o artificial light is good when observing, but
sometimes under the stars we need it — to
help us read a star map, get around or find a
piece of lost gear. We have historically used red lights,
and recently we’ve gravitated to deep-red light emitting
diodes (LEDs). But given the range of current options,
is red really the best colour?
The rationale for red is straightforward. Our
nightvisionhasonly^1 / 1000 the sensitivity to red light
as to yellow, and red is the only colour to which our
daytime vision is (slightly) more sensitive than our
night vision (see graph below). So with red light,
we can use our daytime vision without ‘saturating’
our nighttime detectors, and this is why red light
preserves our dark adaptation.
But personally, I have trouble reading under red
light. So I looked into other options. What I found
were not only better insights into our vision but also
a surprising alternative to ruby-tinting our world —
one that’s actually better than the tried-and-(maybe)-
true red torch.Cones and rods
We have two sets of visual detectors in our retina,
called cones and rods. Cone cells work best in bright,
daylight conditions, enabling what’s called our photopic
vision. Rods, on the other hand, provide low-light,
scotopic vision for night. At twilight, both our rods and
cones operate (mesopic vision) and complement each
other by providing a mix of fairly good resolution and
faint-light sensitivity.
Our best resolution occurs with photopic vision,
which can resolve features about 1 arcminute across.
(That corresponds to 60 pairs of closely spaced,
alternating white and black lines per degree.) From a
reading distance of 45 cm, we need a resolution of less
than 10 arcminutes to resolve the structure of the F at
the beginning of this sentence.
In the mesopic range, we have only about half the
resolution of our photopic vision. And when using our
scotopic vision, we have only about a tenth.
The number and spacing of receptor cells
determine the resolution of the image produced in
our mind’s eye. (The same principle applies to the
chip’s pixels in a digital camera.) The cones in our
eyes provide good resolution, or visual acuity, because
they are closely packed in the centre of our view.
Although there are about 20 times more rods than
cones, our reading ability with the rods is marginal
at best. That’s because of the way these cells work
together. Their high sensitivity at night would
produce considerable ‘noise,’ like a poorly tuned
analog television, if they were not interconnectedto suppress false signals to the brain. Hundreds of
these cells combine to produce one signal, but this
interconnectedness reduces their effective resolution to
only about^1 / 10 that of cones. So, if our goal is to read a
star map, our rods alone are insufficient. We need our
hi-res cones as well for that task.Colour
Our cone cells are also the ones that detect colour.
We have three types of cones, each containing one
of three pigments that preferentially absorb different
wavelength ranges: L-cones, which detect the long
wavelengths of yellow to red light (centred on 560
nm); M-cones, which detect mid-range yellow-greenLED vs INCANDESCENT BULB
This article deals primarily with LEDs. An old red incandescent bulb
doesn’t have the same caveats as red LEDs: the bulb’s glowing filament
emits very little blue light, and the filter’s wide bandwidth takes better
advantage of your cones’ sensitivities than red LEDs do.RODS AND CONESRod vision and cone vision have different sensitivities to
different wavelengths of light. Throughout most of the visible spectrum, the rod
system is more sensitive to light than the three-cone system, by as much as a factor
of 1,000. But at wavelengths longer than 620 nm, including the deep red of red
LEDs, the sensitivities are about equal.S&T: LEAH TISCIONE, SOURCE: IDO PERLMAN / WEBVISION AND THE AUTHORRod CellsCone Cells
Relative sensitivityWavelength (nanometres)110100103104105106107400 500 600 700ALL GEOMETRIC BACKGROUNDS: BIGSTOCKPHOTOS.COM