Sky and Telescope - June 2016 - 23

LE D VS . IN C AN D E S C E N T B U LB
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.

Color

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 nighttime. During 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 18 inches, 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 center of our view.
Although there are about 20 times more rods than
cones, our reading ability with our 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 interconnected to 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.

Our cone cells are also the ones that detect color. We
have three types of cones, each containing one of
three pigments that preferentially absorb three diﬀerent wavelength ranges: L-cones, which detect the long
wavelengths of yellow to red light (centered on 560 nm);
M-cones, which detect mid-range yellow-green (530
nm); and S-cones, which detect short-wavelength blue
(about 420 nm).
Although the three types of cones seem to have the
same sensitivity to light, their impact on vision scales
with how many of them we have. Only 6% are S-cones,
meaning we don't see blue-lit objects very well in the
10 7

Rod Cells

10 6
105
10 4
103

Cone Cells
100

Relative sensitivity

S&T: LEAH TISCIONE, SOURCE: IDO PERLMAN / WEBVISION AND THE AUTHOR

Cones and Rods

ALL GEOMETRIC BACKGROUNDS: BIGSTOCKPHOTOS.COM

NO 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 color?
The rationale for red is straightforward. Our night
vision has only 1/1000 the sensitivity to red light as to yellow, and red is the only color 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 was 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 ﬂashlight.

10
1
400

500

600

700

Wavelength (nanometers)
RODS AND CONES Rod 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.

Sk yandTelescope.com June 2016

http://www.BIGSTOCKPHOTOS.COM http://www.SkyandTelescope.com

Sky and Telescope - June 2016

Table of Contents for the Digital Edition of Sky and Telescope - June 2016