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38 AUSTRALIAN SKY & TELESCOPE JULY 2016

(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

daytime. These cells essentially tell our brain where

to put a ‘wash’ of blue in our mental image, and when

combined with the responses of the M- and L-cones,

we get the impression of ‘white’ light.

The average observer has roughly twice as many

L-cones as M-cones, so most of us are more sensitive

to longer wavelengths. However, the ratio of L- and

M-cones varies across the population, and some

people have more M-cones than L-cones — making

these folks even more sensitive to yellow light than

to red. I must be one of them. (This is a different

phenomenon from colour blindness.)

Unlike our cone cells, our rods are honed to see

during the fading light of twilight and nighttime. Rods

are about a thousand times more sensitive to blue-

green light as to red. They only have one type of light-

sensitive pigment, so they can’t tell which wavelength

is stimulating them, hence they essentially see in black

and white. This is why faint stars on a dark night, and

faint deep sky objects through the eyepiece, all have

a neutral silvery grey colour: we’re using only our

scotopic vision.

Sensitivity

As astronomers, we are pushing the envelope

of vision. Unlike our ancestors (or most of our

contemporaries), we need both the light sensitivity of

our rods and the resolution of our cones at the same

time. Although we’ve long turned to red light as the

solution, using deep-red light alone exacerbates our

loss of visual acuity, because this colour has a narrow

bandwidth (only 20 nanometres for red LEDs) and

triggers only our L-cones. This forces us to use brighter

red light in order to see.

But because dark-adapted rods have nearly the same

sensitivity to red light as our L-cones, red light that

is bright enough to read by is sufficient to excite our

rod cells somewhat. Prolonged exposure to it will still

affect our night vision.

If we could take advantage of our M-cones as well

by using broader-band light, could we use just enough

light for hi-res cone vision while actually leaving our

night vision in better shape?

In order to find this balance, we need to know

how much light is enough to read by but not too

much to saturate our rods. The amount of light our

night vision can tolerate depends on how quickly

our rods recover from exposure to it. That recovery

time depends in turn on both the brightness of the

illumination and how long we’re exposed to it. The

dimmer and briefer it is, the faster our rods will

return to the dark-night threshold.

The question is, how fast do we want to adapt? It

LUMINANCE
AND VISION
The diagram
above shows one
divvying up of
how the visual
system operates
in different
illumination
settings. The
lowest light levels
activate only rod
cells. Cones begin
to contribute at
about the level
of starlight, but
practical colour
vision doesn't kick
in until around the
level of moonlight.
Cones are the
only receptor
cells active under
the brightest
conditions; rods
bleach out.

Self experiment

Go outside at night, let your eyes adapt, and illuminate the ground with

your white torch covered with a good yellow or amber filter. Then, look up to

the clear sky. How long does it take for your eyes to recover so that you can

distinguish faint stars? If it takes more than a few seconds, your light is too

bright. Try the same thing with a red LED and see how fast your rods readapt.

Scotopic Mesopic Photopic 50% bleach

Rod saturation Best acuity

begins

Cone threshold

10 -6 10 -4 10 -2 0 10^2                         104                           106108

Luminance (candela/m^2 )

Good colour vision

No colour vision

Poor acuity

Visual function

Absolute

threshold

Damage

possible

S&T:

PATRICIA GILLIS-COPPOLA / SOURCE: D. PURVES ET AL. /

NEUROSCIENCE

,

© 2001 SINAUER ASSOC.

Luminance of white paper in.......... .Starlight ..... Moonlight .............Indoor Lighting ............Sunlight

Night Vision