Sky & Telescope - July 2019 - 36

Screen brightness (lux)

Therefore, seeing intricate marklengths. Cones, on the other hand,
14h
13h
12h
ings on the Moon or planets, or
come in three types in the human
a Cru
separating close individual stars, is
eye: blue, green, and red, correspond-55°
only possible with cone receptors.
ing to the colors they detect. The vast
Rod receptors' role in visual astronmajority of the cones are red or green,
` Cen
CRUX
omy is basically limited to detecting
and these do most of the imaging.
-60°
_ Cru
the presence or absence of a faint star
There are just enough blue cones to
or nebulosity that is not visible in
allow the brain to interpret whether
central vision. This is why observers
an object is blue.
p HOW MANY STARS? After light exposure,
see nebulosity best with their averted
Rods and cones react to color difparticipants counted the number of stars they could
vision, which places the image on the
ferently. Rods are around 1,000 times see with direct vision in the triangle formed by Beta
Centauri and Alpha and Gamma Crucis.
region of maximal rod density, about
more sensitive to green light than
20° out from the visual axis. Rods'
cones are, but rods and cones are
exquisite sensitivity to green may, however, subtly contribute
equally sensitive to red light.
Cones and rods also perform different roles. Cones provide to observers' color perception.
color vision in bright light, known as photopic vision, but
are unable to detect very low levels of light. Rods provide
Dark Adaptation
monochromatic vision in very dim light, known as scotopic
An eye that is fully dark-adapted can be up to 500,000 times
vision, but bright light quickly saturates them. Intermediate
more sensitive than an eye that is fully light-adapted. Pupil
light levels where both cones and rods operate are known as
dilation only plays a minor role in dark adaptation, allowing
mesopic vision.
a rapid increase in sensitivity by a factor of 30. Most of the
One signiﬁcant function of a telescope in visual astrondark adaptation occurs within the rods and cones themomy is to gather enough light for the eye to operate in the
selves. As rods and cones receive photons and create neural
mesopic range, where both rod and cone receptors work.
impulses, they are depleted of the photochemicals that create
Many observers incorrectly think that visual astronomy relies
these impulses. Because these molecules take time to regenersolely on scotopic vision, but rods only provide low-resoluate, the process is a self-regulating sensitivity mechanism tied
tion, monochromatic vision in the peripheral visual ﬁelds.
to light exposure. During periods of darkness, the photoCentral vision falls on the part of the retina called the fovea,
chemicals fully regenerate, and sensitivity increases.
which is exclusively populated by cones packed very tightly
Cones take 15 minutes to fully dark-adapt, while rods take
together. This high density of cones only occurs in the fovea,
at least 30 to 40 minutes. To be able to separate extremely faint
which is why only the fovea can perceive ﬁne detail. The acuobjects from the dark background, rod receptors are interconity of off-axis vision, also known as averted vision, decreases
nected with many other rod receptors, producing a combined
by as much as 50% or more per degree. By 10° off axis,
signal. This acts as a background-noise reduction mechanism
observers' visual acuity is less than 20% that of their central
but leads to further loss of visual acuity in the periphery.
vision. Most of the detailed peripheral imagery we perceive is
When using lights during an observing session, our goal is
actually concocted by the brain.
to ﬁnd the right combination of intensity and color to enable
us to read maps and navigate around our equipment with the
least effect on our dark adaptation. For most people, the red
Brightness Required to Read Chart
cone receptors compose the majority of our cones. As observ1.40
ers depend on cone function to see detail, the use of red light
1.20
may confer no advantage, because we're undermining the dark
adaptation of the very cones we need. Conversely, given how
1.00
sensitive rods are to blue-green light, this color should be the
worst color for protecting rod receptor vision.
0.80
Dick hypothesized that orange (speciﬁcally, "amber") light
0.60
might be optimal, because it stimulates both red and green
cones and therefore may not need to be as bright as red light.
0.40
Orange light also avoids overstimulating the rods as occurs
with blue-green light.
0.20
However, it's worth noting that, while the majority of
0.00
people
have around 70% red cone receptors, researchers have
Red
Orange
Green
White
recently discovered that in people with normal color vision
Screen color
this fraction may vary from as little as 27% to as high as
90%. This would suggest that the optimal color for protecting
p RESULTS: READING Participants required different screen brightdark adaptation might also vary from person to person.
nesses in order to read the test chart, depending on the light color used.
36

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Sky & Telescope - July 2019

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