A (6)

http://www.skyandtelescope.com.au 39

Scotopic

vision

(rod cells)

Photopic

vision

(cone cells)

Red

LEDs

Relative sensitivity

Wavelength (nanometres)

400 500 600 700

Amber

LEDs

takes me less than a minute to physically move from
a star map to the telescope and orientate myself at the
eyepiece. If I want my vision to adapt that quickly, how
low must I go? Published experiments don’t go faint
enough for astronomy, so the easiest way to find out
how much light we can get away with is to try it!
My own experiments show that my rods recover
almost immediately even after a few seconds of
exposure to the full Moon’s illumination on a white
sheet of paper (0.1 lux, where a lux is the unit of
surface brightness, defined as 1 lumen coming from
1 square metre of surface). Our threshold for reading
with broadband light is 10 times higher than with
moonlight, about 1 lux, and my night vision recovered
from that exposure within a few seconds. At this level,
we can also see some colour as well, indicating that
our cones are working.
Above about 3 lux, the rods take noticeably longer
to adapt. This is roughly the threshold above which the
rod cells become bleached.
So, when artificial light is necessary, the best choice
is relatively faint light: about 1 lux, and not above 3 lux.
Incidentally, this range also falls in our mesopic vision
(see facing page).
LEDs’ pure red light (about 650 nm) is detected by
only our L-cones, and it provides poor resolution for
reading or seeing hazards (such as a tripod’s legs). So
we not only want illumination at about 1 lux to save
our rods but also light that covers a wider wavelength
range, between about 500 and 700 nm, to take
advantage of our M-cones as well as our L-cones.
What does this light look like? Between 1 and 3
lux, my impression of the colour is pale ‘amber’ or
‘candlelight’. Aesthetically, it is quite pleasing.

Stamp out blue
This new work goes against what has become common
sense for dark-site observing. Nevertheless, we have
adopted this amber spectrum and 1–3 lux illumination
level for Canadian Dark-Sky Preserves.
Although the use of amber light can help us see
while walking about a campground, it can also help

in cities. We distinguish colours twice as well when

they are illuminated by a smooth amber spectrum

than when illuminated under the golden light of

high-pressure sodium lamps. Those lamps don’t have

a continuous spectrum; their glow is mostly made up

of narrow emission wavelengths specific to sodium,

which only let us see colours that match up with these

spectral features.

Worse, the blue component of ‘white light’ city

lamps provides very little information for our visual

acuity, yet it cripples our night vision, making it

difficult to see into more shadowed areas. So using

amber light, at rational illumination levels, actually

improves our ability to see at night. The International

Dark-Sky Association is on a campaign to educate

lighting installers on this fact.

Where can you get amber LEDs? Most large LED

manufacturers make them. Close variations are

available from Lumiled (Philips), CREE and Nichia.

Before you buy, check the LED’s spectrum and ensure

it does not emit light at wavelengths shorter than

500 nm. The peak wavelength should be about 590 nm,

which is at the boundary between orange and yellow.

It must be emphasised, of course, that any artificial

light will compromise our night vision. Observers

pushing their visual limits would not use any artificial

light! But amber gives us more visibility with less light.

So, when you have to see in the dark, choose amber. ✦

Robert Dick is a professional engineer and part-time

astronomy professor. His interest in the bio-impact of light

began in the late 1990s, when he developed the Canadian

Dark-Sky Preserve Program for the Royal Astronomical

Society of Canada. This culminated in the creation of the

Canadian Scotobiology Group, Inc. For this and other

contributions, he was named Fellow of the RASC in 2015.

AMBER vs RED

LEDS Our rods

and cones have

different peak

sensitivities: rods

in the green,

cones in the

yellow. These

are compared in

the graph above

with the spectra

of amber and

red LEDs. Red

LEDs trigger only

our L-cones, so

in order to be

bright enough

for us to read by

their light, red

LEDs must also

be bright enough

to bleach rod

cells. Conversely,

amber light excites

both the M- and

L-cones. Using

it, we can see

and read well at a

lower illumination

level that will

permit rods to

quickly recover.

S&T

: LEAH TISCIONE, SOURCE: THE AUTHOR

COLOUR

TEMPERATURE

Another way to select LEDs is by their

colour temperature. Colour temperature

is the approximate temperature of a

solid, hot surface that ‘looks’ like the

desired colour. The colour temperature

for amber is about 1,900–2,100 kelvin.