Original by Philip Gibbs 5-November-1996
The human eye is very sensitive but can we see
a single photon? The answer is that the sensors in the retina
can respond to a single photon. However, neural
filters only allow a signal to pass to the brain to trigger
a conscious response when at least about five to nine
arrive within less than 100 ms. If we could consciously
see single photons we would experience too much visual
"noise" in very low light, so this filter is a necessary
adaptation, not a weakness.
Some people have said that single photons can
be seen and quote the fact that faint flashes from
radioactive materials (for example) can be seen.
This is an incorrect argument. Such flashes produce a large
number of photons. It is also not possible to determine
sensitivity from the ability of amateur astronomers to
see faint stars with the naked eye. They are limited
by background light before the true limits are reached.
To test visual sensitivity a more careful experiment
must be performed.
The human retina at the back of the eye has
two types of receptors known as cones and rods.
The cones are responsible for colour vision but
are much less sensitive to low light than the rods.
In bright light the cones are active and the iris
is stopped down. This is called photopic vision.
When we enter a dark room the eyes first adapt by
opening up the iris to allow more light in. Over
a period of about 30 minutes there are other
chemical adaptations which make the rods become
sensitive to light at about a 10,000th of the
level needed for the cones to work. After this
time we see much better in the dark but we have
very little colour vision. This is known as scotopic
vision.
The active substance in the rods is Rhodopsin, A single
photon can be absorbed by a single molecule which changes shape
and chemically triggers a signal which is transmitted to the
optic nerve. Vitamin A aldehyde also plays an essential
role as a light absorbing pigment. A symptom of vitamin
A deficiency is night blindness because of the failure
of scotopic vision.
It is possible to test our visual sensitivity by
using a very low level light source in a dark room.
The experiment was first done successfully by Hecht, Schlaer and Pirenne
in 1942. They concluded that the rods can respond to single
quanta during scotopic vision.
In their experiment they allowed human subjects to have
30 minutes to get used to the dark. They positioned a controlled light source
20 degrees to the left of the point on which the subjects eyes were
fixed so that the light would fall on the region of the retina
with the highest concentration of rods. The
light source was a disk which subtended an angle of 10 minutes
of arc and emitted a faint flash of 1 millisecond to avoid too
much spatial or temporal spreading of the light. The wavelength
used was about 510 nm (green light).
The subjects were asked to respond "yes" or "no" to say whether
or not they thought they had seen a flash. The light was gradually
reduced in intensity until the subjects could only guess the answer.
They found that about 90 quanta had to enter the eye for
a 60% success rate in responding. Since only about 10% of photons
which arrive at the eye actually reach the retina this means
that about 9 photons were actually required at the receptors.
Since the photons would have been spread over about 350 rods
they were able to conclude statistically that the rods must
be responding to single photons even if the subjects were not
able to see them when they arrived too infrequently.
In 1979 Baylor, Lamb and Yau were able to use rods from toads
placed into electrodes to show directly that they respond to
single photons.
Julie Schnapf, "How Photoreceptors Respond to Light",
Scientific American, April 1987
S. Hecht, S. Schlaer and M.H. Pirenne, "Energy, Quanta and
vision." Journal of the Optical Society of America, 38, 196-208 (1942)
D.A. Baylor, T.D. Lamb, K.W. Yau, "Response of retinal rods to
single photons." Journal of Physiology, Lond.