We are visual creatures, and our eyes are marvelous at detecting light.
For us to see, light enters our eyes, passes through the lens, then reaches the retina.
The retina contains cells called photoreceptors that can detect light and other cells transmit
this information to the brain. Though we might not realize it, when it
comes to seeing in the dark, our vision approaches or even reaches the fundamental physical limits of
detection. In this video, let’s learn about how our eyes work, to answer the question:
how well can we see in the dark and is our night vision really
that great? [Intro music]
Photoreceptors are located at the back of the retina and consist of two types of cells: cones
and rods, so named because of their shape. Cones allow us to see in bright light and in
color, as there are 3 types which are sensitive to red, green, and blue. In dimmer light, we rely
on the rods, as they are much more sensitive to light. In this video, we’ll be focusing on rod
cells, as we rely on them for our night vision. Light is composed of discrete packets called
photons. In bright light, we’re bombarded by a deluge of photons, like raindrops falling
during a downpour. A single rod or cone cell can receive millions of visible photons per second. As
the light becomes dimmer, there are fewer photons, until there are no visible photons in the dark.
We can only see visible photons that span the spectrum from red to purple.
We can’t see all photons: radio or infrared are also made of photons and are all around us,
but we are blind to them because our rods and cones can’t detect them.
For us to see in the dark, our rods absorb and detect visible photons.
The physical limit for the dimmest light that a rod cell could possibly detect is a single photon.
A rod can’t detect half a photon any more than a parent have half of a child or you could score
half of a point in baseball. Is it possible for a rod cell to detect a single photon?
We could test this by measuring the dimmest flash of light that you can see.
This was done in the 1940’s by 3 biologists named Hecht, Shlaer and Pirenne;
references in the description below. First you need to dark adapt for about 45
minutes. As you know, when you enter a dark room everything seems dark at first.
Over time, your eyes adapt and you can see more easily because your pupils dilate, your retina
switches to using your rods for vision, and the rods increase their sensitivity to light.
Next, stare straight ahead and flash a light off to the side. When we look straight at something,
the light is focused onto a region of the retina that is almost all cones—cones
are great for seeing in bright light, but not in the dark. Our peripheral vision—about 20 degrees
off to the side--has the greatest density of rods and the best night vision. You might have
noticed this—in the dark, you might be able to see things in the periphery, but when you stare
straight at it, it becomes harder to see. So what is the dimmest light that most people
can see? A flash where the retina received only a single photon is too dim for anyone to perceive.
People CAN notice flashes where less than a dozen photons are absorbed and detected by the rods.
But there is a second, important result: We can notice a flash even if each of these dozen photons
are received by different rods. This means a rod cell can see a single photon, but it takes several
rods each receiving a photon for us to notice. The ability of rods to detect single photons has
been directly verified. Rods are filled with light sensing molecules called rhodopsin.
When a photon is absorbed by rhodopsin, it activates the rhodopsin
and ultimately produces a brief electrical signal that is passed on to the brain.
Let’s visualize this electrical signal. In a dark adapted rod cell, the absorption of a
photon produces a small brief change in voltage in the rod. I’ll show the rod as blue while the
electric signal is present. The signal rapidly fades away so that the rod can report the next
photon that arrives. These visualizations are slowed down to make it easier to see;
the real response to a photon lasts less than a second.
A rod cell can detect individual photons, and generate electric signals in response.
But it takes a brighter flash that activates many rods for us to notice.
Why? The reason is noise: Rhodopsin—the molecule that detects photons—occasionally makes a mistake
and reports seeing a photon that wasn’t there. This happens a few times per minute per rod
and results in the same electrical signal that a visible photon would produce.
Here we visualize this noise in complete darkness. Our brain has to rely on the electrical signals
from rods as reporters of light. With the many rods in the retina there’s always some
reporting seeing a photon even in complete dark. If there’s a dim flash that results in a single
rod detecting a photon, there’s no way the brain can distinguish that from the background noise.
But, suppose there’s a brighter flash detected by a dozen rods. It is unlikely that random
chance will produce this many nearby rods all seeing photons at the same time. While rods
can detect single photons, our brain requires corroboration from several rods simultaneously
for us to notice the flash. But you might have noticed that when you’re completely dark adapted,
you might see a faint shimmering, like TV static, in your peripheral vision.
This is partially due to this noise. So, why not just make the rhodopsin better
so there’s less noise? Each rod contains about 100 million rhodopsin molecules each waiting to absorb
a photon. If there’s a couple of noise events per rod per minute, how often does a single rhodopsin
molecule produce a noise event, on average? The answer works out to be once in 100 years.
Rhodopsin is tremendously reliable. Compare that to the film used in old film cameras with an
expiration date of about 2 years. The film would go bad quicker if stored at body temperature,
which is how we store our rhodopsin. Cold blooded animals such as frogs
often have a lower body temperature than us and can have better night vision.
Frog rods have lower background noise, and they can see better in the dark than we can.
Our vision isn’t perfect though, and there are several ways that things could be improved.
While rods can detect individual photons, they don’t detect every photon. Our rods
usually detect less than 10% of the photons that reach the surface of our eye. Some of
the photons are reflected off of the eye, or pass through the retina without being
absorbed. Noctournal animals such as cats have a reflective layer behind the retina : if a photon
passes through the retina without being detected, it bounces off the back and passes through again,
giving it a second chance to be seen. Also, the color of the light matters: our rods
have difficulty detecting red photons, and see blue-green photons best. This is why red light
is sometimes used by navigators or astronomers to read charts. Red light doesn’t activate our rods
and cause us to loose our dark adaptation. Notice that rods and cones are on the back side
of the retina, and light passes through several cells before reaching the rods. These cells can
block some of the light. Its kind of like having a camera with some of the wires and electronics
between the lens and the sensor or film. However, all vertebrate eyes are arranged this way.
Despite this, our eyes are astonishingly sensitive. The amount of energy carried by photons
is miniscule. Carl Sagan wrote in 1980 that “The total amount of energy from outside the solar
system ever received by all the radio telescopes on the planet Earth is less than the energy of a
single snowflake striking the ground”. For comparison, how sensitive are our rods?
Imagine that you took all 8 billion people on the planet with normal eyesight and had them
dark adapted. Show a blue flash of light just bright enough to be seen by all of these people.
The total amount of energy received by the rods of all the people on Earth put
together would be less than the energy of that single snowflake striking the ground.