When it's late at night and I turn off all the lights I often stand in the darkness and wait for magic. Because, like magic, my eyes adjust, and in a few moments what was pitch black becomes gradually clearer. Forms, shapes in shadows-become furniture and walls. Dark windows become transparent again and bright with the light of other houses, moonlight or car beams. Colors emerge and I can tell the difference between the brown hands on the hanging clock and the bright green numbers on the microwave:


Outside, I can see clouds drift across the sky in front of the stars, high above houses and trees-dancing across the horizon. Small jewels of white light in between the blinking reds and greens of low flying planes.

Everything is clearer late at night, in the dark. The silence accentuates the contrasts. Of course, if someone would turn on a light, I would be blinded-and the person flipping the switch could see everything I missed.

I would be left there, leaning against a wall, eyes squinting. Lost in the light defensless. A forgetful vampire at dawn.

Night vision units are Electro-optical devices which do not rely on their own light source. They simply amplify (or intensify) available light. Night vision units are sensitive to a wide range of the spectrum, from visible to infrared. (You can even purchase infrared beam casters that dramatically increase infrared sensitivity, without displaying anything visible to the naked eye. Cool Stuff. You'd be amazed at how it looks through the lens. It looks like you're shining a spotlight, but when you look at it with the naked eye, you see nothing. )

When you look through something equipped with night vision, you're not really looking "through" it per se - you're actually looking at the amplified image which is being displayed in real time on a phosphor screen, which has been colored green. The green color is intentional, as the human eye can discern a greater number of shades of green than any other phosphor colors. The fact that you're actually looking at a display rather than the actual physical world through a lens is why looking through a night-vision device severely impairs depth perception.

This is a (quite crude) drawing I did of how a light vision unit works:

         |  |____|__
Light -> |  |____|__|
         |__|_|__| |
         `    |  ` |
              |   Eyepiece
          Image Intensifier

                  |                  |
    (Light -> [ ] | -> [ ] -> [ ] -> | [ ])
               |        |      |        |
             Lens       |      |        |
                 Photo Cathode |    Eyepiece
                        Phosphor Screen

The top diagram shows the construction of a typical night vision unit. Light enters the unit through the objective lens, and strikes the photo cathode. The photo cathode has a high energy charge from the built-in power supply, usually consisting of a rechargable battery unit. The energy charge from the power supply accelerates across a vacuum inside the image intensifier, and strikes the phosphor screen where the image is displayed. The eyepiece simply magnifies the image displayed on the screen.

The bottom diagram shows the layout of the image intensifier. The phosphor screen works just like the screen in your television or monitor - except there's only one photon gun in the night vision unit, as opposed to three in your TV or monitor.

There are different phases to gaining night vision. The pupil widens, hungry for light. Rhodopsin in the rods in your retina increases, slowly, changing the chemistry to make the seeing possible. You learn to look for edges instead of taking them for granted.

Playing kick the can in the dusk and dark, you learn this about night vision: it's all shades of grey. Cones are for color. Rods are for peripheries and darkness. The center of your vision, in the dark, is dimmer than the rest (there are fewer rods there). The looker can be made to believe her eyes are playing tricks on her, if she looks right at you. You can become a tree. You can become a rock. You can get closer and closer. Beware, though - motion makes things easier to see. So stand still.

Night vision is a gift. If you have it, you are invisible. You don't need a light. You can see the monsters that aren't there and tell them with authority to go away. You can walk quietly past sleepers and not trip over things left carelessly.

And so a flashlight, though well-intended, is a curse. Walking in the dark, you want to hide from people with flashlights, so they won't shine it in your face. Avert your eyes from headlights. It takes seconds to lose night vision and up to twenty minutes to regain it, all the while stumbling, fearful, and clumsy.

From Suzanne Vega's 1987 album, Solitude Standing.

By day give thanks
By night beware
Half the world in sweetness
The other in fear

When the darkness takes you
With her hand across your face
Don't give in too quickly
Find the thing she's erased

Find the line, find the shape
Through the grain
Find the outline, things will
Tell you their name

The table. the guitar
The empty glass

All will blend together when
Daylight has passed

Find the line, find the shape
Through the grain
Find the outline, things will
Tell you their name

Now I watch you falling into sleep
Watch your fist uncurl against the sheet
Watch your lips fall open and your eyes dim
In blind faith

I would shelter you
Keep you in l
But I can only teach you
Night vision
Night vision
Night vision

Night blindness, the inability to gain much (or any) night vision, has a number of potential causes, one of which is a Vitamin A deficiency. This is why mothers and others prescribe carrots for your eyes (not just a tale!). The deficiency might be dietary lack or something worse, like cirrhosis of the liver - at any rate, carotene may help. Some varieties of night blindness have been successfully experimentally treated with concentrated doses of Vitamin A, but it does not conclusively work in all cases.

It's not listed anywhere (scientists can't measure it) but an aspect of night vision is patience and the will to see. If you can't sit/stand quietly and wait, but have to turn on the lights, run, hide from the dark - this relation to the shapes around you won't come. You won't learn to look with your ears and your eyes, with your sight slightly averted so the central blind spot won't fool you. And you certainly won't win at kick-the-can.

The Absolute Limits of Night Vision

This famous experiment first performed by Selig Hecht, C. Shlaer, and M. H. Pirenne in 1942 demonstrated that the rhodopsin in our eyes can detect single photons, but that the brain will not register a response unless several are detected in a short period of time.

Before the experiment can be conducted, scotopic vision must be induced in the subject. Most of the time, we see in photopic mode which uses the light receptors called cones. The cones can see in vivid colors but need a lot of light to do so. Scotopic vision, however, uses the rod receptors, which cannot detect colors but require very little light and are distributed to give much better peripheral vision.

To induce scotopic vision, the subject must be subjected to very dark conditions for some time. First, and relatively quickly, the irises constrict, dilating the pupils and letting more available light into the eyes. Another change that occurs is chemical in nature, and takes much more time. Rhodopsin is the light-sensing molecule in the rods, and each time a rhodopsin molecule senses light, it breaks down into retinal and opsin. The retinal and opsin spontaneously recombine, but very slowly, with a "half-life" of about five minutes (much too slow for the rods to be of any use in daylight). Waiting for 30-40 minutes will allow about 99% of the rhodopsin to be available for sensing light.*

Next the subject must have their head in some kind of stabilizing contraption, so that a precise light source can be accurately aimed at one eye. The best place to aim the light source is 20° away from where the subject's eye is focused. The rods are actually most dense on this part of the eye, resulting in better peripheral night vision than direct night vision. (You can observe this effect by looking at a very dim star at an angle; it will seem to disappear if you look right at it) The light source should be aimed at a 10 arcminute wide spot (about 1/3 the apparent width of the moon from earth), meaning that there will be about 300 rods in that area connected to the same nerve ending.

Now it is time to actually send photons into the subject's eye. Of all the photons that enter the subjects eye, only about 10% of them will actually change a molecule of rhodopsin. Others are either reflected, do not hit a rod, or fail to cause the molecule to change. This uncertainty means that it is neither necessary nor practical to use single photon LEDs (only a recent invention). It can be done statistically with any very dim light source capable of putting out a few hundred photons in less than 100 milliseconds. The lamp should be set for a few hundred photons per pulse, and randomly flashed on or left off. Each time the subject should be asked if they saw it or not. (Note: The lamp must be completely silent, otherwise a subject with good hearing could skew the results) If the subject can get an accuracy better than guessing, then the lamp is turned down and the experiment repeated until the subject gets an accuracy of about 60%. This may sound very low, but because the lamp itself is not perfect, it may sometimes put out fewer photons than expected, so some of the inaccuracy is not the subject's fault.

In the end, the lamp only has to emit about 90 photons for the subject to see it. This corresponds to about 9 photons entering this small area of the eye. It is extremely unlikely that two of these photons hit the same molecule of rhodopsin, so the conclusion is that our eyes can detect individual photons, but any fewer than 9 in a short time will not trigger a brain response.

*Note: Rhodopsin also spontaneously splits into retinal and opsin, but with a half-life of about 6 years. Because there are about 4 million molecules of retinal on each rod, and 300 rods in each 10 arc-minute, it is entirely probable that 9 will decompose within 100 ms of each other. This is why even in complete darkness you can sometimes see random flashes of light.

http://math.ucr.edu/home/baez/physics/Quantum/see_a_photon.html (Background information)
http://www.dur.ac.uk/c.a.heywood/netnotes3.txt (Description of the experiment)

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