Hack11.See in the Dark

Hack 11. See in the Dark

Have you ever wondered why all cats are gray in the dark?

Our eyes function in two entirely different modes, depending on how much light is available. In daylight or bright artificial light, our eyes function in day vision mode. After dark, our eyes shift to night vision mode. The physiological changes that occur in our eyes during the shift from day vision to night vision are called dark adaptation. Dark adaptation occurs slowly, typically requiring 25 minutes for 80% adaptation and 60 minutes for 100% adaptation. That's why astronomers get upset when someone shows a bright light.

When we move from dim light to bright light, our eyes undergo physiological changes called light adaptation. But while dark adaptation occurs slowly, light adaptation occurs quickly, in two phases. During aadaptation, which requires about 1/20th of a second, the sensitivity of the retina drops by 50% or more. During badaptation, which requires from one to several seconds, the sensitivity of the retina drops more gradually, and we recover full color vision and visual acuity.

There are many misconceptions about night vision and dark adaptation, even among astronomers. To understand the process of dark adaptation, you need to understand something about the physiology of the human eye. Our eyes have two types of light sensors, called rods and cones. Rods provide monochromatic vision, but are very sensitive to light. Cones provide full color vision, but are relatively insensitive to light.

Cones and rods are unevenly distributed over the surface of the retina. Cones predominate in the fovea, the center of the retina, where they are densely packed. The fovea contains about 200,000 cones in an area of about one square millimeter, and thus provides acute resolution of fine detail. The entire retina contains about only 7,000,000 cones. That means cones are very sparsely scattered outside the fovea, just enough to show brightness and color with little detail in your peripheral vision. Rods predominate outside the fovea. The entire retina contains about 130,000,000 rods. They are less densely packedat about 90,000 per mm²than cones in the fovea, but much more densely packed than cones outside the fovea. Accordingly, rods provide poor resolution of fine detail relative to the cones in the fovea, but much higher resolution than the sparsely scattered cones outside the fovea.

The uneven distribution of cones and rods explains, for example, why you have to look directly at this book to read it; if you glance at it from the corner of your eye, you can't resolve sufficient detail to read the words. Conversely, the paucity of light-sensitive rods near the center of your eye explains why it's easier to see dim objects by looking to one side rather than directly at them, a phenomenon astronomers call averted vision.

Rods and cones detect light by using dyes to absorb it. As light is absorbed, the dyes bleach and a signal occurs to indicate that light has been sensed.

There is only one type of rod, which is why rods provide monochrome vision. There are three types of cones, one for each of the primary colors of light: red, green, and blue.

Rods

Rods use the dye rhodopsin, and have peak light sensitivity at a wavelength of about 498 nm, in the blue-green part of the spectrum.

L cones

L cones, also called red cones, use the dye erythrolabe and have peak sensitivity at about 564 nm, in the yellow-orange part of the spectrum.

M cones

M cones, also called green cones, use the dye chlorolabe and have peak sensitivity at about 533 nm, in the yellow-green part of the spectrum.

S cones

S cones, also called blue cones, use the dye cyanolabe and have peak sensitivity at about 437 nm, in the violet part of the spectrum.

Nearly all humans have only rods and three types of cones. A tiny percentage of women have a fourth type of cone. Presumably, they see colors they can no more explain to the rest of us than we can explain colors to a blind person.

2.2.1. Vision Modes

Broadly speaking, there are three modes of vision:

Photopic mode

Photopic mode(day vision) occurs at moderate to high lighting levels, 1 milliLambert (mL) or higher, and uses primarily the cones. Photopic vision provides full-color images and high visual acuity, allowing you to resolve fine detail. Photopic light sensitivity peaks in the green part of the spectrum at 555 nm.

Scotopic mode

Scotopic mode (night vision) occurs at low lighting levels, below about 0.001 mL, and uses the rods exclusively. Scotopic vision is monochromeyou see only shades of gray. It provides low visual acuity, making it difficult to resolve fine detail. Scotopic light sensitivity peaks in the blue-green part of the spectrum at 505 nm. (Although blue-green light stimulates rods most efficiently, that blue-green light is still visible only as gray because the rods do not convey color information to your optic nerve and brain.)

Mesopic mode

Mesopic mode occurs at lighting levels in the transition zone between 1 mL and 0.001 mL, or about the brightness range of a moonlit land-scape. At these light levels, cones and rods both contribute to vision. Color and finer detail is visible in the more brightly lit or more reflective areas, while objects in shadow are visible only as murky gray.

Technically, mesopic mode isn't a separate mode, but a combination of photopic mode and scotopic mode. Mesopic mode occurs when part of your eye functions in photopic mode and part in scotopic mode. Here are two examples of mesopic mode as it applies to astronomy:

Seeing color in bright nebulae

If you use a medium to large scope to observe the brightest nebulae, such as the Orion Nebula (M42) and the Ring Nebula (M57), you may see parts of the object in a greenish-gray cast. You can see this color because the amount of light striking your retina is just barely sufficient to trigger some of the green cones. (Some young people can see tinges of blue and red in M42. We want their eyes.) Other objects in the field appear gray because their light is sufficient to trigger only your rods.

Seeing star colors

With the naked eye, most stars appear white because their light is insufficient to trigger your cones. The red color of a few of the brightest red stars, such as Betelgeuse and Antares, may be visible to the naked eye because they are just bright enough to cross the transition from scotopic to photopic vision. If you use a binocular or telescope, which brightens the image from hundreds to thousands of times, many stars have distinct colors. When you view a colorful star in the same field of view as a faint fuzzy, you are using mesopic visionphotopic (cones) for the star and scotopic (rods) for the faint fuzzy.

2.2.2. Night Vision Fallacies

Here are some common fallacies about night vision and dark adaptation:

2.2.2.1 Dark adaptation is all-or-nothing

Wrong. Many astronomers believe that any exposure to light damages overall dark adaptation. In fact, not only does each eye dark adapt separately, but each cone or rod also adapts individually and cones do so separately from rods. That means you can keep one eye fully dark adapted even if the other eye is exposed to bright white light. Also, because cones adapt separately from rods, you can use photopic vision for viewing charts, recording observations, etc., without harming the scotopic dark adaptation of your rods.

2.2.2.2 Pupil diameter is critical to dark adaptation

Not true. The human pupil varies from as small as 2mm in diameter under bright lighting to as large as 8mm under dark conditions, a range of only 4:1 linearly and 16:1 areally. In fact, the usual range is less. It's very rare for a person more than 20 years old to be able to dilate to 8mm; 7mm is the more usual maximum in young adults, 6mm at age 35 to 45, and 5mm is common in people older than 55 or 60. At most, then, the range of brightnesses controlled by pupil diameter is 16:1, and a range of 12:1 or even 6:1 is more usual. In fact, directional sensitivity reduces that factor still more, into the range of 10:1 to 4:1. The range of brightnesses detectible by the human eye is about 10,000,000,000:1, so pupil diameter plays only a miniscule role compared to the sensitivity level of the rods. Also, the pupil constricts and dilates very quickly compared to the time needed for rods to recover their dark adaptation.

It is true that it's important to match the exit pupil of your instruments to the entrance pupil of your eye [Hack #7]. For example, if your maximum entrance pupil is 5mm, using an eyepiece or binocular that provides a 7mm exit pupil simply wastes light. If your entrance pupil is 7mm, the exit pupil of a 7X50 binocular almost exactly matches your entrance pupil. Someone whose entrance pupil is only 5mm would be better served by a 7X35 or 10X50 binocular, with a 5mm exit pupil to match the entrance pupil.

It is also true that a constricted pupil impairs night vision. For example, a pupil that dilates to 7mm receives nearly twice as much light as one that dilates to only 5mm, because the first pupil has nearly twice the area (7²:5²=49:25). One magnitude corresponds to a brightness difference of about 2.51, so a young person whose pupils dilate to 7mm can typically see nearly one full magnitude deeper than an older person whose eyes dilate only to 5mm. In fact, it's worse than that, because the eye lens yellows and darkens as we age. The eye lens of a 15-year-old typically transmits three times as much lightanother full magnitudeas the eye lens of someone who is 75.

2.2.2.3 A dim green light is the best choice for preserving night vision

Nope. This myth probably persists because the military uses dim green lighting in some tactical situations and because military night-vision scopes produce a dim green image. In fact, the military uses dim green light because the photopic (cone) vision needed to provide high visual acuity is most sensitive at green wavelengths. But any green light bright enough to trigger your cones is much more than bright enough to destroy the dark adaptation of your rods, eliminating your night vision.

2.2.2.4 A bright red light destroys night vision

Red light preserves dark adaptation for a simple reason. The rhodopsin pigment in rods is completely insensitive to light at wavelengths longer than about 620 nm, which is to say deep red. Although the erythrolabe dye present in your L cones has peak sensitivity near 564 nm, its sensitivity extends far into the red part of the spectrum. That means you can use a very bright red light without damaging your scotopic vision at all.

This is an excellent reason to use a red LED flashlight rather than a standard flashlight with a red filter. LED flashlights emit light at one specific wavelength, and red LED flashlights emit at a wavelength to which rhodopsin is insensitive. Red filters, on the other hand, also transmit a fair amount at light at shorter wavelengths, so a bright red-filtered flashlight can impair your night vision.

Interestingly, rhodopsin sensitivity peaks very close to the 486 nm H-Beta and 496/501 nm O-III lines emitted by many nebulae. Were it not for this coincidence, many faint fuzzies would be even fainter.