The human eye is a marvel of design. All the parts work together to provide us with a vivid and colorful mental picture of our surroundings. As an astronomer, one aspect of human vision that I find particularly amazing and useful is the ability of the eye to adapt to extremely different lighting conditions. At night, the eye adjusts to be able to see stars that are 12 trillion times fainter than the sun. The way this mechanism works is ingenious and is merely one example of the cleverness of the Lord and of His grace toward us.
Light and Dark Adaptation
When you first step outside on a clear, moonless night far away from city lights, you can see a few of the brighter stars. After a few minutes, more stars become visible. And after a half hour, the sky appears much brighter with thousands of stars and the faint cloud of the Milky Way stretching from one horizon to the other. The eyes have adjusted to a low-illumination setting: a phenomenon called dark adaptation. Conversely, when you first step outside from a dark movie theater on a bright sunny day, the sky is so bright it is almost painful. But then after a few minutes, the eyes adapt and it doesn’t seem so overwhelmingly bright. This process is light adaptation.
In a previous article, we discussed how the pupil of the eye can change size to adjust for rapid changes in light intensity. The pupil is the dark center of the eye, surrounded by the iris – the colored part of the eye. The iris can contract or expand to change the size of the pupil; this regulates how much light enters the eye. The pupil can be contracted down to 2 millimeters in diameter or expanded up to 8 millimeters – a factor of 4. Since the light entering the eye through the pupil is proportional to the area and thus the square of the radius (or diameter), the pupil can change the light entering the eye by a factor of at least 16. The rate at which the iris contracts or expands is quite rapid. Therefore, you can adjust to a brightness change of a factor of 16 almost instantly!
But your eyes have another way of adjusting to much greater changes in ambient illumination. The retina has the capacity to adjust the sensitivity of the light-detecting cells in the retina – the rods and cones. This process is much slower than the rapid adjustments of pupil size. But its range is much greater. The changing sensitivity of the rods and cones in the retina is why you can see clearly on a bright sunny day, or a dark starry night.
Sensitivity refers to the amount of signal that is generated by rods or cones when they receive a given amount of light. Although rods can apparently detect a single photon of light, they are wired together so that they will not send a signal to the brain unless at least 5 to 9 photons are detected within a timeframe of 100 milliseconds. This is a noise-reduction mechanism; it prevents you from seeing a distracting pattern of “static” at night like you might see on an old television that is not tuned to any broadcast station. For this reason, there is a minimum illumination needed to see anything at all.
Just above the minimum level of illumination, the rod will send a signal of 1 instead of 0 (the number 1 representing the weakest non-zero response). As the illumination increases, so does the signal. Perhaps 20 photons within the timeframe will generate a signal of 2. The response is non-linear. So it might take 50 photons to generate a signal of 3, rather than just 30.
In any case, the sensitivity of the rods and cones can be changed. They become more sensitive under dark conditions – only faint light results in a large signal. And they become less sensitive under bright conditions – a stronger light is needed to result in a given signal. So, while 50 photons may generate a signal of 3 at night, during the day perhaps 50,000 photons are necessary to generate a signal of strength 3. In fact, the minimum illumination necessary to generate any signal at all will be much higher in bright conditions than in dark conditions. The way in which the retina adjusts its sensitivity is ingenious, and is due to the chemical processes by which light is transformed into an electro-chemical response.
The Chemistry of Light Detection
Rods have a protein within them called rhodopsin. This chemical has two components: opsin and retinal, which are bonded to each other. When light strikes rhodopsin, it causes the protein to change shape, and then break into opsin and retinal. This process is called bleaching because it literally changes the color of the retina from red to orange, yellow, and finally, transparent. Bleaching triggers a series of steps that result in (potentially) an electro-chemical signal to the brain.
Before the bleached retinal and opsin can once again convert light into an electrical signal, they must be reunited into the correct form of rhodopsin – a process called pigment regeneration. This process requires the enzyme vitamin A, which is converted to retinal and is contained in a layer called the pigment epithelium which lies just behind the rods (and cones). The body can produce vitamin A from beta carotene which is found in plants. Beta carotene is what gives carrots their distinctive color. So, carrots really are good for your vision.[1]
The Chemistry of Dark Adaptation
Exposure to light is constantly breaking down rhodopsin in the rods. However, retinal from the pigment epithelium is constantly being used to restore that rhodopsin. These two processes eventually reach an equilibrium condition in which the rhodopsin destruction rate matches its creation rate.
Now consider what happens in a very bright setting. With all that light, most of the rhodopsin in the rods is quickly depleted, having been converted into opsin and retinal. Although the rods are constantly replenishing the rhodopsin, the influx of so much light breaks down the protein faster than it can be re-built, resulting in relatively few rhodopsin proteins at any given time. Therefore, most of the light that enters the rod does not have the opportunity to strike a rhodopsin protein (since there are so few), but instead passes through without generating a signal. The bleached rods have become relatively insensitive to light. They generate only a modest signal even in the presence of very bright light.
Now consider what happens in a very dark setting. Since there is very little light to bleach the rhodopsin, the rods have plenty of time to produce the maximum amount of this protein. After about 30 minutes or so, the rods have as much rhodopsin as they can store. So, what happens when even a very small amount of light enters the rod? Since there is abundant rhodopsin, the likelihood that the light will strike one of these proteins is very high, thereby (potentially) resulting in a signal. The rods have become very sensitive to light. So, even a small amount of light results in a relatively strong signal. Since it takes about 30 minutes to produce the full amount of rhodopsin in the rods, this is roughly how long it takes to dark adapt. However, additional slight increases in sensitivity can occur for the next hour and a half.
Although it takes about 30 minutes for the rods to fully dark adapt, it takes only seconds for them to light adapt. The process of bleaching the rhodopsin is due solely to the amount of light that reaches the rods per unit time. This is why it is a bad idea to turn on lights when you are stargazing once you have become dark adapted. Bright light will spoil that adaptation in seconds, requiring another 30 minutes to fully re-adapt.
Cones can also dark adapt using the same kind of process. But they cannot reach the low-light sensitivity of the rods. Instead of rhodopsin, each of the three cones has a particular photopsin that breaks and triggers a signal when exposed to certain wavelengths of light. These photopsins are replenished in the cones in the same way as rhodopsin is in the rods. But, when both are maximally dark adapted, cones are not nearly as sensitive to light as rods. In fact, after about 3 or 4 minutes in a pitch-black setting, the cones are as dark-adapted as they can get. After about 7 to 8 minutes in a dark setting, the rods have surpassed the cones in sensitivity, and you are relying primarily on your rods to see at that point. Fully dark-adapted rods are able to detect light that is hundreds of times fainter than fully dark-adapted cones.
This is also why astronomers typically use red flashlights. The rods are most sensitive to light of medium wavelengths, in the “green” part of the spectrum. Rods cannot detect long wavelength (“red”) well, but the L-cones can. Thus, using a red flashlight allows an astronomer to briefly use the cones to read something without ruining the dark adaptation of the rods.
As mentioned in a previous article, the part of the retina corresponding to our center of vision is called the fovea, and is packed with a very high density of cones. This is why we have such a crisp view of something only when we look directly at it. However, to make room for all those cones, the fovea has no rods at all. This results in an interesting situation under very dark settings. Under conditions too faint for the cones, the rods provide us with a picture of our surroundings except for the center of our vision. Essentially you have a “blind spot” in the center of your field of view – as far as your night vision is concerned.
To experience this, try the following experiment on a clear, starry night, far away from city lights. After 15-30 minutes in the dark, your rods will be well-adjusted and you will be able to see very faint stars. Pick a faint star in your peripheral vision, maybe 15 to 30 degrees away from the center of your field of view. Then look directly at it. If the star is faint enough, it will seem to disappear! Look away a bit, and the star will reappear in your peripheral vision. This may seem very strange at first, but astronomers quickly become adept at using averted vision: looking not directly at an object, but a bit to the side. This provides the best view of faint objects – those too dim to be detected by the cones.
A Versatile System
The eye is superior in its adaptability. The Lord designed it with a suite of data processing cells to reduce unwanted noise, and photoreceptors ingeniously distributed to provide both a wide field of view and a high-resolution image for objects near the center of vision. The two eyes work together to provide us with a nearly instantaneous 3-dimensional color mental representation of our surroundings. So that we can see both during the day and at night, the Lord has equipped our eyes with a system of both cones and rods. The cones deliver a full-color experience in bright settings, allowing us to enjoy the beauty of a rainbow. Conversely, the rods are grayscale, but can adjust their sensitivity to detect very faint sources of light. This allows us to enjoy the beauty of the night sky. We are indeed fearfully and wonderfully made (Psalm 139:14).
[Next, the inverted retina in part 5].
[1] Vitamin A can also be taken as a supplement. However, care must be exercised since a vitamin A overdose can lead to problems. Eating beta-carotene is a better option because the body then regulates vitamin A production.