SENSITIVITY OF THE VISUAL SYSTEM

The luminous efficiency of radiant energy is dependent on the ability of various receiving and measuring devices. The spectral response characteristics of the human eye vary between individuals so that it is not feasible for any one individual to act as a standard observer. Therefore, experiments had to be conducted on thousands of subjects to determine the average response characteristics of the human visual system.

The spectral sensitivity of the human eye to the entire visible spectrum can be stablished experimentally. The scotopic (dim-light) experiment was initially conducted by Hecht and Williams in 1922. In 1932 Gibson and Tyndall conducted a bright-light experiment to obtain the photopic curve. These experiments involved measuring the threshold response to stimulus at various wavelengths. The results are indicated on the left in the figure. Threshold can be thought of as the yes/no point of vision, that is, the point at which some critical detail just disappears, the "I see it/I don't see it" point of vision.

The interval between the absolute threshold of visibility (scotopic curve) and the initial appearance of hue for a given homogeneous wavelength (photopic curve) is called photochromatic interval. This interval is greatest at the shorter wavelengths (blue) and near zero at the longer wavelengths (red). Mesopic vision occurs at intermediate levels of luminance, where rods and cones are believed to work together.

By definition, sensitivity has a reciprocal relationship to threshold. That is,

This results in the threshold curve being inverted, as on the right side of the figure, and allows one to speak in terms of sensitivity, which is a more meaningful term to most individuals.

Four major facts can be derived from the sensitivity curves:

1.Neither the rods nor cones are uniformly sensitive across the visible spectrum.

2.The region of maximal sensitivity is 555 nm for cone vision and 510 nm for rod vision.

3.The rod function lies above the cone function, indicating that throughout most of the spectrum the rods require less energy for vision (have a greater sensitivity) than the cones.

4.The rods and cones are equally sensitive to radiant energy in the long-wavelength (red) end of the spectrum.

Visual stimulation at scotopic levels does not produce color vision anywhere along the spectrum. Color vision is possible only with light levels of sufficient magnitude to activate the cone system. When only the rods are functioning, all wavelengths are seen as a series of lighter or darker grays. Weak light is visible, but hue is absent.

Relative Sensitivity of the Visual System

The curves showing light sensitivity of the human eye are commonly presented in the form of photopic and scotopic relative luminosity curves (relative spectral sensitivity curve or standard observer curve). They are shown in the figure below. The notation used is:

Despite the gross difference in absolute sensitivity between rods and cones, the two functions are plotted on the same graph by performing a simple arithmetic adjustment to put them on a relative basis.

The cone sensitivity curve (see previous figure) must be raised a vertical distance of approximately 1.6 log units to make it comparable to the rod sensitivity curve on a relative basis. The relative sensitivity curves are used for photometric problems to represent the response of a standard human observer.

The Purkinje effect is a shift in the maximum sensitivity of the eye from photopic to scotopic vision. The relative spectral sensitivity curve for cones (photopic curve) peaks at 555 nm. During rod or scotopic vision, the relative spectral sensitivity curve shifts 45 nm toward the 400-nm (blue) end of the spectrum so that the peak occurs at 510 nm. This shift results in an increase in sensitivity to shorter wavelengths (400 nm) and a decrease in sensitivity to longer wavelengths (700 nm) for the rod system. Even though objects will be colorless under rod vision, a blue object will appear brighter or more intense during scotopic (rod) vision than a red object of equal reflectance, due to this shift in sensitivity.

Astrolights for Visual Work

Go For The Green

With regard to the preservation of scotopic (night) vision; The intensity (brightness) of the light used to read charts is more important than the color. Inexpensive polychromatic green LEDs, which closely match the spectral response of the human eye, are now widely available. In combination with the popularity of LED astrolights, the deep sky observer now has the opportunity to drastically cut their visual recovery time and improve their perception of faint objects.

There is widespread astronomical mythology concerning the "correct", "proper", or "best" type of light to use while observing. Conventional wisdom dictates that your chosen, appropriate, source of illumination should be primarily red although spectrally pure red is even better. The conventional wisdom is wrong! Millions of observers are unnecessarily compromising their night vision each time they use a red light to read their charts. Why? Because monochromatic red light must be many times brighter than polychromatic green before we can see with it.

It is unfortunate that the red myth, perpetuated by half baked logic, mis-interpreted facts, and unsupported assumption, has been needlessly forced upon generations of unsuspecting stargazers. Maybe the origin of the red light myth has roots in the early days of photography, when early emulsions were insensitive to red. Much of the myth has certainly been supported by the existence of numerous studies showing red light, of a given intensity, has less effect on night vision than other colors. While this is true, a light source that has the least effect on night vision is not the same thing as a light source that will allow visual perception at the lowest possible level of illumination.

This whole issue is apparently quite confusing. Many observers can't understand why, after being repeatedly told to use red light, it isn't the best. There are many misconceptions about low light vision and the factors affecting the performance of the eye under those conditions. One thing, that can't be overemphasized, is: To preserve your night vision, you need to work your eyes as hard to see the charts as you do for faint deep sky objects. This means dim, not just hard to see! The minimum amount of light necessary to see charts is exactly that, the point at which you couldn't read charts if the light was imperceptibly dimmer.

Once again; we should use the absolute minimum amount of light. This discussion is only about very low levels of light, any benefit of using green light to read charts will not be realized when the illumination level is excessive.

Refer to the graph detailing the threshold of vision. This threshold curve, on the left, is a "now you see it, now you don't" graph. It is reciprocal to the more common sensitivity curve shown on the right. The vertical scales are a logarithmic indication of light intensity, or brightness. The horizontal scales are a linear representation of light by color. The upper curve shows the minimum amount of light that is required to see with your cone, or photopic, visual receptors. The upper curve also shows the full spectrum response averaging the different cone sensitivities. The lower curve shows the minimum amount of light, again by color, that is required to see with your rod, or scotopic, receptors.

Four very important pieces of information are evident on the graph. As we study the graphics, we start to see the point at which the poorly stitched logic of the red myth begins to unravel. First, the intensity of light required to see anything in the red end of the spectrum is much higher than the level needed to see light of any other color. The visual threshold for perception of red illumination with the low light rods is many times higher than the green threshold of the bright light cones! Second, recognize that there's also very little difference between the sensitivity of the cones and rods at the red end of the spectrum. This shows why red light must be excessively bright to see with. Third, note the difference of minimum illumination intensity required to see red as opposed to green. At all other colors, especially green, there is a large variation between the photopic and scotopic threshold / sensitivity. Fourth, even though we can't distinguish color with low light rod vision, the rod cells respond to all colors. Our low light vision may appear to be monochromatic , but our spectral sensitivity to low intensity illumination is polychromatic! Any light source that is only one color will need to be excessively bright for visual perception to occur.

There are three cone type receptors, each has a different spectral sensitivity. The cones are used for bright light color vision. This type of photoreceptive cell is also tightly packed near the center of your visual field, permitting high resolution color vision. There is only one type of rod (night vision) cell in your eye, so it is not possible to see color at low light levels. Additionally, the rod cells are not tightly packed together (producing low resolution vision) and they are not located at the center of your visual field. This is why averted vision and higher magnification (within reason) improve the perception of faint deep sky objects.

All too often observers crank up the brightness of their red lights attempting to make the charts nearly as visible under red light as they are under white. This level of illumination is excessively high, it's not necessary to use such bright light. Your night vision will be compromised to a much greater degree than with a more appropriate level of illumination. Turning up the light level too high is really a natural reaction caused by an inability to see clearly under red light and the need to activate (for visual resolution) additional photopic sensors in the eye. The spectral response of each type of cone is not sharp, there is considerable overlap. If pure red is bright enough, it will enhance visual resolution by activating the green cone cells. This is why so many observers use such bright red lights to read charts.

If you want to preserve your hard won dark adaptation; Why would you chose a light source that must be brighter, than any other color, to see your charts? Wouldn't it be much better to use the minimum amount of light necessary to see the charts? Of course it would! Although color does have an effect on night vision at a given level of illumination, the overall intensity of the light is more important.

Maybe you're thinking that the charts also show that red is the color with the minimum affect on night vision because of reduced sensitivity. That logic only applies to an illumination source at a given brightness level, not sensitivity level. To illustrate the fallacy of this interpretation, ask yourself if high levels of infra-red or ultra-violet would either have less effect on scotopic vision or improve star chart visibility. Such logic also does not apply to any attempt to see by reflected light. Due to the fact that we are more sensitive to green, than red, the illumination intensity can be much lower and still allow us to read our charts.

What about the Purkinje effect, that 45 nanometer spectral shift between maximum cone and rod sensitivity? This overall spectral sensitivity shift is from yellow green (photopic) to blue green (scotopic). It is caused by the poor response of the rod cells to red light. When you see the color red, the light level is many times brighter that the illumination intensities represented on either graph. The Purkinje effect has no relevance when reading charts, even though it is important for visual stellar magnitude estimates.

You will never be able to read a star chart when fully dark adapted. It is not possible when using only low light, low resolution, rod vision. At least not until charts are printed with two inch tall type characters. Mesopic vision is required, this type of vision involves both the rods and cones. There must be enough light to activate the high resolution cones in your eye if you want to read a star chart in dim light. As shown on the graph, cone vision can be activated with a lower intensity of green light than with any other color.

With mesopic vision, at minimum illumination levels, you will see a slightly greenish tinge of color from white or green objects, every thing else will look gray or black. Remember: If you can see color, then your night vision has been compromised! Since a much greater intensity of red (than green) light is required to see, you are compromising your night vision to a greater degree when using red.

It is important to remember that the benefit of using green light to read your star charts is only evident at very low levels of illumination. Any light that is excessively bright will compromise your night vision more than a dimmer light source, regardless of color.

A useful comparison of green light vs. red light for reading star charts in the dark can be made with a simple experiment. If you already own a red LED type flashlight, buy a green LED at Radio Shack, part no. 276-303, and replace the red LED already in your flashlight. If you also need a red LED, the RS part number is 276-310. Then try reading your star charts under both green and red light. The results should be illuminating.

NOTE: A similar article first appeared in the REFLECTOR, The Newsletter of the Astronomical League, August 1997.