How Color Affects VSO

As you know, the human eye is an extraordinary detector of color under bright conditions. You also know that the color receptors, the cone cells, do not function well at night and as a result you will usually see no color. You have no doubt noticed that you are unable to distinguish the colors of vehicles or of the clothing of people walking in parking lots at night or along a street that is poorly illuminated.

Because rod cells and cone cells are each most sensitive to different colors, the perceived brightness of an object near the transition light level when your eyes are switching from cone cells to rods cells, for example when the sun is setting, can depend on its color. This Perkinje effect is known, but not well understood, to variable-star observers who often have to compare two stars of different colors. As you will discover, it's not easy to find a comparison star that is the same color as the variable that you are observing.

The Perkinje effect is named for Jan Evangelista Purkinje, a pioneering Czech experimental physiologist whose studies within the fields of histology, embryology, and pharmacology helped create a modern understanding of the eye and vision. The Perkinje effect describes the following phenomenon: As light intensity decreases, red objects are perceived to fade faster than blue objects of the same brightness.

In 1819, Purkinje noticed that as the light faded on his garden the red poppies became black but the blue flowers remained blue and the green leaves stayed green. Purkinje discovered that when light levels decrease, the human eye becomes more sensitive to blue and green light. Purkinje theorized,

The implication is that this twilight zone is the most dangerous one: lighting conditions are varying quickly; nocturnal predators are appearing: and fatigue is setting in. It might well be more adaptive to have better vision during this brief but dangerous period than to optimize reception to the spectrum of moonlight when the light levels are too low to allow any advantage to be had from it.

Basically, he said that perception is more important than color.

Despite their individual weaknesses, the rod and cone systems make a great team, working together as two hunters who have agreed that one will scan the countryside for movement but must allow the other to identify what it is that's moving. Of course, we do not notice that we have two receptor systems because our brain seamlessly combines their outputs.

All of this will become important when you begin to observe bright red stars, such as the Mira and semiregular type variables or when you must compare a variable with a comparison star of a different color. Later, we will take a look at some of the methods you can use to reduce the Perkinje effect when observing variable stars.

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