Eyes (Level 5)

The stimulus for color hasn't disappeared at night. The problem is that there simply isn't enough light for us to perceive colors. I took this picture of Yosemite Falls at night (about 11:00PM) on an early spring evening with a nearly-full moon. I set my camera exposure time to about two minutes in order to capture enough light to make the image. You can see that it was taken at night by looking at the stars in the sky and seeing that they actually moved a little bit (well actually the earth rotated a little) during the long exposure time. This full-color night-time image shows that all the colors are still there under moonlight, but we just can't see them. The sky is blue, the water white, trees green and brown, rocks gray and brown, etc. When I was in the original location, I could only see a black and white version of the scene with my naked eyes. That is because there was only enough light for my rods to function and not my cones.

Why Can't I See Colors at Night?

You can't see colors at night because our visual systems are not designed to see colors when there isn't very much light in a scene. We actually have two visual systems that work in parallel to help us survive in the world. When there is plenty of light, we use our cone photoreceptors. There are three types of cones roughly sensitive to red, green, and blue light and we can compare the images captured with these three systems to perceive the colors in the scene. We can also see fine detail with our cones.

However, the ability to see colors and detail with our cone system means that the cones cannot be very sensitive to light. As the light levels decrease at night, we reach a point where our cones can no longer respond because there simply is not enough light for them to produce a response. In this situation, our visual system automatically switches to a second set of photoreceptors known as rods. There is only one type of rod receptor, so that means we can only see in shades of gray when our rods are working and our cones are not. The rods also gang up together to capture light over relatively large areas. This helps them to be very sensitive to the small amounts of light available at night, but it means that they cannot possibly allow us to resolve fine details.

Thus, it is our switch from the color-sensitive, but light-insensitive, cone system to the color-insensitive, but very light-sensitive, rod system that causes us to loose our color vision at night. Or as it was once written by the rock band, The Moody Blues:

Cold hearted orb that rules the night
Removes the colours from our sight
Red is gray and yellow, white
But we decide which is right
And which is an illusion

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Ever wonder ... How do digital cameras detect colors?

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Updated: June 3, 2010