The Purkinje Effect

When I teach beginning lighting design classes, I always have a week of learning about the eye – how your rods and cones balance each other, what their respective jobs are, how the images you see are translated to the brain, and how, as lighting designers, we can use the strengths and weaknesses of the eye to heighten the audience's experience.  Physiology always comes into play when you're designing a lighted environment, and students always seem to enjoy finding out what different colors of light do to the body – blues are soothing (even to both genders), reds increase blood pressure and respiration rate, yellows make your eyes tire faster, and black is a submissive color.  None of the really "alternative" students appreciate being told that their black fingernails, eyeliner, and dyed hair color are really signs of compliance.

Inevitably, a question that comes up is "why do we see blue and gray at dusk and when the sun is setting?"  It's a great question, especially because it lends itself to explanation of the photopic (lots of light, optimal conditions) and scotopic (low light, monochromatic) vision systems, and how rods and cones operate and cooperate.

The Purkinje Effect, as it's called, is when the sun goes down, and we see a whole lot of blued out and grayed out colors.  This happens primarily at dusk, and in very low light conditions, where the color receptors (cones) basically leave it to the light receptors (rods) for detail for the brain.  When the light goes down, the color information is lacking, so the rods have to compensate.  Our vision becomes monochromatic because our color receptors, which do not interpret intensity information and respond best to yellow-ish light pass the workload on to the light receptors, which respond best to green/blue light and allow us to see shape and contrast.  Just without color.

There's the lecture for the day.