Binocular Colour Vision

'Sky Blue' Observation

Sky Blue Viewing
Related Effects including the Purkinje Effect
Real World Parallel including Anaglyphs
Historical Reference Link
Origin of the name 'Sky Blue' plus "Why is the sky blue?

Sky Blue Viewing

There are times in real life when you do not see the same colour in both eyes.  Have one eye in the light and the other shaded, then alternately open and close the eyes as in the animated GIF image that follows.  (The figure shows two lights but one light source is usually sufficient.  One of the best places to test this observation is in a car with the setting or rising sun off to the side.)
    Your first observation will be that the eye on the bright side will have a darker field of view.  As you continue to alternate the view between the eyes, you should become aware of an overall colour cast to what is being observed, with one eye seeing a bluish tint to the scene and the other eye a reddish tint.
    Once you have established that you are not seeing the same colours in both eyes in the alternating observations, open both eyes.  The proper colours will return to the scene.

The Purkinje Effect is described in the literature.  In dim light, blue objects appear bright relative to red ones.  Under these conditions the rods, which are more sensitive than cones to blue wavelengths, add an extra sensitivity to blue objects.  As the light gets stronger, the blue objects will become less bright relative to the red ones as cone vision becomes dominant.
    The Sky Blue effect appears to be the opposite of Purkinje Effect.  The eye in dim light seems to be attuned to red and the eye in bright light to blue.

Real-World Parallel

The ability of the eye to see the world in proper colour through a red-blue mixture is shown in the real-world application of anaglyphs.
    Anaglyphs create 3-D images from specially prepared flat pictures using coloured filters set in paper viewers resembling glasses.  The images for each eye are differently coloured and printed on top of each other on the page.  The image for each eye is selected by the appropriately coloured filter.  Early comic books - especially line drawings - used the red-green complementary colours.  This made theoretical sense since this offered the greatest visual difference between the images.
    When fully coloured images were wanted, it was found that red-blue (or red-cyan) filter combinations were better.

Historical Reference

Helmholtz recorded observations similar to the 'Sky Blue' effect.  Both he and others (including more recent authors*) saw the effect as red-blue.  Despite this, the explanation given by Helmholtz is for a red-green effect.

(*Foundations for a Stereoscopic Cinema, Lenny Lipton;1982. ISBN 0-442-24724-9)

Origin of the name 'Sky Blue'

The title of this page comes from the sophomore philosophy question "Is the sky blue?"; i.e. do you see the same 'blue' that I do?  Since there is no way to hard-wire one brain into another, there is no way to be sure that my sensation of blue is the same as yours.  (This is also a subject for serious philosophers and colour theorists.)   With information to the brain being organized into Red-Green and Yellow-Blue opponent pairing, it is possible that some people could see in complementary colours as if in a colour negative. (See also "pseudonormal" vision.)

Why is the Sky Blue?
    The usual explanation for the sky being blue is dust particles or water droplets scattering blue light more strongly than red light.  This is called Tyndall light scattering.
    This is incorrect because the blue colour also appears in clear air.  The light scattering is a result of forced harmonic oscillation of nitrogen or oxygen molecule dipoles by light.  The interaction is a fourth-power relationship.  Thus extreme blue is scattered 16 times more efficiently than extreme red.  Since the interaction is with linear molecules (dipoles), the scattered light is seen to be polarized tangent to the sun by the observer.  This is called Rayleigh light scattering.

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