|
I suspect the illusion is due to "lateral inhibition" if my memory of retinal physiology is correct. Neighbouring visual receptors mutually inhibit each other when active. When some cells are not stimulated ie are in the dark, they cease to inhibit their stimulated neighbours, thereby increasing the activity/output of the stimulated cells. This accentuates the perception of edges. If you look at a sheet of alternate dark and light bands, you will see that the edges of the bright bands look brighter than their middles and the edges of the dark bands look darker than their middles.When you look throught the tube, the receptor cells receiving the light from the wall are not inhibited by those that are focused on the dark tube, making the wall appear brighter.
What does it mean for eyepieces? I'm not sure, but I imagine the effect would be operating all the time anyway - ie bright bits appear bright against a dark background. But it maybe part of why smaller fov eps seem to give a stronger contrast than wider fields.
|