Re: Comparitive visibility of red and green lights

From: andrew Judd (andrewedwardjudd_at_hotmail.com)
Date: 06/19/04 

Date: 19 Jun 2004 03:54:01 -0700

There are a couple of other things that might be relevant here

It has been suggested that the human eye uses the difference in image
intensity found at the retina between red and blue light to focus
accurately. So that focus moves between the red end and the blue end
of the spectrum to automatically focus.

>From the experiments i have done with an arrary of pure red green and
violet diodes at a distance most people will see the red diode more
clearly focused than the green and see the violet quite noticably
unfocused. To make this effect more clearer i placed tiny slivers of
black tape on each diode so it was clear if it were seen perfectly or
less than perfectly.

There are studies that suggest that pure colour light creates problems
for the human eye because it cannot automatically change focus.

Another factor here is that in darkness the human eye tends to become
myopic as it assumes the dark focus of accomodation as it cannot
detect any contrast. Ie there is an active component to focusing an
image on the retina rather than it just being relaxed for distant
vision as was once thought.

This would mean that the dark focus adapted eye would tend to see red
better than green because red is less intensely refracted than green
by the optics of the eye and in the myopic condition just happens to
be better focused **on the retina** than green light which focuses
more in front of the retina than red.

Pilots get a similar problem at night because of myopic dark focus of
accomodation. The USAF has experimented with using glasses at night
to correct this problem without much success so that an
instantaneously appearing object can be seen. Interestingly one
group of US navy pilots (the guys who land at night on carriers) were
found to have no dark focus of accommodation.

Whadyareckon?

Andrew

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