MadSci Network: Anatomy
Query:

Re: How come it hard to drive with an interior car light on??

Date: Mon Jan 15 15:35:32 2001
Posted By: Kenneth Mitton, Post-doc/Fellow, Molecular Development Retina, Cataract, Dept Ophthalmology / U. Mich
Area of science: Anatomy
ID: 978225930.An
Message:

In the bright interior of the car with the dome lite on, you have the exact 
same problem as the camera, and a new problem as well.....
Firstly, you force your eye to close its pupil adjusting for the light. This 
lets less light into the eye, and thus looking out at night, you see 
nothing, 
or very little. Secondly, in the light of the car, or in sunlight, if you 
can 
see colors, then you are using your color sensitive cone-vision. Cone cells 
of the retina come in short, medium and long spectral sensitivity bands to 
give you color vision. But, cones are not very light sensitive compared to 
rod cells (rods give us only black and white, gray scale). Rod-vision is 
more light sensitive, and that is why we cannot see the colors in dim 
lighting, but we can see at night with our rod-vision. In bright light the 
rod cells are so activated that they are "bleached" and not giving 
information. Conversely in the dark, our cone cells are not being activated 
as the light intensity is below the threshold required to give visual 
signals through the optic nerves. So, in the bright light of the car...you 
have bleached out your nite-vision rod cells, and closed down your pupil 
size to let in less light...therefore you cannot see outside. That is why in 
aircraft cockpits, instruments are often red-lit. The red light activates 
red sensitive color vision, but the rod cells are not as sensitive to red 
light and are not as severely bleached out, therefore the pilot can see 
instruments and outside. Some cars now come with dome lite filters can be 
used to give RED interiors. http://
ken.mitton.com/ern



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