MadSci Network: Neuroscience

Re: Why can't humans see in color at night?

Date: Tue Jan 25 12:32:01 2000
Posted By: Tom Stickel, Grad student, Optometry, Indiana University School of Optometry
Area of science: Neuroscience
ID: 948673550.Ns

  The reason we can't see colors at night is pretty interesting.  It all 
comes down to the fact that humans really have two different visual systems 
at work in our eyes.  Seems a little crowded inside our eyeballs, doesn't 
  Our two different visual systems are built to do two different things.  
One is built to work in the daytime or else when it's bright out at night.  
The other is built to work at night, or else when the lights get very dim.  
First I'll talk about how our dark system works, then I'll move to how the 
bright system works (and why the bright system sees colors but the dark 
  The night time system is called the 'scotopic' system.  I think 
'scotopic' is Latin for 'It's really dark in here' (just joking).  There 
are cells called rods in the back of your eye that make this system work.  
The rods catch light and tell your brain that you saw something.  Anyway, 
the rods aren't built to see colors.  As an aside, rods are also not built 
to see very well.  Try reading a letter chart at night.  Your vision will 
be a lot worse than 20/20, that's for sure.  But our night time system is 
built to make us really sensitive to dim lights, because our rods are very 
sensitive to small amounts of light.
  The reason we can see colors in the daytime is because our daylight, or 
'photopic' system, is at work.  The cells in your eye that detect light for 
your photopic system are called cones.  They are built to see really fine 
details, so that's why you see 20/20 in the daytime.  There are three 
different kinds of cones.  One kind sees red light, one sees blue, and one 
kind sees green.  All colors are made up of a combination of these three 
colors.  So, when you see a color, your three kinds of cones get together 
and decide how much of red, blue, or green is in it, and then your brain 
puts that information together, and you see colors.
  So going back to the night system, we only have one kind of rod. Rods 
don't care what color something is, they only care how dim or bright it is.  
So they don't tell your brain anything about color.
	Hope that this is helpful.  If you need any more info, feel free to 


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