MadSci Network: Anatomy

Re: Human vision in the near-infrared realm

Date: Tue Jul 4 12:47:38 2000
Posted By: Tom Stickel, Grad student, Optometry, Indiana University School of Optometry
Area of science: Anatomy
ID: 962431578.An

  The human eye is capable of detecting electromagnetic radiation in a 
range from about 380 nanometers (nm) of wavelength to 700 nm.  The 
radiation between 380 and 700 nm is thus called visible light. Wavelengths 
around 400 nm appear violet.  Therefore, wavelengths shorter than 400 nm 
are in the "ultra"-violet (UV) range, and are out of the range of 
detection of the human eye.  
  Wavelengths around 700 nm appear red.  So, beyond 700 nm, the radiation 
is referred to "infra"-red, or IR.  Anything that gives off heat is giving 
off some form of infrared radiation.
  The reason we can see colors at all is because we have three separate 
proteins in the back of the eye, each specialized for a specific 
wavelength.  These proteins reside in cells called cones.  Each cone has 
only one type of color protein in it.  At the 380 nm wavelength the S cone 
(S for short wavelength) is the only one detecting light, because 400 nm 
is out of the range of the other two cones.  Likewise, at 700 nm, only the 
L (L for long wavelength) cone is operating, because the other two cones 
don't detect light of that wavelength.
  Natural variation being what it is, some people will probably have 
slight variations in their L cones that give them better color vision than 
others in the range of red colors.  So it seems likely that some people 
will be able to see longer wavelengths than others. Likewise, some people 
can see shorter wavelengths than others.  But either way, as long as it 
can be seen, it's visible light, and therefore not infrared (or UV on the 
short wavelength side).  Physically, there is very little difference 
between 700 nm light and 725 nm light, except humans can detect one and 
not the other.
  There are arbitrary cutoffs for where visible reds end and near IR 
begins. For instance, Steven Schwartz in his book "Visual Perception" 
states that the range of visible light is between about 380 and 700 nm.  
Notice he says 'about' because there is variation between people at the 
ends of the ranges. As I said before, as long as it can be seen, it's not 
technically IR, it's still red.  
  Anyway, due to natural variation, some people can probably see a tiny 
ittle way into the near IR range as given by some arbitrary definition.  
But that doesn't give them any special powers or capabilities.  It just 
lets them see reds that are very slightly dimmer than you or I could.  I 
don't know what the record is for how far into the near IR range anyone 
has detected red light, but it's probably only a few nm farther than the 
population mean.
  So that's the story with near IR and your eyeballs.  Hope that helps,


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