25 Jul '13 17:37>4 edits
I have been puzzling for some time over the claims of many websites of how many times the eyes of various nocturnal animals are more sensitive to dim light than the human eye because, when I plug in the figures they give into the physical equations, the numbers don't add up and I don't see how you could make those figures add up without breaking the known laws of physics!
What I hope here is for somebody to either tell me exactly where in my mathematical calculations or assumptions I have gone wrong or confirm that all of those website claims are wildly wrong!!!
For any given particular aperture area of an eye (usually this equates with the pupil area -but I will keep referring it to the 'aperture' here to emphasize the physical rather than the biological aspects of it ) , there must be a fundamental physical limit to how sensitive the eye can be to dim light. That is because light is made of discrete particles called photons and, for a given amount of intensity of light, at any given moment of time, there would not an infinite but rather a finite average number of photons that just happen to be heading exactly towards the aperture of the eye and thus only a finite maximum average number of photons can be entering through the aperture of the eye each second that can then all be detected by the retina. This puts an absolute theoretical upper limit to the sensitivity of dim light of the eye defined by, given the aperture area, the proportion of photons which just happen to be heading towards the aperture that would both enter that aperture, reach the retina, be absorbed by the retina and be detected by the retina.
For example, if there is only one such photon heading towards the aperture per second, then the absolute theoretical upper limit to the sensitivity of that very dim light would be an average one photon detected per second and that cannot be improved on without increasing the aperture area.
Now, I have deduced from the info on a website (specifically, this one: http://www.schoolphysics.co.uk/age16-19/Medical%20physics/text/Eye_/index.html
“...In dim light 5% of the light energy falling on the eye is reflected by the cornea, 35% goes through the cornea and is absorbed by the black pigment on the inner surface of the choroids, 50% is absorbed by the lens and humours, and the final 10% is absorbed by the rods. ...” ) that the human eye, given its typical maximum aperture area, once it has been adapted for dim light by being in darkness for more than half an hour, has about 10% of the theoretical absolute maximum sensitivity of light. This is because, out of all the photons heading towards the aperture area, about 10% are absorbed by the rod cells and I am assuming here that the the proportion that are absorbed by the rod cells that are actually detected by them is close to 100% .
This above means that NO EYE, no matter how good in dim light, providing its aperture is never much more than that of a human eye and assuming the wavelengths it can detect are not much different either, can be much more than 10 times more sensitive to dim light than the human eye.
Now, the problem here that I perceive is this: according to many websites, there are many examples of nocturnal animals with eyes that obviously couldn't have an aperture area much greater that that of the human eye and yet, according to these same websites, their eyes have vastly greater than 10 times greater sensitivity to dim light!!!!
Example:
http://en.wikipedia.org/wiki/Gecko
“....Nocturnal species have excellent night vision; their eyes are 350 times more sensitive to light than the human eye ...”
-well, according to my calculations, for it to be 350 times more sensitive to light than the human eye, The gecko's eye aperture would have to be at least 35 times greater (actually, it MUST be greater than this! ) which means its diameter would be almost 6 times greater in dim light than that of a humans in dim light! I don't think any gecko eye pupil can be that large!
And another example:
http://www.chegg.com/homework-help/questions-and-answers/owls-large-sensitive-eyes-good-night-vision-assume-thatthe-pupil-owl-s-eye-diameter-85-mm--q239098
“...Owls have large, sensitive eyes for good night vision. Assume thatthe pupil of an owl's eye can have a diameter of 8.5 mm (as compared with a maximum diameter ofabout 7.0 mm for humans). In addition, the owl's eye is about 100times more sensitive to light of low intensity than a human eye,allowing owls to detect light ...”
-but, again, even taking account of that greater aperture size (of owl eyes in this case ), I cannot make the numbers add up!
Anyone?
What I hope here is for somebody to either tell me exactly where in my mathematical calculations or assumptions I have gone wrong or confirm that all of those website claims are wildly wrong!!!
For any given particular aperture area of an eye (usually this equates with the pupil area -but I will keep referring it to the 'aperture' here to emphasize the physical rather than the biological aspects of it ) , there must be a fundamental physical limit to how sensitive the eye can be to dim light. That is because light is made of discrete particles called photons and, for a given amount of intensity of light, at any given moment of time, there would not an infinite but rather a finite average number of photons that just happen to be heading exactly towards the aperture of the eye and thus only a finite maximum average number of photons can be entering through the aperture of the eye each second that can then all be detected by the retina. This puts an absolute theoretical upper limit to the sensitivity of dim light of the eye defined by, given the aperture area, the proportion of photons which just happen to be heading towards the aperture that would both enter that aperture, reach the retina, be absorbed by the retina and be detected by the retina.
For example, if there is only one such photon heading towards the aperture per second, then the absolute theoretical upper limit to the sensitivity of that very dim light would be an average one photon detected per second and that cannot be improved on without increasing the aperture area.
Now, I have deduced from the info on a website (specifically, this one: http://www.schoolphysics.co.uk/age16-19/Medical%20physics/text/Eye_/index.html
“...In dim light 5% of the light energy falling on the eye is reflected by the cornea, 35% goes through the cornea and is absorbed by the black pigment on the inner surface of the choroids, 50% is absorbed by the lens and humours, and the final 10% is absorbed by the rods. ...” ) that the human eye, given its typical maximum aperture area, once it has been adapted for dim light by being in darkness for more than half an hour, has about 10% of the theoretical absolute maximum sensitivity of light. This is because, out of all the photons heading towards the aperture area, about 10% are absorbed by the rod cells and I am assuming here that the the proportion that are absorbed by the rod cells that are actually detected by them is close to 100% .
This above means that NO EYE, no matter how good in dim light, providing its aperture is never much more than that of a human eye and assuming the wavelengths it can detect are not much different either, can be much more than 10 times more sensitive to dim light than the human eye.
Now, the problem here that I perceive is this: according to many websites, there are many examples of nocturnal animals with eyes that obviously couldn't have an aperture area much greater that that of the human eye and yet, according to these same websites, their eyes have vastly greater than 10 times greater sensitivity to dim light!!!!
Example:
http://en.wikipedia.org/wiki/Gecko
“....Nocturnal species have excellent night vision; their eyes are 350 times more sensitive to light than the human eye ...”
-well, according to my calculations, for it to be 350 times more sensitive to light than the human eye, The gecko's eye aperture would have to be at least 35 times greater (actually, it MUST be greater than this! ) which means its diameter would be almost 6 times greater in dim light than that of a humans in dim light! I don't think any gecko eye pupil can be that large!
And another example:
http://www.chegg.com/homework-help/questions-and-answers/owls-large-sensitive-eyes-good-night-vision-assume-thatthe-pupil-owl-s-eye-diameter-85-mm--q239098
“...Owls have large, sensitive eyes for good night vision. Assume thatthe pupil of an owl's eye can have a diameter of 8.5 mm (as compared with a maximum diameter ofabout 7.0 mm for humans). In addition, the owl's eye is about 100times more sensitive to light of low intensity than a human eye,allowing owls to detect light ...”
-but, again, even taking account of that greater aperture size (of owl eyes in this case ), I cannot make the numbers add up!
Anyone?