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I'd like to build an infrared tachometer for my hamster's wheel, but I don't want to distract him with a light source. Therefore I want to choose the wavelength of the LED for my hamster not to see it. Being a nocturnal animal, I suspect him of having some IR sensing capabilities.
What spectrum of light does a hamster notice? As a bonus question: are they colorblind as many sources say?
So... First, a quick biology lesson on cones and rods.
Rods are photoreceptive cells that are very light responsive and are responsible for most night vision capability. However, very dim light results in colorless vision and rods don't capture color.
Cones are photoreceptive cells that are bright light responsive and are responsible for most color vision in good light.
Hamsters, it turns out have 97% of their eyes as rod cells. This gives them very good low light capability, which is handy in a nocturnal animal, but poor performance during the day. As a result, hamsters have very little to no color capability, so yes, colorblind. As an additional note, vision from rods tends to be less sharp, and so hamsters also have a fuzzier vision.
Nevertheless, they do have cones, but the type and distribution of the cone cells means that hamsters tend see more into the green/blue/UV end of the spectrum. Infrared is in the opposite direction and so less visible to hamsters.
Oddly enough, since you commented on it, I would have actually expected nocturnal animals to tend more to the ultraviolet end. However, it turns out that the green part of the spectrum is where many nocturnal animals see (hopefully that Google excerpt link will work) and most are blind to blue and red light where the vision falls off.
All of which boils down to: you should be fine with the LED.
From a more technical perspective: IR is a notion for a wide range of wavelenghts. IR vision can be in near-IR or thermal which is on the other end of the range. Near-IR vision still needs the objects to be illuminated by an IR light source, and common IR LEDs are in this range (800-1000nm). And photons in the thermal IR range carry much less energy, so it's more difficult to register them.
Those could be reasons why IR vision has evolved only in some specific species.
Biological night vision rather relies on better utilizing low light intensities, for example by having a reflective tissue layer in the back of an eye, packing rods more densely and optimizing them for low light conditions.
From practical point of view, I construct such a hamster wheel tachometer and I'm happy to share the details, if anyone is interested. The hamster doesn't seem disturbed at all by the IR light and is happily using the wheels for hours every day. Her top speed is about 1m/s (3.6km/h) and currently she runs 10km or more each night.
