I have done some very minor research -- I'd call it 'reading' rather than research though as it wasn't inything serious.
I tend to agree with RZL36 though, based on what I remember.

It isn't what they DO see only, but also what they DONT see.
Many snakes like Garters see reasonably well in normal daylight, like we do. What they DONT see, as I understand it, is dark blue and dark red lighting, which is why the recomended lightbulbs used for night heating are these colors.

Also as RZL36 mentioned, boids and pythons DO have infrared pits, which provide the majority of their visual perception. Their eyesight is quite poor.

Back to Garters, I don't believe they have ANY infrared spectrum, or 'heat' detectors, but I could be wrong there. They might actually use their tongues for this..

Anyone?