Cats cannot see in absolute darkness. If zero light enters the eye, photon absorption cannot occur. However, their night vision sensitivity requires only one-sixth the illumination that a human requires to navigate space comfortably.
This low-light performance stems from two specific physiological adaptations. First, feline pupils are vertical slits operated by muscular calipers that can dilate up to 135-fold in surface area, flooding the posterior chamber with ambient photons. Humans dilate their circular pupils only about 15-fold by comparison.
Second, cats possess the tapetum lucidum, a specialized retroreflective layer of tissue positioned directly behind the retina. Composed of organized arrays of zinc and guanine crystals, the tapetum lucidum acts like a miniature mirror. Unabsorbed light passes through the photoreceptor layer, hits this crystalline surface, and bounces back across the rods a second time. This gives the rods a dual chance to register the incoming photon. This biological mechanism produces the eerie green-gold eye shine visible when a flashlight or vehicle headlamp catches a cat at night.
