Testing indicates that a cat's vision is greater at night in comparison to humans, and inferior in daylight[citation needed]. Cats, like dogs and many other animals, have a tapetum lucidum, which is a reflective layer behind the retina that sends light that passes through the retina back into the eye. While this improves the ability to see in darkness, it appears to reduce net visual acuity, thus detracting when light is abundant. In very bright light, the slit-like iris closes very narrowly over the eye, reducing the amount of light on the sensitive retina, and improving depth of field. Big cats have pupils that contract to a round point. The tapetum and other mechanisms give the cat a minimum light detection threshold up to seven times lower than that of humans. Variation in color of cat