Rods can act as light detectors even in extremely low levels of illumination but are ineffective-they are known to "saturate"-in bright light. Photoreceptors for night vision are called rods. (Cones are unable to respond to light reliably in dim illumination, however.) The first, the cones, evolved for day vision and can respond to changes in brightness even in extremely high levels of illumination. The human retina can perform its light-detection function in an astounding range of light intensities, from bright sunlight to dim starlight, by relying on two types of light-sensitive cells, or photoreceptors. This phenomenon is known as "dark adaptation," and it typically takes between 20 and 30 minutes to reach its maximum, depending on the intensity of light exposure in the previous surroundings. As time goes by, however, we gradually become able to detect the room's contents. If we go from the outdoors on a bright sunny day into a very dimly lit room, we are hardly able to see our surroundings at first. Rafael Caruso, an investigator in the National Eye Institute's Ophthalmic Genetics & Visual Function Branch in Bethesda, Md., guides us to an answer.
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