Hämäränäköa

Hämäränäkö, also known as scotopic vision, is the ability of the eye to see in low light conditions. This type of vision relies on specialized photoreceptor cells in the retina called rods. Unlike cone cells, which are responsible for color vision and function best in bright light, rods are highly sensitive to light and are capable of detecting very dim illumination. However, rods do not distinguish between colors, which is why vision in very low light appears in shades of gray.

The transition from daylight vision (photopic vision) to dim light vision (hämäränäkö) is not instantaneous. It involves a process called dark adaptation, where the rods gradually increase their sensitivity to light over a period of time, typically taking around 30 minutes for full adaptation. Conversely, when moving from darkness to bright light, vision initially appears washed out as the rods become saturated and less effective, with cones gradually taking over.

Hämäränäkö is crucial for activities that occur at dawn, dusk, or during the night, such as nocturnal animal navigation or observing the night sky. The human eye's ability to adapt to low light conditions is a remarkable biological feature that allows us to perceive our surroundings even when visible light is scarce. Impairments in hämäränäkö can be an indicator of certain medical conditions, such as vitamin A deficiency or retinitis pigmentosa.