Rod cells are one of two types of photoreceptors in the retina of the eye. They are responsible for scotopic vision, the ability to see in dim light. Rod cells are very sensitive and can detect even the smallest amounts of light. They are insensitive to differences in colour, and are not able to distinguish between different wavelengths of light. Rod cells are only active in low light and become inactive in brighter light. They are essential for night vision and peripheral vision. The human eye contains approximately 100 million rod cells, which makes up about 90% of the photoreceptors in the retina.