Grin lens (also named Gradient Index Rod Lens) refers to one kind of cylindrical optical Lens whose index of refraction varies gradually along the radial. It has the ability to focus and form real image with plane surface.

When encountering different material in the air, light ray will change its direction due to the different refractive index of the material. The conventional Lens redirects light towards a point of focus by controlling the abrupt change in the index of refraction.

The difference between Grin Lens and conventional lens is that the material of Grin Lens can refract light that spreads along the axis and gradually reduces the index of refraction. In this way imaging light can be smoothly and continually redirected towards a point of focus.