That part of the electromagnetic spectrum that the human eye can perceive, between the ultraviolet and the infrared (range = .4 microns to .76 microns).
SPHERICAL VS. ASPHERIC LENSES
HOW THEY’RE USED IN IMAGING APPLICATIONS
Optical Lenses are often categorized by their shape. Two common surface shapes are spherical and aspherical. Each offers unique characteristics that make it suitable for different use cases.