Light, or the visible part of the electromagnetic radiation spectrum, is the medium through which human beings receive a major portion of environmental information.

Evolution has optimised the human eye into a highly sophisticated sensor for electromagnetic radiation.

Joint performance between the human eye and visual cortex, a large part of the human brain, dwarfs recent technical and scientific developments in image processing and pattern recognition. In fact a major part of the information flow from external stimuli to our brain is transferred visually.

Photometry deals with the measurement of this visible light energy.

However, optical radiant energy not only encompasses visible 'light' but radiation invisible to the human eye as well.

The term optical is used because this radiation follows the laws of geometrical optics.

Radiometry deals with the measurement of all optical radiation inclusive of the visible portion of this radiant energy

http://www.light-measurement.com/spectrophotometer/