Photometry is the measurement of visible light as perceived by the human eye. Photometry is a name formed by combining the words photo and metry. Photo is a prefix meaning light. Metry refers to the process of measurement.
Light consists of small packets of energy called photons. Light energy is a form of electromagnetic radiation and is the only form of energy visible to the human eye. Light energy travels in waves and is very fast. The sun, stars, lamps, lasers, and hot objects are examples of light...
Energy or particles emitted from a source and moving in the air are called radiation. Radio waves, microwaves, X-rays and gamma rays are types of electromagnetic radiation. Visible light is also a kind of electromagnetic radiation. However, this constitutes only a small part of the electromagnetic spectrum.
Light is a phenomenon that allows living things to see as a result of a beam hitting objects or reflecting directly after it leaves the light source. Light refers to visible light with its widespread use. Objects that emit light on their own are called light sources.