Radiometry and photometry (from light to color)

Radiometry is the science of physical measurements of electro-magnetic quantities such as the energy or power of radiation. It measures the amount of light at each wavelength using the SI unit of power[watt =W] and is concerned with the flow of light through the environment.

Photometry is the name for the science that deals with this psychophysical perception of light. Radiometry only deals with objective quantities which can be measured in standard SI units. On the other hand, the response of the human visual system varies strongly with the wavelength of the radiation.

Because the eye has a different relative sensitivity for radiation at different frequencies, we must translate radiometric quantities to the equivalent photometric quantities. This mapping is described by the luminous efficiency function V. It translates between the radiant power measured in watt[W] and the luminous power measured in lumen[lm]. The units of the function V are therefore [lm/W].


"light는 color다! color도 light다!."
과거 내 머리속을 지배해 왔던 정의 중에 하나 였다. 처음 lighting을 이해하는데 도움을 줄지는 모르지만 위의 글에서도 볼 수 있듯이 오류를 유도 할 수 있는 잘못 된 정의이다. CGI 관점에서 바라 봤을때 자칫 크나큰 오류를 안고 가게 되는 것도 같다.

color space와 light(illumination) space는 서로 다른 공간이며 존재하는 방식 또한 다르다. 하지만 sensors라는 conversion을 통해 연결되는데 그냥 linear하게 전환 되는 것도 아니다. 이런 미묘한 response & transfer functions in terms of human visual perception을 알아내는 것이 foundation of color science의 묘미 중에 하나가 아닐까 생각 한다.