Radiometry & Photometry

Editor : Optomon

 Until now, I explained about sunlight but from now on, I am going to talk about visible light and its measurement method. Among our products, OLED & Flat Panel Display Test System(M-Series) and LED & Optical Experiment Test System(L-Series) use measurement technologies for visible light. So, I would like to explain about definitions and experiment methods.

 Before going into the details, we need to summarize background knowledge. I believe that many of you have seen names of equipment which seems similar but seems different at the same time such as Radiometer, Spectroradiometer, Photometer, Spectrophotometer and Colorimeter. I was quite confused as well at first. I will try to summarize them as simple as possible.
(I referred to information from Monitor4U ( and Standard System of Units of KRISS (Korea Research Institute of Standards and Science)

 Regarding name of Radiometry ‘radio’ means ‘of or relating to electric currents or phenomena (as electromagnetic radiation)’ and ‘?metry’ means ‘science of measuring’. Thus, radiometry measures Optical Radiation of light. To be more specific, it measures Electromagnetic Radiation in the range of 3 x 1011 Hz ~ 3 x 1016 Hz corresponding wavelengths ranging from 10nm to 1,000 ?m and this range includes UV, Visible, Infrared.

Fig 1. Electromagnetic Radiation

 Photometry measures light visible in Electromagnetic Radiation Energy. In other words, it measures Visible Light only whose range is 380 ~ 780nm, so UV or IR which is not visible to human eyes. Thus, difference is the fact that Radiometry can measure characteristics of larger range of light than Photometry can do.

Memory of Candle: Birth of Candela and its evolution

 History of standard for measuring brightness of light goes all the way back to early 19th century. In those days, measurement was done by comparing luminous intensity between standard candle and a certain light source. Since candle’s flame was a standard of brightness, ‘candle’ became a unit. A while ago, ‘candle power’ was used which was originated from candle as horse power is being used to express power of vehicle.

 However, standard candle was gradually replaced by oil lamp and method of using platinum point blackbody was suggested in 1909. This method was accepted as a standard by CIE (International Commission of Illumination) in 1921, then accepted by CGPM (General Conference on Weights & Measures) in 1948 and named as “candela” which is Latin word. CGPM defines Candela as below.

“The candela is the luminous intensity, in the perpendicular direction, of a surface of 1/600000 square meter of a blackbody (full radiator) at the temperature of freezing platinum under a pressure of 101325 newton per square meter.”

 By defining candela, consistent measurement became possible internationally but there were various troublesome issues. In the middle of 1950’s, suggestion has been made to remove complex issues by defining candela in conjunction with optical power (watt). Finally, the new definition of candela was accepted by CGPM in 1979 at 16th CGPM. The new definition is as below.

“The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 × 1012 hertz and that has a radiant intensity in that direction of (1/683) watt per steradian.”

 ‘cd’(lower case) must be used as a symbol.

Fig 2. Definition of candela(from KRISS)

 Anyway, in this way, Photometry was able to be defined by units of Radiometry. You may understand relationships more easily by comparing terms used in Radiometry and Photometry.

Photometric unit = k(λ) x Radiometric unit
k(λ) = 683 V(λ)
V(λ) : Standard relative luminous efficiency


To be continued on April’s issue…