In practice, for any bundle of rays mapping an emitter to a detector, the radiance seen at the detector will be diminished by the light which is absorbed along the way or scattered out of the solid angle of the bundle of rays reaching the detector. Radiance is a conserved quantity in an optical system so that radiance measured as watts per unit area per unit solid angle incident on a detector will not exceed the radiance at the emitter.
Often, as in the example above we use radiance of a source to mean the radiance averaged over a finite sized aperture and over some solid angle of interest. Strictly speaking, radiance is defined at every point on the emitting surface, as a function of position, and as a function of the angle of observation.
The radiance of a source is increased by increasing its emitted power, by making the emitting area of the source smaller or by emitting the radiation into a smaller solid angle. As shown in Figure 1, the radiance (R) of the source emitting area (A) equals the radiation power (P), which is emitted from A and propagates in solid angle Ω, divided by the area A and the solid angle Ω: R = P / (A x Ω). Since many radiation sources used in laboratories have emitting area in the square millimeters range, the unit of milliwatts per square millimeter per steradian is often used for radiance. The SI unit of radiance is watts per square meter per steradian. An example of brightness perception will be given in the following section. Radiance is often casually called “brightness”, a term also used in photometry to describe the perception of human eyes looking at a light source. These most commonly used units are: (1) Radiance (2) Irradiance and (3) Radiant Flux. The SI System (Système International d’unités) defines six radiometric units, of which three are most commonly used for describing the effectiveness of radiation coupling between a light source and an optical system. Definition of the frequently used Radiometry Units On the other hand, industrial lighting and household lighting products (lamps) and illumination designs are characterized using photometry and its units. It is a quantitative science based on a statistical model of the human visual perception of light (eye sensitivity curve) under carefully controlled conditions.īroadband radiation sources emitting UV-VIS-IR wavelength radiation, such as Energetiq’s Laser-Driven Light Sources (LDLS™), are characterized using radiometry and its units. Photometry is the science of measuring visible radiation, light, in units that are weighted according to the sensitivity of the human eye. In practice, the term is usually limited to the measurement of ultraviolet (UV), visible (VIS), and infrared (IR) radiation using optical instruments. Radiometry is the science of measuring radiation energy in any portion of the electromagnetic spectrum. In addition, we will illustrate how the radiometric terms help in selecting an appropriate light source for a particular application. This Application Note attempts to explain radiometry terms and units, to differentiate them from photometry terms, and to clarify the non-standard terms commonly heard. Finally, photometry terms such as luminance are often misused when discussing radiometry situations. In addition, non-standard terms such as brightness, radiant power, flux, and intensity are often used casually without explanations. To those new to the field, the units and terms, such as Radiance, Irradiance and Radiant Flux, may be unfamiliar. Understanding Radiance (Brightness), Irradiance and Radiant FluxĮvaluation of the performance of a radiation source has to involve radiometry – the measurement of quantities associated with radiation.