Radiometry is the science of measuring radiation energy in any portion of the electromagnetic spectrum. In practice, the term is usually limited to the measurement of ultraviolet (UV), visible (VIS), and infrared (IR) radiation using optical instruments. Broadband radiation sources emitting UV-VIS-IR wavelength radiation, such as Energetiq’s LDLS, are characterized using radiometry and its units.
The SI unit of radiance is watts per square meter per steradian [W·m-2·sr-1]. Since many radiation sources used in laboratories have emitting area in the square millimeters range, the unit of milliwatts per square millimeter per steradian [mW·mm-2·sr-1] is often used for radiance. 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).
Figure 1. Radiance (R) of source is the Power (P) emitted from the source emitting Area (A) and propagated in the Solid Angle (Ω).
The steradian [sr] is the SI unit for measuring solid angles, defined by the solid angle (Ω) that projects on the surface of a sphere with a radius of r, having an area (A) equal to r2 (Ω = A/r2 = r2/r2 = 1 [sr]). It describes angular spans in three-dimensional space, analogous to the way in which the radian [rad] describes angles in a two-dimensional plane. The total solid angle for a point in space is 4π steradians.
Figure 2. Steradian [sr] is a unit for measuring solid angles (Ω) defined by the solid angle that projects on the surface of a sphere, with a radius of r, having an area of A = r2 (Ω = A/r2 = r2/r2 = 1 [sr]).
Irradiance is the radiometry term for the power per unit area of electromagnetic radiation incident on a surface. The SI unit for irradiance is watts per square meter [W/m2], or milliwatts per square millimeter [mW/mm2]. Irradiance is sometimes called “intensity.”
If a point radiation source emits radiation uniformly in all directions and there is no absorption, then the irradiance drops off in proportion to the distance squared from the source, since the total power is constant and it is spread over an area that increases with the distance squared from the radiation source. To compare the irradiance of different sources, one must take into account the distance from the source. A 50cm distance is often used for such measurements.
Irradiance is a useful measure for applications where power must be delivered to large areas. For example, illuminating a classroom or a football field is primarily a question of delivering a certain number of watts per square meter. This can be achieved by using a single high power source. However, since irradiance does not depend on solid angle, multiple sources can be combined, illuminating the walls or the field from different angles.
Mufid Alfaris is Energetiq’s Laser-Driven Light Source (LDLSTM) applications engineer. He provides solutions to couple LDLS with a variety of optics to ensure the optimized use of LDLS. He also enjoys assisting with testing and data collection in our laboratory. An interest in the universe led him to study physics and astronomy. He is passionate about learning everything. If not working on a technical solution, Mufid can be found reading a new book or technical paper. His reading hobbies vary from technical papers to science fiction, mystery, and adventure books.
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