Supplement 1.7: Radiation quantities and radiometry      (1/5)

The SEOS learning units often use similar-sounding radiation quantities such as irradiance, radiance or radiation intensity. Their different meanings are not always obvious. This also applies to the methods used to measure them, known as radiometry. Some quantities are referred to differently in astronomy or nuclear physics than in physical optics, which is our focus here.

In addition, some units have similar names in different languages but do not mean the same thing, which can be very confusing. For example, intensity in English corresponds to radiant power (watts) in a solid angle (steradian), but the German word Intensität is not defined in physics and is only used colloquially; intensity in German is Strahlstärke.

Unlike radiant quantities, physiological quantities refer to the subjective perception of light by the eye and can be explained by spectral visual acuity: in blue and green, the same physical radiant power is perceived as having different brightness levels, which is very important in lighting technology. Examples of such quantities from lighting technology or photometry are the lumen for the visual brightness of a lamp and the lux for its illuminance on a surface. This is of rather minor importance in physical optics and will therefore not be considered further here.

The radiant power

Radiant power, also known as radiant flux, corresponds to the power of electromagnetic waves or photons present in all directions. This can be the emission from a light source, radiation falling on an object, or even radiation from distant sources present at a specific point in space.

Formula symbol: $\Phi$ or P; we use $\Phi$
Unit of measurement: Watt, $\left[\Phi \right]=\text{W}$

Examples

• The total radiation emitted by the sun into space has a power of 387.5·10²⁴ W.
• The radiation from the sun that reaches us at midday on a cloudless summer's day is an impressive 1000 W/m².
• The radiant power of an LED lamp that consumes 9 W of electrical power is approximately 4 W. This corresponds to an efficiency of 40 to 50%, which is much higher than the efficiency of an incandescent lamp, which is only about 5% because it emits mainly in the infrared range.

Measurement of radiant power

The radiant power of spectrally broad light sources can be measured directly with thermal detectors, as their sensitivity does not depend on wavelength. Absorption on a blackened surface is independent of wavelength and can therefore be used advantageously for this purpose. This is where the wave model of light comes into play, in which wavelength plays no role in intensity (or radiation power). The heat generated by absorption changes the properties of the detector, for example its electrical resistance, from which the radiation power is calculated. You can find more information about thermal detectors in a further supplement.

In quantum detectors, which include semiconductor photodiodes, for example, the photon energy and thus the wavelength are decisive for the measurement and must be taken into account when measuring radiant power. For power measurement of narrow-band emitters such as lasers, the spectral sensitivity of the photodiode is taken from the data sheet or determined by calibration for precision measurements. For broadband radiation sources, the entire spectrum must be measured with a spectrally calibrated spectrometer and integrated over all wavelengths; spectrally independent thermal detectors are advantageous here.

Both types of detectors can also be used to measure the temperature of blackbody emitters. They are then referred to as pyrometers.