Principles of radiation measurement

P RINCIPLES OF R ADIATION M EASUREMENT

®

RADIATION MEASUREMENT
Much confusion has existed regarding the measurement of radiation. This report presents a comprehensive summary of the terminology and units used in radiometry, photometry, and the measurement of photosynthetically active radiation (PAR). Measurement errors can arise from a number of sources, and these are explained in detail.Finally, the conversion of radiometric and photometric units to photon units is discussed. In this report, the International System of Units (SI) is used unless noted otherwise (9). of sunlight plus the diffuse component of skylight received together on a horizontal surface). This physical quantity is measured by a pyranometer such as the LI-200SA. Unit: W m-2. Direct Solar Radiation is theradiation emitted from the solid angle of the sun's disc, received on a surface perpendicular to the axis of this cone, comprising mainly unscattered and unreflected solar radiation. This physical quantity is measured by a pyrheliometer. Unit: W m-2. Diffuse Solar Radiation (sky radiation) is the downward scattered and reflected radiation coming from the whole hemisphere, with the exception of thesolid angle subtended by the sun's disc. Diffuse radiation can be measured by a pyranometer mounted on a shadow band, or calculated using global solar radiation and direct solar radiation. Unit: W m-2.

RADIOMETRY
Radiometry (1) is the measurement of the properties of radiant energy (SI unit: joule, J), which is one of the many interchangeable forms of energy. The rate of flow of radiant energy, inthe form of an electromagnetic wave, is called the radiant flux (unit: watt, W; 1 W = 1 J s-1) Radiant flux can be measured as it flows from the source (the sun, in natural conditions), through one or more reflecting, absorbing, scattering and transmitting media (the Earth's atmosphere, a plant canopy) to the receiving surface of interest (a photosynthesizing leaf) (8). Terminology and UnitsRadiant Flux is the amount of radiation coming from a source per unit time. Unit: watt, W. Radiant Intensity is the radiant flux leaving a point on the source, per unit solid angle of space surrounding the point. Unit: watts per steridian, W sr -1. Radiance is the radiant flux emitted by a unit area of a source or scattered by a unit area of a surface. Unit: W m-2 sr -1. Irradiance is the radiant fluxincident on a receiving surface from all directions, per unit area of surface. Unit: W m-2. Absorptance is the fraction of the incident flux that is absorbed by a medium. Reflectance and Transmittance are equivalent terms for the fractions that are reflected or transmitted. Spectroradiometry: All the properties of the radiant flux depend on the wavelength of the radiation. The prefix spectral isadded when the wavelength dependency is being described. Thus, the spectral irradiance is the irradiance at a given wavelength, per unit wavelength interval. The irradiance within a given waveband is the integral of the spectral irradiance with respect to wavelength (8). Unit: W m-2 nm-1 . Spectral measurements can be made using the LI-1800 Portable Spectroradiometer. Global solar radiation is thesolar irradiance received on a horizontal surface (also referred to as the direct component

PHOTOSYNTHETICALLY ACTIVE RADIATION
In the past there has been disagreement concerning units and terminology used in radiation measurements in conjunction with the plant sciences. It is LI-COR's policy to adopt the recommendations of the international committees, such as the Commission Internationalede I'Eclairage (CIE), the International Bureau of Weights and Measures, and the International Committee on Radiation Units. The International System of Units (SI) should be used wherever a suitable unit exists (9). Units The SI unit of radiant energy flux is the watt (W). There is no official SI unit of photon flux. A mole of photons is commonly used to designate Avogadro's number of photons...