Shortwave Instrumentation

Instruments

The pyranometer is an instrument used to measure direct and diffuse shortwave radiation arriving from the whole hemisphere (usually a complete sky). Some pyranometers can be inverted to measure the reflected shortwave radiation from a surface. In addition, with a shading device to obstruct the direct solar beam, it can measure the diffuse radiation. Examples of widely used pyranometers are those manufactured by Eppley, of which there are two types, and a silicon cell pyranometer manufactured by LI-COR. A limited region of the shortwave spectrum, specifically photosynthetically active radiation (PAR) (0.4-0.7 µm), can be measured with a specialized pyranometer called a quantum sensor. A photo-diode quantum sensor is manufactured by LI-COR.

a) Epply Precision Spectral Pyranometer, PSP

This instrument is considered to be the most accurate radiometer produced commercially for the measurement of global sun and sky radiation, totally or in defined wavelength bands. It can be inverted to measure reflected shortwave radiation (from which the albedo can be calculated).

The PSP has a circular multi-junction thermopile of the plated (copper-constantan) wire wound type which is temperature compensated in order to get a response that is independent of ambient temperature. The temperature sensed by the detector is nearly linear with the flux density of incident radiation. The instrument is supplied with a pair of removable precision ground and polished concentric hemispheres of Schott optical glass. The inner hemisphere is transparent to a wavelength of 0.285-2.800 µm. The outer dome can be replaced by Schott glass hemispherical filters which transmit within specified bandwidths. Desiccant is placed in the side of the instrument to absorb humidity inside the glass domes. Measurements are reported in Wm^-2.

Sample Data

b) Epply Black & White Pyranometer

This pyranometer also measures global, reflected, and diffuse shortwave radiation. The detector is a differential electroplated (copper-constantan) thermopile with the cold junction receivers whitened and the hot junction receivers blackened. The black absorbs and the white reflects radiation, developing temperature differences. The greater the temperature difference that develops between the white and black areas, the greater the flux density. Measurements are reported in Wm^-2.

c) Li-Cor Silcon Chip Pyranometer ( LI - 200SA )

Silicon photo diode technology has made possible relatively low cost pyranometers of reasonable accuracy. The LI-COR pyranometer is cosine corrected up to 80 angle of incidence. (Cell response deviates from Lambert's cosine law. This can be corrected for, in part, by the use of a plastic diffuser, anti-reflection coating or by texturing the surface (Iqbal, 1983).)  The response of the photo diode sensor (Figure 1) is not ideal, covering 0.400-1.100 µm instead of a spectral response in 0.280-2.800 µm range. This does not cause serious error provided the photo diode is used only for solar radiation and not under conditions of altered spectral distribution. Thus, they should not be used to measure reflected radiation, vegetation-transmitted shortwave radiation or artificial light sources such as metal halide lamps used in greenhouses. Although the accuracy is not as high (due to spectral characteristics) as the Eppley pyranometer it is still adequate for most purposes and the LI-COR instrument's response is virtually instantaneous. Measurements are reported in Wm^-2.
 

Figure 1 - LI-200SA LI-COR pyranometer spectral response curve.

d) Li-Cor Quantum Sensor ( LI - 190SA )

A silicon photo diode with an enhanced response in the visible wavelengths is used as a PAR sensor. This type of sensor was selected because it approximates the photosynthetic response of plants (Figure 2) A visible bandpass interference filter in combination with color glass filters is mounted in a cosine corrected head. Error calculations indicate that under sun-and-sky radiation, and various natural or artificial light sources, the relative errors in measurement are less than ±5%. Therefore, this sensor can be used within or inverted over canopies and in greenhouses, controlled growth chambers and confined laboratory conditions. Generally this instrument measures hemispherical incoming radiation, however, when placed within a canopy the instrument acts as a point source detector. Measurements are reported in µmol m^-2 s^-1.

Figure 2 - LI-190SA LI-COR quantum sensor spectral response curve

e) Li-Cor Line Quantum Sensor ( LI-191SA )

A Line Quantum Sensor  measures spatially over a one meter length. The spectral response of this instrument is similar to the ordinary quantum sensor, however, the cosine correction of the line quantum sensor is not nearly as good as that of the LI-190SA (Figure 3.) Measurements, for both instruments, are reported in units of µmol m^-2s^-1. (Full sun plus sky photosynthetic photon flux density is approximately 2000 µmol s^-1m^-2.)

Figure 3 - Performance of instruments (LI-COR pyranometer, quantum sensor and line quantum sensors) with cosine corrections with a true cosine corrected instrument.