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How It Works

Measurement Principle CSR-Sensor

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The Circumsolar Radiation

Direct sunlight scattering is caused by the presence of water vapor and aerosols in the atmosphere. The scattering causes a fraction of the sunlight to divert from the direct path through the atmosphere (Fig. 1).

Fig. 1: High circumsolar ratio due to a high content of water vapor and ice particles in the atmosphere. In this photo, the CCD camera sensor is oversaturated, which makes the brightness difference between the circumsolar region and the rest of the sky well noticeable (Photo: Jaus, Val Thorens, France).

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 An observer on the earth sees this part of the sunlight not coming from the sundisc, but from its surrounding. By convention, the area between the outer rim of the sundisc (+/- 0.26°) and the DNI definition angle of 2.5° is considered "circumsolar" (Fig. 2).

 CSR Skizze1

Fig. 2: Definition of circumsolar radiation.  The ratio between the fraction of the DNI radiation not coming from the sun disc is often referred to as "cirumsolar ratio CSR".

The circumsolar radiation is measured with common pyrheliometers and is therefore counted as direct radiation DNI for the power rating of solar power plants, but many high concentration CSP or CPV power plants have much smaller acceptance angles. Therefore most of the circumsolar radiation can not be used in those systems. Even in conditions with more than 200 W/m2 direct solar radiation DNI, up to 40% of the radiation can be counted towards circumsolar radiation. More details on typical CSR scenarios can be found in the publications of Neumann et. al., see end of this article.

Measurement Principle of BPI-CSR1

The BPI-CSR1 measures the circumsolar radiation with an array of two similar sensor-channels: One channel serves as a direct measurement of the circumsolar radiation, the other channel is the reference channel to determine the circumsolar radiation (Fig. 3).

CSR Sensor Principle Fig. 3: Operational Principle of the CSR1 sensor. The sensor is based on an array of two detectors: One channel, the CS-channel, measures the Circumsolar Radiation (CS). It has a special collimator with a CS baffle that filters out the direct sunlight. The Reference channel is used to calculate the circumsolar ratio (CSR), which relates the circumsolar radiation to the overall DNI available.  

For more details on the definitions, measurement principles and a comparison to other possible measurement technologies, the article of S. Wilbert: "Circumsolar Radiation and Beam Irradiance Measurements for Focusing Collectors" is very helpful. Download file Download Abstract.


For more details on this topic, the following papers may be considered:

C. Gueymard: Download file Spectral Circumsolar Radiation Contribution to CPV , Proceedings of CPV-6, Freiburg, Germany. 

D. Grether et al. Download file "The Effect of Circumsolar Radiation on the Accuracy of Pyrheliometer Measurements of the diret solar radiation" , Lawrence Berkeley National Laboratory 1981.

A. Neumann, A. Witzke, S. A. Jones und G. Schmitt, "Representative terrestrial solar brightness profiles", Journal of Solar Energy Engineering, vol. 124 (2), pp.·198-204, 2002.

A. Neumann, B. von der Au und P. Heller, "Measurements of circumsolar radiation at the Plataforma Solar (Spain) and at DLR (Germany)", Proceedings of Solar 98: Renewable Energy for the Americas, Albuquerque, New Mexico, USA, pp. 429-38, 1998.

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Last Updated on Wednesday, 13 March 2013 14:31