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How It Works

Measurement Principle IT-Sensor

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Spectral Changes

The solar spectrum changes frequently during the day: The red glare of the sun during sunrise and sunset are well known. During these times, the path of the sunlight through the atmosphere is very long. The blue light is much stronger diffracted and absorbed by the atmosphere then the red light (red and blue are corresponding to short and long wavelengths). The uneven diffraction and absorption between blue and red light causes the visible shift of the sunlight color.

But not only during sunset and sunrise, but also during the day the sunlight's spectrum is constantly changing. The relevant absorption and diffraction processes depend a lot on the current composition of the atmosphere, e.g. the fraction of Ozone, water vapor, and microscopic dust and ice particles ("Aerosols"). Also relevant is the sun zenith angle which determines the path length of the sunlight through the atmosphere ("relative Air Mass (AM)").

The composition of the atmosphere is not only depending on the time of the day, but also on the annual season and the geographic location. Therefore, the spectrum of the sunlight is highly spectrum dependent. This can be also be observed by the human eye, as is demonstrated in the following two photos, one taken close to the sea, one in a high altitude mountain environment:

pointedelamasse bluerich web sandiego redrich web
Abb. 1: Blue-rich spectrum in high altitude environments with low air mass and aerosol content (Photo: JJ, Point de la Masse, France). Abb. 2: Red-rich spectrum in a low altitude location with high aerosol loading from adjacent deserts.
(Photo: JJ, Coronado Beach, San Diego, USA)

The solar spectrum has significant influences on the performance of solar power plant, a good monitoring setup of the solar spectrum is an important part of the operational instrumentation of a solar power plant. Black Photon Instruments offers robust solutions based on isotype cells for this purpose.

Measurement Principles of Isotype Sensors 

IT-Sensor Principle v01

The isotype sensor is used to measure spectral variations of the solar irradiation. It can be used to measure the spectral variations of the natural sunlight or those of solar simulators that are used to characterize solar modules.

This type of characterization is of particular importance for multi-junction solar cells. In these cells, several solar cells are manufactured on top of each other and each subcell is specialized on a particular region of the solar spectrum. Due to the manufacturing process of depositing layers on top of each other, all subcells are connected in series and the subcell with the lowest  resulting current determines the current of the whole stack. 

An isotype cell has a similar composition, but here only one of the subcells is electrically active, as shown in the picture for the bottom cell ("bot") of a triple-junction cell. The other subcell-layers act as optical filters so that the active subcell receives the same spectral irradiation as the subcell in a "normal" multi-junction cell. In a measurement setup, for every subcell of a multi-junction solar cell a corresponding isotype cell is manufactured. This enables the currents of all three subcells to be measured independently.

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Last Updated on Tuesday, 16 October 2012 17:18