Solar radiation


The precise measurement of natural solar radiation is an important requirement for atmospheric research, the measurement of the total ozone column and the determination of UV index. Additionally, renewable energy research and the use of artificial sources such as solar simulators require accurate optical radiation measurement data. Gigahertz-Optik GmbH has been an active partner of the solar community since 1986 as both a manufacturer of measuring instruments and a service provider. Some example applications of natural and artificial solar radiation measurement with Gigahertz-Optik GmbH products are given below.

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Traceable calibration of spectroradiometers for measuring solar radiation

The measurement quantity for determining the intensity of optical radiation incident on a reference surface is irradiance, measured in terms of W/m². Spectroradiometers used for atmospheric research must be calibrated for their spectral irradiance sensitivity. Standard lamps are used as the reference for calibrating spectroradiometers. Their spectral irradiance is certified by a calibration traceable to a national metrology institute.

Gigahertz-Optik GmbH has been producing the standard lamp model BN-9101 for spectral irradiance since 1991. 

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The design of this 1000 W quartz halogen lamp was made in collaboration with the Physikalisch-Technische Bundesanstalt, PTB. The FEL lamps manufactured by both General Electric and Osram Sylvania are used. All lamps are examined for spectral anomalies before being calibrated.

Selection before lamp ageing

Graph 1: For lamp A, an absorption in the wavelength range around 280 nm can be seen. It amounts to a maximum of about 5%. Thus, a possible spline interpolation of the data leads to undesired deviations. Such lamps are selected and are not approved as a calibration lamp.

Some noteworthy features of the construction are the thermally stable cementing of the lamp in its holder and the precise setting of the filament to the crosshairs of the accompanying alignment tool.

Since the accreditation of its calibration laboratory for the spectral irradiance in 1993, Gigahertz-Optik GmbH has supplied accredited calibrations, initially certified by DKD-K-10601 and since 2010, by DAkkS certificate D-K-15047-01-00. As part of the maintenance of the calibration standards used within the DAkkS certified calibration laboratory, its standard lamps are regularly calibrated by PTB Braunschweig and their measured data are compared with those of Gigahertz-Optik GmbH. These comparisons ensure the accuracy of traceability.


Determined deviations of spectral irradiance are within the measurement uncertainty of PTB

Graph 2: The determined deviations of the spectral irradiance are within the measurement uncertainty of the PTB.


The standard lamp BN-9101 is used in many single applications, but also serves in round robin comparisons as a transfer standard to compare the institutions involved. References [1] to [6] below cite the important role of the BN-9101 in a selection of solar radiation measurement scenarios.


[1] Adaption of an array spectroradiometer for total ozone column retrieval using direct solar irradiance measurements in the UV spectral range

[2] Calibration and evaluation of CCD spectroradiometers for ground-based and airborne measurements of spectral actinic flux densities

[3] Influence of clouds on the spectral actinic flux density in the lower troposphere (INSPECTRO): overview of the field campaigns

[4] A new actinic flux 4π-spectroradiometer: instrument design and application to clear sky and broken cloud conditions

[5] Photolysis frequency measurement techniques: results of a comparison within the ACCENT project

[6] Aalto University - Instruction Manual of Operating Standard Lamps

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Climatic challenges for outdoor optical radiation measurements

Instrumentation for measuring solar radiation is often subject to extreme climatic conditions as much research is necessarily located high in mountain areas or other hostile environments such as the Arctic and Antarctic regions. For example, the High Altitude Research Station, Jungfraujoch (HSFJG) [1] Switzerland is located 3456m above sea level. It has been the location for solar radiation measurements with both broadband radiometers and spectroradiometers since the 1980’s. One research project undertaken during the months of October and November investigated whether the broadband device WPD-UVA-03 (Gigahertz-Optik GmbH) was suitable as a monitor detector for relatively slow scanning spectroradiometers such as the Bentham DTM300 double monochromator.

The broadband measuring device was exposed to very demanding weather conditions.

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Air temperatures on the Jungfraujoch ranged from + 5 °C to -18 °C with wind speeds of up to 150km/h. In addition, the device had to withstand sporadic icing. Third party comparison work by CMS Schreder concluded that the UV-A broadband meter had sufficient stability even under extreme weather conditions. Thus, it was very well suited as a monitor detector for quality control of spectroradiometers used for global solar radiation. A suitable broadband monitor detector can be used to reduce the inherent measurement uncertainties resulting from the long measuring times of scanning spectroradiometers (typically 2 minutes or more) which are due to short-term events such as passing clouds.


Comparison of solar measurements with BTS2048 UV S and a double monochromator

Comparison of solar measurements with BTS2048-UV-S and a double monochromator.


Conventionally, double monochromator based scanning spectroradiometers have been used for solar UV measurements due to their high scattered light performance. In 2017, Gigahertz-Optik GmbH introduced the weatherproof UV spectroradiometer BTS2048-UV-S-WP for outdoor solar radiation measurements. Its BiTecSensor provides the benefits of a fast, high-resolution array spectroradiometer, but with excellent scattered light rejection combined with the properties of a broadband detector. The weatherproof BTS2048-VL-TEC-WP spectroradiometer is also available for outdoor solar radiation measurements in the visible and near-IR wavelength regions (to 1050nm).


[1] High Altitude Research Station Jungfraujoch

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Measurement of solar irradiance to calculate the UV index

The global solar UV index [1], UVI, is an internationally recognised measure primarily intended to inform the public about UV radiation health risks and sun protection. The UVI uses simple integer values, typically 0 to 11+, to describe the level of solar UV radiation at the Earth’s surface. The potential for damage to the skin and eyes increases and the time it takes for harm to occur will decrease as the UV index value increases. UVI values change continually throughout the day and vary widely depending on location [2] and the time of year. Daily histories and forecasts of UVI are generally reported for maximum values i.e. clear sky conditions at the local solar noon, when the Sun is highest in the sky.

Local UVI can be most accurately determined with measurement data from a suitably configured spectroradiometer.  

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Exposure risk UVI Range
Low 0 – 2
Moderate 3 – 5
High 6 – 7
Very High 8 – 10
Extreme 11 +

UVI Exposure categories 

Absolute solar UV radiation levels on the ground are measured in terms of W/m2, the radiometric measure of irradiance. The CIE erythema action spectrum [3] models the tendency of caucasian skin to sunburn (erythema). UVI is derived from the effective erythema irradiance which is determined by integrating the actual UV irradiance, weighted by the CIE erythema action function. A UVI value of 1 equates to an effective erythema irradiance of 25mW/m2.

CIE 1998 Erythema Action Spectrum

CIE 1998 Erythema Action Spectrum

It is not possible to implement the erythema function very precisely with filter based broadband radiometers due to its wide dynamic range and steep decline in effectivity of more than 3 decades over a narrow spectral range. Consequently, public health and other research applications are best served by spectroradiometric measurements of solar irradiance. The resulting spectral irradiance data, W.m-2.nm-1, can be weighted by the exact erythema function to provide a precise evaluation of the effective erythema irradiance and hence UVI. 

The extremely large intensity difference between the irradiance in the ultraviolet region and longer wavelengths within the solar spectrum is also a challenge for the spectroradiometer itself. Inevitably, some of this visible and near infrared radiation is ‘scattered’ within the instrument and may be erroneously detected as if it were UV radiation. Even seemingly negligible stray light becomes significant when weighted by the erythema function. Therefore, spectral measurements of UV solar irradiance have been made conventionally with double monochromator based spectroradiometers, due to their excellent scattered light rejection specification.

Comparison of solar measurements with BTS2048 UV S and a double monochromator

Comparison of solar measurements with BTS2048-UV-S and a double monochromator. 

However, with its contemporary, CCD-based UV spectroradiometers BTS2048-UV-S (laboratory version) and BTS2048-UV-S-WP (weatherproof version), Gigahertz-Optik GmbH offers a compact and cost-effective alternative. For more general purpose use the XD-9506 broadband radiometric detector with effective erythema irradiance responsivity is also available.


[1] WHO Global Solar UV Index – Practical Guide

[2] WHO Typical UVI values worldwide

[3] ISO 17166:1999(E)/CIE S 007/E-1998, “Erythema Reference Action Spectrum and Standard Erythema Dose”

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Measurement of the total ozone column from direct solar irradiance

Ground level solar irradiance is dependent on various atmospheric parameters such as cloud cover, total ozone and aerosol content. Most of the atmosphere’s ozone is contained in the stratosphere, providing essential protection from the Sun’s harmful UV radiation. Ground-based spectroradiometer measurements of direct solar UV irradiance can be used to determine the Total Ozone Column, TOC [1]. This requires a spectroradiometer capable of tracking the solar zenith angle and measuring spectral irradiance within a narrow field of view. TOC can be determined by applying the Beer-Lambert law in conjunction with the known absorption characteristics of ozone over the 290-350nm wavelength range. Suitable modelling is required to minimise the influences of the other atmospheric attenuators [2].

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The compact BTS2048-UV-S-WP weatherproof UV spectroradiometer is ideally suited for use on sun-trackers to perform direct solar irradiance measurements. During an international inter-comparison [3] of total ozone measurements the instrument was verified with measurements of direct solar irradiance and subsequent TOC evaluations showing deviations of less than 1.5% to most other reference instruments.


[1] M. Huber et al., “Total atmospheric ozone determined from spectral measurements of direct UV irradiance,” Geophys. Res. Lett.22, 1995.

[2] A. Vaskuri et al., “Monte Carlo method for determining uncertainty of total ozone derived from direct solar irradiance spectra: Application to Izaña results” 

[3] R. Zuber et al., “Adaption of an array spectroradiometer for total ozone column retrieval using direct solar irradiance measurements in the UV spectral range”, Atmos. Meas. Tech 2017