Solar Radiometers: Spectral Selectivity vs. Spectral Error

The ISO 9060 standard for solar radiometers underwent a significant update in 2018, introducing major changes in the classification of solar radiation measurement quality.

Solar Radiometers: Spectral Selectivity vs. Spectral Error

Image Credit: OTT HydroMet - Solar Energy

A cursory look into ISO 9060:2018 would suggest this is primarily a renaming of the radiometer classifications introduced in the original 1990 version, but a number of specific details within the standard are causing confusion within the solar industry.

This article explores the implications of the updated standard and highlights the difference between spectral selectivity and the newly introduced spectral error.

ISO 9060 is titled ‘Solar energy – Specification and classification of instruments for measuring hemispherical solar and direct solar radiation.

This standard includes definitions detailing the use of a pyranometer for measuring global horizontal or global tilted irradiance (GHI and GTI) and, when shaded, diffuse horizontal irradiance (DHI).

The standard also defines the use of a pyrheliometer for measuring direct normal irradiance (DNI).

Changes in the Guidance

There are a number of notable changes in pyranometer specifications between ISO 9060:1990 and ISO 9060:2018 - the latest update. The 2018 version defines the newly introduced Class A as ‘roughly corresponding’ to the 1990 Secondary Standard, comparing Class B to First Class and Class C to Second Class.

‘Roughly’ is an appropriate term in this instance, as performance parameters and testing requirements exhibit a number of differences between the 1990 and 2018 standards. There is some uncertainty surrounding field irradiance measurements according to ISO 9060 pyranometers, however:

Table 1. Source: OTT HydroMet - Solar Energy

ISO 9060:1990 ISO 9060:2018
Class C
Second Class Spectrally Flat Class C
First Class Spectrally Flat Class B
Secondary Standard Spectrally Flat Class A

 

Understanding the differences between the 1990 and 2018 versions of the ISO 9060 standards can be challenging, particularly around the issue of spectral response, error, sensitivity and selectivity.

In general, the 2018 update has introduced a new parameter designed to better characterize the spectral properties of a radiometer in terms of how it reacts to various parts of light - particularly photons with different wavelengths.

The Spectral Error (different from the Spectral Selectivity defined in the 1990 version) considers the key fact that the composition of sunlight - its spectrum - differs depending on the time of day. This is considered more relevant when applying solar measurements.

The new 2018 version offers an improved reflection of the actual wavelength range of sunlight versus its predecessor, while the introduction of the new Class C on the entry-level allows the standard to cover well-built photodiode radiometers.

ISO 9060:1990: Spectral Selectivity

The ISO 9060 standard specifies the minimum performance requirements for pyranometers and pyrheliometers in three classifications when these are employed for solar energy purposes.

Specified parameters are:

  • Response time
  • Zero offsets
  • Non-stability
  • Non-linearity
  • Directional response (not applicable to pyrheliometers)
  • Spectral selectivity
  • Temperature response
  • Tilt response

Approximately 97% to 99% of all the solar radiation (GHI) arriving at the Earth’s surface falls in the wavelength range between 300 nm and 3000 nm (0.3 µm and 3.0 µm), though this does depend on sky conditions to some degree.

Much of this light lies outside the wavelengths that are viewable with the human eye - the range of visible light spanning from 380 nm to 800 nm.

A solar energy radiometer should ideally exhibit a flat response across a wide spectral bandwidth, allowing it to measure all available incoming solar energy independent of the types of solar collectors or PV modules used.

The primary wavelength range used by photovoltaic materials lies between 350 nm and 1500 nm (0.35 µm and 1.5 µm). ISO 9060:1990 defines this ’flatness’ of response as ‘spectral selectivity’ and regards this as the deviation from the mean response ranging from 350 nm to 1500 nm.

ISO 9060:1990 defines the limits for pyranometers as:

Table 2. Source: OTT HydroMet - Solar Energy

. .
Secondary Standard ± 3%
First Class ± 5%
Second Class ± 10%

 

In order to adhere to this requirement, pyranometers will typically include a ‘thermoelectric’ type detector with a black coating. This coating absorbs incoming radiation and heats up a thermopile before converting the rise in temperature into a small voltage.

The heating properties of the coating covering the detector are, therefore, vital to a radiometer’s quality, while its spectral response is essential to the construction of an accurate and reliable radiometer.

The graph below illustrates the constant absorption properties of Kipp & Zonen thermopile detectors in the range from 350 nm to 1500 nm.

Constant spectral response properties of the Kipp & Zonen coating is key to build an accurate and reliable radiometer.

Constant spectral response properties of the Kipp & Zonen coating is key to build an accurate and reliable radiometer. Image Credit: OTT HydroMet - Solar Energy

Photoelectric sensors, such as silicon cells and photodiodes, are only able to offer a limited and uneven spectral response which fails to meet the spectral selectivity requirements for a pyranometer (or pyrheliometer) outlined in ISO 9060:1990.

Due to this, these devices must be described as ‘Silicon Pyranometers’, or using similar terminology.

The graphs below illustrate a commonly encountered clear sky solar radiation spectrum at sea level, showcasing the response of an entry-level glass-dome thermopile pyranometer, for example, the CMP3 and SMP3 models from Kipp & Zonen.

The graphs also feature the response of a typical silicon photodiode sensor, including the Kipp & Zonen SP Lite2 and RT1. The spectra presented here have been normalized to the peak/maximum of 100% to enable easy comparison.

The spectral selectivity is a function of the spectral absorptance of the black coating and the spectral transmittance of the dome/window and/or diffuser material and of any optical filters that are fitted.

The spectral selectivity is a function of the spectral absorptance of the black coating and the spectral transmittance of the dome/window and/or diffuser material and of any optical filters that are fitted. Image Credit: OTT HydroMet - Solar Energy

The complete range of Kipp & Zonen CMP series and SMP series pyranometers – as well as the CM4 high temperature pyranometer – boast a spectral selectivity of < 3%, confidently meeting the ISO 9060:1990 Secondary Standard requirement.

ISO 9060:2018: Spectral Error

The updated Second Edition of ISO 9060 was first published in November 2018. The most notable update to the guidance is related to spectral response – more specifically, a change from the ‘spectral selectivity’ of 1990 to ‘clear sky irradiance spectral error’ in 2018.

When making this change, the standard’s authors considered the impact that weather conditions and the time of day had on the characteristics of sunlight.

Another notable change was made to allow specific types of photoelectric sensors (including well-designed photodiodes and silicon cells) to be classified as entry-level ‘pyranometers’.

The introduction of the 2018 standard provided an improved and more realistic understanding of potential field measurement errors resulting from the spectral response of a radiometer under various conditions.

Relative air mass can be understood as the thickness of the atmosphere. This changes with the solar zenith angle, meaning that when the sun is low in the sky, the direct beam cuts through more atmosphere, changing the spectrum of the light.

The Air Mass relates to the thickness of the air layer the sunlight has to pass. It is defined as: Air Mass = 1/Cos ? (Solar Zenith Angle).

The Air Mass relates to the thickness of the air layer the sunlight has to pass. It is defined as: Air Mass = 1/Cos θ (Solar Zenith Angle). Image Credit: OTT HydroMet - Solar Energy

Table 3. Source: OTT HydroMet - Solar Energy

. .
Sun at Solar Zenith Angle of  0° AM = 1
Sun at Solar Zenith Angle of  48.2° AM = 1.5
Solar Zenith Angle of 85° AM = 11.4

 

The amount of air also increases as the sun hangs low at the horizon, causing the geometrical relationship to break down beyond a Solar Zenith Angle of 85° - equivalent to a setting sun.

The Spectral Shift

The spectrum of sunlight changes as it travels through the atmosphere. Short wavelengths (ultraviolet and blue) are absorbed and scattered, causing the spectrum to shift towards longer wavelengths (infrared and red).

This effect is also triggered by cloud cover, which increases the concentration of aerosols and particulates in the atmosphere, reducing visibility. With this in mind, the fixed approach from the 1990 version did not accommodate this key factor; therefore, an update was required.

Spectral shift in diffuse irradiance with sky type.

Spectral shift in diffuse irradiance with sky type. Image Credit: OTT HydroMet - Solar Energy

Spectral Error Calculation

In order to better understand how different sky and atmosphere conditions impact the sunlight spectrum, a total of nine different test spectra (representing a range of atmospheric and daytime scenarios) were compared against the reference spectrum from the IEC 60904-3 (2016) standard.

These test spectra are included on the ISO website under the ISO 9060 Second Edition.

The spectral error is calculated by comparing the respective radiometer’s relative spectral response to the reference spectrum to the response for the test spectra.

Spectral response is a quality attribute that is specific to each radiometer. For each individual test spectrum, an error is calculated, which is dependent on the instrument’s reaction to the wavelength distribution.

Of these nine spectra, the largest recorded error is regarded as the radiometer’s spectral error.

Table 4. Source: OTT HydroMet - Solar Energy

. .
Class A ± 0.5% (0.1%)
Class B ± 1% (0.5%)
Class C ± 5% (1%)

 

The CMP and SMP series pyranometers and the CM4 high temperature pyranometer from Kipp & Zonen confidently meet the appropriate classification limits for spectral error Class A by a significant margin, offering spectral errors of <0.16%.

Defining ‘Spectrally Flat’

Should the pyranometer exhibit a spectral selectivity of less than 3% (guard bands 2%) in the wavelength range between 350 nm and 1500 nm (0.35 µm and 1.5 µm), then this can be regarded as ‘spectrally flat’.

This is effectively the criteria for an ISO 9060:1990 Secondary Standard pyranometer and is considerably more stringent than the First Class and Second Class pyranometers limits defined in 1990.

Kipp & Zonen Pyranometer ISO 9060 Classifications

Kipp & Zonen’s CMP and SMP series pyranometers, as well as the company’s CM4 high temperature pyranometer, meet the relevant classification limits for spectral error and offer a spectral selectivity of <3%.

This means that they meet the ISO 9060:2018 Spectrally Flat criteria.

It should be noted that ISO 9060:2018 Class A pyranometers should be individually tested to confirm that their temperature and directional responses are in line with the standard’s requirements.

There are a range of international standards to consider in terms of solar energy, PV plant operation and maintenance. This article aimed to summarized those standards.

Table 5. Source: OTT HydroMet - Solar Energy

. . . . .
ISO 9060:2018 Class C Spectrally Flat
Class C
Spectrally Flat
Class B
Spectrally Flat
Class A
ISO 9060:1990 Not allowed Second Class First Class Secondary Standard
Performance Lower Higher
Passive pyranometers SP Lite2
(Fast Response)
CM4
CMP3
CMP6 CMP10
CMP11
CMP21
CMP22
Smart pyranometers RT1 SMP3 SMP6

SMP10
SMP11
SMP21
SMP22

 

Acknowledgments

Produced from materials originally authored by Martin Maly from OTT HydroMet.

This information has been sourced, reviewed and adapted from materials provided by OTT HydroMet - Solar Energy.

For more information on this source, please visit OTT HydroMet - Solar Energy.

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