Solar Radiation Measurement Instruments: Types & Uses | NiuBoL

Solar radiation measurement instruments measure radiant energy from the sun or specific components of the radiation received at the Earth's surface. Different instruments are used to measure global solar irradiance, direct solar radiation, diffuse radiation, reflected radiation and the spectral distribution of sunlight.

The most common instruments include pyranometers, pyrheliometers, albedometers, net radiometers, spectroradiometers and PAR sensors. Each instrument measures a different quantity or uses a different optical arrangement.

Selecting the correct instrument is important for photovoltaic performance monitoring, solar resource assessment, meteorological observations, agricultural research and environmental studies. The measurement required by the project should determine the sensor type, installation method and data acquisition equipment.

What Instruments Are Used to Measure Solar Radiation?

Solar radiation instruments can be compared according to their measurement quantities, spectral response and optical field of view.

These instruments are not interchangeable. For example, a sunshine recorder measures sunshine duration, while a pyranometer measures irradiance. A PAR sensor measures photosynthetically active light, which is different from total broadband solar irradiance.

1. Pyranometer: Measuring Global Solar Irradiance

A pyranometer is one of the most widely used instruments for measuring solar radiation at ground-based weather and solar monitoring stations.

A pyranometer measures the solar irradiance incident on a receiving plane from the hemisphere above that plane. Its measurement depends on the orientation of the sensing surface and whether direct sunlight is blocked.

Working Principle

Many broadband pyranometers use a thermopile sensing element. Incoming solar radiation is absorbed by a coated sensing surface, producing a temperature difference that generates a small thermoelectric voltage.

The voltage is converted into irradiance using the instrument's calibration relationship. Other pyranometers use silicon-based photoelectric detectors with different spectral and directional response characteristics.

An important characteristic of a pyranometer is its response to the angle of incident radiation. Suitable instruments are designed to approximate the cosine response required for irradiance measurement on a plane surface.

GHI, POA and DHI Measurement

A horizontally installed, unshaded pyranometer measures Global Horizontal Irradiance (GHI), which includes the direct solar component projected onto the horizontal surface and diffuse radiation from the sky.

When a pyranometer is installed parallel to an inclined photovoltaic module, it measures Plane-of-Array (POA) Irradiance.

With suitable shading equipment that blocks the direct solar beam, a horizontal pyranometer can measure Diffuse Horizontal Irradiance (DHI).

The sensor does not automatically separate these components. The measured quantity depends on installation geometry and auxiliary equipment.

For available instruments, see the NBL-W-HPRS Pyranometer Solar Radiation Sensor.

2. Pyrheliometer: Measuring Direct Normal Irradiance

A pyrheliometer measures Direct Normal Irradiance (DNI), which is the solar irradiance received on a surface perpendicular to the incoming direct solar beam.

Unlike a pyranometer, a pyrheliometer uses an optical arrangement with a narrow field of view centered on the sun. This allows it to measure the direct-beam component, including the circumsolar contribution admitted by its defined field of view.

How Does It Work?

A typical pyrheliometer uses a collimating tube or aperture arrangement to restrict incoming radiation to a narrow angular region. The radiation entering the instrument reaches a detector, often based on a thermoelectric measurement principle.

For continuous measurements throughout the day, the instrument is normally mounted on a solar tracking mechanism to keep its optical axis aligned with the sun.

DNI is particularly important in concentrating solar energy systems and detailed solar resource studies.

NiuBoL also lists the NBL-W-FARS Direct Radiation Sensor. Its specific measurement configuration, tracking arrangement, field of view and output should be verified against the selected model documentation when planning a DNI measurement project.

For a detailed comparison, read Difference Between Pyranometer and Pyrheliometer.

3. Measuring GHI, DNI and DHI

Three important solar radiation measurements are Global Horizontal Irradiance, Direct Normal Irradiance and Diffuse Horizontal Irradiance.

The relationship between these components on a horizontal surface is commonly expressed as:

GHI = DNI × cos(θz) + DHI

Here, θz is the solar zenith angle. The cosine term converts the direct normal component into its contribution on the horizontal receiving plane.

A single unshaded pyranometer cannot independently measure all three components. A complete measurement arrangement requires suitable additional instruments, shading equipment or validated estimation methods.

4. Albedometer: Measuring Reflected Solar Radiation

An albedometer is used to determine surface albedo, which describes the fraction of incoming shortwave solar radiation reflected by a surface.

A common albedometer arrangement uses two pyranometers. The upward-facing instrument measures incoming shortwave irradiance, while the downward-facing instrument measures reflected shortwave irradiance.

Surface albedo is calculated as the ratio of reflected to incoming irradiance under appropriate measurement conditions.

Albedo = Reflected Shortwave Irradiance / Incoming Shortwave Irradiance

Albedo is dimensionless and may also be expressed as a percentage. It is used in land-surface energy studies, snow research, agricultural meteorology and environmental monitoring.

The installation should provide a representative view of the surface being studied. Shadows, uneven terrain and nearby structures can influence downward-facing radiation measurements.

5. Net Radiometer: Measuring the Surface Radiation Balance

A net radiometer measures or determines the balance between incoming and outgoing radiation at a surface.

A four-component net radiometer commonly uses two pyranometers for shortwave radiation and two pyrgeometers for longwave radiation. Upward-facing and downward-facing detectors measure radiation arriving from the sky and leaving or being reflected by the surface.

Net radiation can be expressed as:

Rn = (SW↓ − SW↑) + (LW↓ − LW↑)

SW represents shortwave radiation, while LW represents longwave radiation. The arrows indicate incoming and outgoing directions relative to the surface.

This measurement is important for surface energy-balance studies, evapotranspiration research and agricultural meteorology.

Technical distinction: A conventional shortwave pyranometer is not a replacement for a pyrgeometer. A pyrgeometer is specifically designed for terrestrial longwave infrared radiation measurements.

6. Spectroradiometer: Measuring Solar Radiation by Wavelength

A spectroradiometer measures the spectral distribution of radiation over a defined wavelength range.

Unlike a broadband pyranometer, which provides an integrated irradiance measurement over its spectral sensitivity range, a spectroradiometer separates the incoming radiation into wavelength bands.

Depending on the instrument configuration, spectral data may be reported as spectral irradiance in W/m²/nm or another appropriate radiometric quantity.

Spectroradiometers are used in atmospheric research, optical material testing, photovoltaic spectral-response studies and plant-light research.

The instrument's wavelength coverage, spectral resolution, optical geometry and calibration determine whether it is suitable for a particular measurement objective.

7. PAR Sensors for Agricultural Light Monitoring

A Photosynthetically Active Radiation (PAR) sensor is commonly used in agriculture, greenhouses and plant research to measure light relevant to photosynthesis.

Conventional PAR measurement covers the 400–700 nm wavelength range. A quantum PAR sensor typically reports photosynthetic photon flux density (PPFD) in μmol/m²/s.

PPFD represents the number of photons received per unit area and unit time within the specified photosynthetically active waveband.

PAR measurements should not be confused with total broadband solar irradiance or illuminance expressed in lux. These quantities use different spectral definitions and weighting methods.

For agricultural monitoring, see the PAR Sensor.

8. Ultraviolet Radiation Sensors

Ultraviolet (UV) radiation sensors measure radiation within a specified ultraviolet spectral region. UV measurements are used in environmental monitoring, material exposure studies and other specialized applications.

UV-A and UV-B measurements refer to different wavelength ranges. An instrument measuring one UV band should not automatically be assumed to measure all ultraviolet radiation.

Some instruments report physical UV irradiance, while other devices estimate a UV index using an appropriate spectral weighting relationship.

For project selection, the required spectral range, measurement quantity and sensor calibration method should be confirmed.

9. Sunshine Recorders and Historical Pyranographs

A sunshine recorder measures the duration of bright sunshine rather than continuously measuring broadband solar irradiance.

A traditional Campbell–Stokes sunshine recorder uses a glass sphere to focus direct sunlight onto a recording card. The resulting trace provides an estimate of sunshine duration.

Modern electronic sunshine duration instruments can use optical detectors and defined irradiance thresholds to identify periods of bright sunshine.

A pyranograph is a historical recording arrangement associated with solar radiation measurement. The term does not describe a fundamentally different solar radiation quantity from a pyranometer.

Modern stations commonly use a pyranometer connected to an electronic data logger rather than a mechanical chart recorder.

10. Satellite Solar Radiation Data and Other Instrument Terms

Satellite observations can support solar radiation mapping and resource estimation over large geographic areas. Satellite-derived solar irradiance datasets are generally produced using observations together with retrieval algorithms and models.

Satellite-derived values are not identical to direct measurements from a ground-mounted pyranometer. Their uncertainty depends on the data source, atmospheric conditions, spatial resolution and retrieval method.

The term solarimeter is sometimes used broadly for solar radiation measuring instruments. The actual measurement should be identified by the specific instrument type and technical specification.

An infrared radiation thermometer measures temperature by detecting thermal radiation from a target. It should not be treated as a standard replacement for a pyranometer or pyrheliometer in solar irradiance monitoring.

How to Select Solar Radiation Measurement Instruments

The correct instrument depends on the required radiation quantity, installation location, spectral range and accuracy requirements.

For outdoor solar measurements, also consider the following factors: installation orientation, shading, environmental exposure, sensor cleanliness, calibration interval, output interface and data acquisition requirements.

A radiation measurement should be clearly identified as irradiance in W/m² or accumulated irradiation in units such as Wh/m² or kWh/m². These quantities are related but not interchangeable.

Frequently Asked Questions

What instrument is used to measure solar radiation?

A pyranometer is commonly used to measure global solar irradiance at meteorological and solar monitoring stations. Other instruments are required for direct, diffuse, spectral or net radiation measurements.

What is the difference between a pyranometer and a pyrheliometer?

A pyranometer measures irradiance received over a hemispherical field of view on a defined plane. A pyrheliometer uses a narrow field of view to measure direct normal irradiance from the sun.

Can one pyranometer measure GHI, DNI and DHI?

A conventional unshaded pyranometer cannot independently measure all three quantities. GHI is commonly measured horizontally, DHI requires appropriate direct-beam shading, and DNI requires a dedicated measurement or an appropriate derived estimate.

Is a PAR sensor the same as a solar radiation sensor?

No. A PAR quantum sensor measures photons in the photosynthetically active range, commonly 400–700 nm. A broadband solar radiation sensor measures radiant power over a different defined spectral range.

What is the difference between solar irradiance and irradiation?

Solar irradiance is radiant power per unit area, expressed in W/m². Solar irradiation is accumulated radiant energy per unit area over a specified period, often expressed in Wh/m² or kWh/m².

NiuBoL Solar Radiation Sensors

NiuBoL provides solar radiation sensors and related environmental monitoring equipment for meteorology, photovoltaic applications, agricultural research and system integration.

The NBL-W-HPRS Pyranometer is designed for broadband solar irradiance measurement. Depending on the selected output version, it can be connected to compatible data loggers or monitoring systems.

For direct solar radiation measurement, the NBL-W-FARS Direct Radiation Sensor is listed separately. The measurement geometry and tracking requirements should be confirmed for the intended DNI application.

For solar radiation monitoring projects, specify whether the required quantity is GHI, DNI, DHI, POA irradiance, reflected radiation or another measurement. Also provide the output interface, data logger requirements, mounting arrangement and project quantity.

Solar Radiation Sensor Instruction Manuals

The following original NiuBoL instruction manuals provide product specifications, installation information and communication details for the listed solar radiation sensors.

NBL-W-HPRS-Solar-Radiation-Sensor-Instruction-Manual-V3.0.pdf

NBL-W-SRS-Solar-radiation-sensor-instruction-manual-V4.0.pdf

These manuals relate to the specified NiuBoL sensor models. Product-specific measurement ranges, spectral response, calibration information and output settings should be verified using the corresponding document.