Important Facts on Pyrheliometer

Important Facts on Pyrheliometer

Pyrheliometer is a device used to monitor the direct beam of solar radiation regularly. It has a tracking system that allows it to track the sun indefinitely.

A pyrheliometer is a device that measures direct sun energy at a particular location. It is generally mounted on a monitoring device that records the sun’s movements. It helps to ensure that the device is pointed towards the sun. 

A thermopile converts sunlight into electrical electricity after it reaches the pyrheliometer. The voltage can then be adjusted to watts per square metre, the conventional unit for solar irradiation. Pyrheliometers can be used for solar panel placement and scientific study. On the surface of the Earth, they can measure sun irradiance after it passes through the atmosphere. 

In 1893, Knut Angstrom, a physicist from Sweden, invented the pyrheliometer. His was the first instrument that could detect both indirect and direct sun radiation.

Parts of a pyrheliometer

Acceptance angle

To measure solar radiation, we have to position the device at a certain angle and capture the sun’s light. The instrument’s acceptance angle should be the closest to 0 degrees, allowing the lens to bend and focus light on the black body. This angle is commonly 5 degrees.

Long collimator tube

A long collimator tube used to arrange the absorbers and limit a beam of X-rays or gamma rays to specific dimensions.

Alignment indicator

The angle of the device is set using the alignment indicator. 

Thermopile junction

The thermopile junction is positioned beneath the black absorber plate. The heat from the black body is measured and recorded for the system.

 Pivot for two-axis rotation

The entire system is mounted on it. It helps track the sun during the day. The instrument’s direction can be adjusted based on the direction of radiation.

Black absorber plate

A black absorber plate is present inside the tube to absorb the radiation.

Structure and working of a pyrheliometer

Since the pyrheliometer is a long tube, its outer structure resembles that of a telescope. Using this tube, the radiance can be calculated by aligning the lens with the sun. 

The radiation from the sun passes through the lens, then the tube, and finally through the last portion, which has a black object at the bottom.

Solar irradiance passes through this system through a crystal quartz window and reaches a thermopile directly. Energy can then be converted from heat to a recorded electrical signal.

Once the mV signal has been converted to the radiant energy flux, a calibration factor can be applied. It is estimated in W/m² (watts per square metre). This type of data can be utilised to improve insolation maps. It is a measurement of the solar energy received on the surface of a region at a specific moment. The isolation factor for different locations proves extremely beneficial when installing solar panels.

Applications of the pyrheliometer

There are many uses of the pyrheliometer. Some of its major uses are listed below:

The power consumption is quite low.

It can work with a wide range of voltages.

It is rigid and stable.

Since it solely measures direct radiation, it is commonly employed in solar tracking systems. 


A pyrheliometer is made of a radiation-sensing device contained in a case (collimation tube) with a small hole that allows only direct sun rays to pass through. Radiation reflecting off a cloud of particles in the atmosphere does not make it to the detector through this small hole and collimation tube. It must be aimed directly at the sun, using only a solar tracker to make observations all day. Pyrheliometers are used to evaluate the effectiveness of solar panels erected to harness the sun’s energy, as well as to aid meteorological research.