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Pyranometer vs Pyrheliometer: What’s the Difference?

Solar radiation is the electromagnetic radiation that comes from the sun. Its measurement is important for understanding the sun’s intensity and its effects on Earth.

There are many fields that need solar radiation measurements, such as meteorology and climatology, renewable energy technology, and health science.

Therefore, instruments that measure solar radiation, like pyranometers and pyrheliometers, are essential.

Before we get into the instruments that measure solar radiation, we need to understand what the different types of solar radiation are.

When we say the “types” of solar radiation, we aren’t referring to the different wavelength ranges that make up the electromagnetic spectrum. Instead, we are talking about the different ways the sun’s radiation interacts with Earth’s atmosphere.

The two types of solar radiation are direct radiation and diffuse radiation.

Direct Radiation

Direct radiation is the sun’s rays that travel in a straight line from the sun to Earth with no interaction with Earth’s atmosphere.

This type of radiation is also called “beam radiation.” When direct radiation hits Earth’s surface, it is called “direct beam solar irradiance.”

Diffuse Radiation

Diffuse radiation is the sun’s rays that bounce off of particles in the Earth’s atmosphere before reaching the surface.

Diffuse radiation is sunlight that has been scattered by the atmosphere. This type of radiation is also called “diffuse sky radiation.” When diffuse radiation hits Earth’s surface, it is called “diffuse solar irradiance.”

Global Radiation

Global solar radiation isn’t a third type of solar radiation. Rather, it is the sum of direct and diffuse solar irradiance. Global solar radiation is the total amount of sunlight that hits the Earth’s surface.

Global radiation is important for many applications, including solar power, solar hot water, and agriculture.

Because global solar radiation includes both direct and diffuse sunlight, it is a more accurate measure of the sun’s energy than either direct or diffuse solar irradiance alone.

The ratio of direct to diffuse radiation

During clear, sunny weather, direct radiation accounts for about 85% of total insolation, while diffuse radiation accounts for about 15%.

With each degree of descent of the sun’s position in the sky, the amount of diffuse radiation increases until it reaches 40% at 10° above the horizon.

Clouds and pollution can also increase the amount of diffuse radiation in the atmosphere.

Diffuse radiation makes up the vast majority of the sun’s rays on a cloudy day. As a general rule, the higher the percentage of diffuse radiation, the lower the total insolation will be.

There is a significant difference between the amount of sunlight reaching Earth at different latitudes and the amount reaching Earth at different times of the day.

As a result of the cloudier, higher-latitude climates, the amount of total radiation emitted in the winter is higher than in the summer. There is less seasonal variation in the ratio of diffuse and direct radiation in the sunniest places.

Comparing London’s wet and mild climate to Aden’s dry and hot climate is a good example of the difference between the two cities’ climates.

Daily irradiation in London’s most sunny month (June) is about 5.5 kWh/m2, with diffuse radiation accounting for about half of it. Radiation levels fall below 1 kWh/m2 in December, and the vast majority of that radiation is scattered.

Average daily radiation is approximately 7 kWh/m2, with less than 30% of it diffuse, in May, Aden’s sunniest month. About 35% of the irradiation in December is diffuse radiation, which is 5.25 kWh/m2 on average.

Which type of solar radiation is more important for solar power?

Direct radiation is more important for solar power because it is the sun’s rays that travel in a straight line from the sun to Earth with no interaction with Earth’s atmosphere.

Diffuse radiation is the sun’s rays that bounce off of particles in the Earth’s atmosphere before reaching the surface. Diffuse radiation is sunlight that has been scattered by the atmosphere.

The steeper your solar panels are inclined, the less of the sky they face and the less diffuse radiation they receive.

If your solar collectors are slanted at a 45° angle, for example, they are looking away from roughly a quarter of the sky and will only capture about three-fourths of the diffuse radiation in the sky.

Nonetheless, because direct radiation is significantly stronger than diffuse radiation, the amount of energy lost by tilted solar panels is often more than offset by the extra radiation collected by tracking the sun.

Solar hot water and photovoltaic systems can use both direct and diffuse radiation. However, concentrated solar power (CSP) systems require direct sunlight and will not work with diffuse radiation.

Now that we understand the different types of solar radiation, let’s move on to the instruments that measure them.

There are two types of instruments that measure solar radiation: pyranometers and pyrheliometers. We have already published dedicated posts on pyranometers and pyrheliometers, which you should definitely check out.

But in case you don’t have time to read those posts, we will give a brief summary of what pyranometers and pyrheliometers are in this post.

What is a pyranometer?

A pyranometer is an instrument that measures the total amount of solar radiation (global radiation), both direct and diffuse, incident on a surface.

A pyranometer typically consists of a light-sensitive detector, such as a thermopile or photodiode, that is mounted in a housing that allows the detector to face the sky.

The detector converts the incident radiation into an electrical signal that is proportional to the intensity of the radiation. This electrical signal is then amplified and recorded by a data logger.

Pyranometers are used to measure the amount of solar radiation incident on a surface for a variety of applications, including solar energy, meteorology, and climatology.

What is a pyrheliometer?

A pyrheliometer is an instrument that measures the amount of direct solar radiation incident on a surface.

Pyrheliometers are similar to pyranometers in that they consist of a light-sensitive detector mounted in a housing that allows the detector to face the sun.

However, pyrheliometers typically have a cosine corrector that ensures that the detector is perpendicular to the sun’s rays.

This cosine corrector is necessary because the amount of solar radiation incident on a surface is directly proportional to the cosine of the angle between the sun’s rays and the surface.

Pyrheliometers are used to measure the amount of direct solar radiation incident on a surface for a variety of applications in addition to solar energy, meteorology, and climatology.

What are the differences between pyranometers and pyrheliometers?

Here are the key differences between pyranometers and pyrheliometers:

Pyranometers measure the total amount of solar radiation (global radiation), both direct and diffuse, incident on a surface. Pyrheliometers only measure the amount of direct solar radiation incident on a surface.

Pyranometers do not require a cosine corrector because they are measuring the total amount of radiation, both direct and diffuse. Pyrheliometers typically have a cosine corrector because they are only measuring the amount of direct solar radiation.

Pyranometers look up at the sky and measure the radiation coming from all directions. Pyrheliometers look directly at the sun and only measure the radiation coming from one direction.

Pyranometers don’t require much maintenance because they don’t have any moving parts. Pyrheliometers require more maintenance because they have a cosine corrector that needs to be regularly calibrated.

Pyranometers are less expensive than pyrheliometers. Pyrheliometers are more expensive than pyranometers because they are more complex and require more maintenance.

A typical pyranometer is a glass dome that has a hemispherical view of the whole sky. While a pyrheliometer has a telescopical view of the sun that is focused on the instrument’s detector.

Both pyranometers and pyrheliometers have thermopile or photodiode as a light-sensitive detector that is mounted in a housing. The electrical signal is then amplified and recorded by a data logger.