What Is the Solar Spectrum? (2024)

Sunlight.

It’s something we tend to take for granted…

But without it, life on Earth could not exist.

Plants rely on it for photosynthesis.

Humans and entire ecosystems rely on it for heat.

Renewable energy sources that use the photovoltaic effect to transform sunlight into electricity — like solar panel systems — can’t function without the photons contained in the sun’s rays.

Passive solar energy systems rely on the sun to produce heat.

What we experience as sunshine on a cloudless day is actually composed of multiple different types of energy.

One of the tools scientists use to measure solar irradiation is the solar spectrum.

So what is it?

Read on to find out.

(Source: NASA)

What Are the Three Main Regions of the Solar Spectrum?

The light energy the sun produces that reaches Earth is comprised of electromagnetic radiation of three primary types.

Each type of light is part of the solar spectrum that makes up solar irradiance — which measures the total amount of energy available from sunlight.

You’ve probably heard of all three types of electromagnetic radiation that make up sunlight…

But, when it comes to solar panels and photovoltaic systems that produce electricity, one type is far more crucial than the others.

Can you guess which?

Here’s a clue — it’s the only one you can see.

Visible Light

The segment of the solar spectrum that humans can see with the naked eye makes up only a tiny proportion of the electromagnetic radiation that the Earth receives as light.

(Source: SMU.edu)

You’ve undoubtedly seen white light refracted through a prism that splits it into different colors.

By definition, all the colors you can see refracted through a prism are made from visible light.

On average, the human eye can only see wavelengths of light that between between 380 and 700 nanometers.

Again, using refracted light as an example, red is the widest wavelength at 700 nanometers (nm) and violet is the narrowest at 380nm.

Animals like the birds and the bees can see ultraviolet (UV) light.

Snakes and vampire bats can detect infrared (IR) light.

Everything that humans can see, however, is comprised of a relatively narrow wavelength of visible light.

According to Science Direct: “About 40% of the solar radiation received at the Earth’s surface on clear days is visible radiation within the spectral range 0.4 to 0.7nm, while 51% is infrared radiation in the spectral region 0.7 to 4nm.

Very little ultraviolet (UV) light reaches the planet’s surface, less than 10%.

Ultraviolet (UV) Light

Despite making up less than 10% of the solar irradiance that penetrates the Earth’s atmosphere, ultraviolet (UV) radiation has an enormous influence on human health.

On the one hand, UV light is essential for the human production of essential nutrients like Vitamin D.

However, too much UV radiation has been proven to cause skin cancer, malignant and non-malignant melanomas, and eye conditions like cataracts.

Virtually everyone has experienced the less severe — but extremely unpleasant — consequences of excessive exposure to UV radiation: sunburn.

Additionally, UV exposure causes premature aging of human skin, another excellent reason to always wear sunscreen.

Ultraviolet radiation (UV-R) occurs on the solar spectrum between the 100nm and 400nm wavelengths.

(Source: National Institute of Health)

The Ultraviolet Index (UVI) was established to help health organizations worldwide alert the public to the real time danger of UVR, as the levels of radiation vary significantly throughout each day and based on atmospheric, seasonal, and other conditions.

The science of how much UVR is beneficial to humans and how much results in negative outcomes is constantly evolving.

Recently, there have been compelling studies on the mental and physical benefits of viewing low-angle sunlight at sunrise and sunset.

One thing that’s clear is that sunburns should be avoided at all costs.

Infrared Radiation

Infrared radiation (IR) from the solar spectrum has been harnessed to produce and store heat by humans for centuries.

But the energy is traditionally captured and stored as heat — not as electricity.

Passive solar systems often capture infrared radiation and store it in water as heat, such as in a solar-assisted heat pump.

Aside from providing heat, infrared light has numerous other applications in a wide variety of applications, including:

• Thermal imaging for devices like night-vision goggles
• Security systems
• Detecting astronomical objects in deep space

You and I can’t “see” infrared light, but we can certainly feel it as heat.

(Source: Pexels)

Why Is Understanding the Solar Spectrum Important?

The sun is fundamental to our solar system’s existence and essential for all life on earth.

As the necessity to find renewable sources of energy and understand changes to the Earth’s climate and systems grows increasingly urgent, tools that help us understand how our sun “works” are taking on even greater importance.

The history of the solar spectrum dates back to 1666 when Isaac Newton discovered that the white light of sunlight is segmented into seven visible colors when refracted through a prism.

In subsequent centuries, solar radiation wavelengths invisible to humans — like ultraviolet (UV) and infrared (IR) light — were discovered and considered as part of the solar spectrum.

The photovoltaic effect was first discovered in 1839 and it remains the foundation of solar panel systems that convert sunlight into electricity.

While many of the basic principles behind the solar spectrum — and solar power — are centuries old, the instruments and techniques used to measure solar irradiance have grown far more sophisticated.

(Source: NREL Measurement and Instrumentation Data Center (MIDC)

Satellites and earthbound devices like pyranometers, thermopiles, and Campbell-Stokes recorders break down the solar spectrum and solar irradiance into microscopic units of measurements like nanometers.

Using such devices, scientists are able to accurately measure global horizontal solar irradiance which is essential to designing both industrial-scale and residential photovoltaic systems.

Sophisticated systems also rely on this data to predict weather events and understand our changing climate.

And to think it all started with Newton’s humble prism!

What Wavelength Do Solar Panels Use?

Visible light accounts for about 40% of solar irradiance that reaches the Earth’s surface.

But it provides by far the most usable solar energy that commercially available photovoltaic cells convert into direct current (DC) electricity.

That’s one of the primary reasons that solar panels don’t work at night.

Only 4% of the solar irradiation that reaches the Earth’s surface is ultraviolet.

While some PV panels can utilize UVR for a tiny portion of their total output, it turns out that UV rays can be almost as harmful to solar panels as they can be to humans.

According to Science Direct: “One of the significant environmental stress factors for degradation in the PV module is UV irradiation exposure in the field during its operational lifespan.”

Any additional electricity production from UVR in traditional photovoltaic panels is likely to be significantly outweighed by the negative impact on their longevity.

Infrared light makes up the largest proportion of solar electromagnetic radiation that reaches the Earth’s surface.

However, the photovoltaic modules in by far the most widespread use — those comprised of monocrystalline or polycrystalline silicon solar cells — either reflect IR or allow it to pass through the panel without capturing photons from infrared light.

Because infrared radiation is so effective at transmitting heat, it can actually negatively impact the efficiency of solar panels.

As the temperature of traditional solar cells exceeds 25°C, solar panel efficiency actually begins to decrease — particularly when it reaches 35°C or higher.

Solar farms in areas with many hours of peak sunlight — such as in a desert — frequently have to install cooling systems to combat the decrease in solar panel efficiency that comes with extreme heat.

Recent technological innovations have shown some promise in harnessing IR to produce and store electricity from heat, but they’re likely years away from commercial use.

Frequently Asked Questions

Final Thoughts

Considering how essential the sun is to our very existence, it’s incredible how few people have a reasonable idea of how it “works.”

Hopefully, by scratching the surface of how a fundamental principle like the solar spectrum and the different wavelengths of sunlight, you have at least a slightly better understanding of how solar power works.

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