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Solar Radiation Spectrum

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The concept of SOLAR ENERGY may seem abstract…but in reality it’s what we have always known as HEAT and LIGHT from the Sun.

The invention of photovoltaic cells last century gave us the possibility of powering our civilization with solar energy. With photovoltaic solar panels, light energy is converted into a flow of electricity.

We might know that Solar energy is electromagnetic radiation from the Sun, but “WHERE” on the electromagnetic spectrum is it? – and “HOW MUCH is there?”

While the electromagnetic spectrum is wide and vast, 99.8% of the Sun’s radiant energy is emitted in a very narrow bandwidth from 20 to 2500 nanometers.

The amount of energy we receive, and at which wavelengths, is called Earth’s insolation curve. Insolation – (note the word Sol) – is the amount of energy received in sunlight.

The composition of solar energy is approximately 5% ultraviolet light; 42% visible light and 53% near infra-red radiation.

It is the AREA under the curve that represents the amount of energy at various wavelengths. Note that solar energy peaks in our colour blue.

At sea level we receive solar energy at an intensity of approximately 1000 watts per square meter for many hours each day. That is our solar inheritance. A little over one horsepower per square yard, for many hours each day.

In the past, solar rights were respected when erecting dwellings near one another:

Let It Shine: Solar Design in Ancient Greece

Although Solar energy has been used for millennia in different ways our modern interest historically begins with Horace De Saussure, who, in the 1700’s experimented with measuring the temperature generated by solar energy within an insulated glass-covered box (a design used later in solar cookers).

The mass production of plate glass came on in the early 1700’s. The fact that glass trapped the Sun’s heat behind it led to the production of greenhouses for growing fruits and vegetables in cooler climates than had been possible before.

Horace de Saussure and His Hot Boxes of the 1700’s

Clear glass has a peculiar property: it easily allows sunshine to pass through, but inhibits thermal radiation from doing the same”. 

De Saussure was wondering why it was colder higher up on mountains than lower down. So he climbed and descended, taking measurements both high and low, and discovered: “…the sun shines with almost equal force at higher and lower elevations—as proved by the equal temperatures in the hot box on the mountain and on the plains.”

His conclusion: “At lower elevations there are greater amounts of carbon dioxide and water vapor in the air. This denser atmosphere holds in the solar heat more effectively, retarding its escape into space; so it gets hotter lower down.”


These early investigations were the background to our current understanding of what is called “The Greenhouse Effect.”

Unfortunately we are in the process of doubling the amount of carbon dioxide in the atmosphere, which will lead to increased temperatures near the ground.

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