Last edited: 18 March 2020      

These graphs show how much of the solar spectrum gets through borosilicate glass of various thicknesses.[1]

Almost all of the sunlight (including non-visible - UV and near infra-red) we get from the sun will go through glass, so we benefit from nearly all of the solar spectrum that hits the earth. The proportion of that solar energy that is within the UV part of the spectrum is very small - only 3 or 4%, so it makes only a tiny contribution to solar cooking. Over half of the solar energy we receive is in the near infra-red, and that will go through glass (borosilicate glass, anyway), so near infra-red and visible are the two sections of the solar spectrum that contribute most of the power we use for solar cooking.

The accompanying graphs, merged together to show how much of the solar spectrum gets through borosilicate glass of various thicknesses. As you see, around 85% of the light we get from the sun (The blue bit of the graph - including a small amount of UV, visible light, and near infra-red light) goes through 3mm borosilicate glass. That applies to the whole solar spectrum. The emissivity of a lot of substances—brick, rough cast iron, etc.—is in the .9 to .93 range. And for most substances, the emissivity approximates the absorptivity. That is, most of the stuff you use for cooking pots in solar cookers have absorptivity in that range - i.e. very high, and right across the solar spectrum.

UV contributes such a small proportion of the energy when a cooking pot in a solar cooker heats up that we could ignore it. The UV index shows us how much UV is getting through the atmosphere to ground level. The highest UV values occur when there is a very clear blue sky (and UV values are higher at the top of mountains because the UV radiation has not had to pass through as much atmosphere and there are no clouds in the way). The only exception occurs when the sun is shining between clouds, and additional UV is reflected from the clouds on either side. So, the UV index is sometimes used as a proxy for the amount of full solar spectrum radiation you can expect to be received by your solar cooker, but UV radiation itself plays very little part in solar cooking. There is just not enough of it to make much difference.

[Our thanks to Dave Oxford for providing much of the information for this article.]

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