How hot is the beam of a Fresnel Lens?
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How to determine the temperature of the Fresnel Beam.
"How hot is that beam of light?"
This is a common question that really does not have an exact answer. The beam of light concentrated by a Fresnel lens or parabolic mirror will have a temperature equal to that of the surrounding ambient temperature. If you are working with a lens in a 60°F climate, the temperature of the beam will be 60°F assuming there are no dust particles in the air. Air molecules and debris are usually present so there may be a slight temperature increase but there is not a way to measure it accurately.
"Where does the heat come from?"
Things really begin to heat up when a dark object is placed in the beam of light, especially at the optimal focal point. Large amounts of sunlight concentrated by the Fresnel lens is focused on a target and as the light is absorbed, heat is the by-product. The larger the Fresnel lens or parabolic mirror, the more potential "power" concentration the solar item has, assuming the optics are of a good quality. A lens that is 1 meter x 1 meter (1 m2) has the potential of 1000 watts of thermal energy concentrated to an area of 1 cm resulting in extreme, rapid temperature increases on the target object. This is where the temperature is measured.
"Why use cement and not steel for maximum temperature estimates?"
Because cement can withstand high temperatures and is a good insulator that darkens as concentrated sunlight melts the material, cement is an idea material for measuring build temperatures (max collected). A build temperature is the maximum peak temperature the solar lens can heat this material to over an extended period of time. Dark colored small masses of steel are used to measure the instant temperature (climb temperature) with a one minute exposure. Steel is an excellent thermal conductor so while it does heat up rapidly, it also sinks the heat into the surrounding atmosphere and prevents maximum build temperatures resulting in a lower temperature number. This gives a good idea of the instant temperature based on flash combustion of different materials. Several materials tested match this temperature measurement. Climb temperatures are also based on several other factors including optimal beam size and related steel testing mass sizes. These are the best methods of determining the maximum heat potential.
"Why not just place a thermometer at the focal point?"
Placing a physical probe thermometer at the focal point does not work because the probe material becomes the "material" test subject. If the probe is shiny stainless steel material, then it will reflect sunlight away resulting in inaccurate temperature readings. If the probe is made from thermal composite material, it will instantly be destroyed. There are very few probes that will survive a one minute exposure in the focal point of a high quality Fresnel lens.
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