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Would it be possible to use a non-laser light source to etch a flat surface? Using lenses or parabolic mirrors to focus the light to a point?

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    $\begingroup$ Have you ever burned something with a magnifying glass on a sunny day? $\endgroup$ – Gilbert Apr 13 '19 at 22:43
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I used to write on wood (by burning it) using a standard lens and the light from the sun. Of course the answer is that it depends on which material you want to work on, but with a good lens and a generously powerful light source you should be able to reach some hundreds of degrees.

Quantitatively if you hit your lens (or mirror) with a power $P$ and you manage to focus all the light in say an area $a^2$ (order less than 1 cm^2 for instance), then neglecting what happens to the air in the middle and the dilatation of the solid material you hit, the material receives a quantity of heat per unit of time equal to $P$.

Therefore at the beginning the temperature will rise as $P/(\zeta a^2 \delta)$ as time passes, where $\zeta$ is the specific heat capacity of the material and $\delta$ is the estimate of the thickness of the material you will have heated up when your definition of "beginning" ends. You should set a goal time (for instance you want to reach the goal temperature in 2 seconds) then make sure the heat can travel through the target material only by a small distance $\delta$, so that the change in the effective area of the "lit-up" portion is negligible ($\delta^2\ll\delta$ and you heat up a volume $V=\delta(a+\delta)^2\sim a^2\delta$).

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