Quote:
Originally Posted by raymo
I, just this minute went outside with my wife's 160mm diam. rotating
mirror, full sun and no wind, so focused on a bedsheet hanging on the line.
About 650-700mm f/l and spot approx 6-7mm diam. After approx 1 second
had to pull my hand away as pain too intense, this would set fire to paper
or material pretty damn quickly.
raymo
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A little bit of basic "order of magnitude" maths:
Solar Radiation Intensity is about 1300 W/m2 (before losses).
A 160 mm mirror has an area of about 0.02 m2.
If the mirror is pointing pretty much directly at the Sun, it receives about 26 watts of incident solar radiation, which it concentrates into a spot less than 10 mm diameter.
Allow for losses (atmospheric absorption / scatter, reflection losses, etc), and we could be looking at something like 10 watts (or more) in a 5 to 10 mm disc - comparable to one of those 10 watt halogen globes for desktop lights - and they get pretty hot!