I imagine it is similar to the method of measuring
the thickness of very thin sheets or coatings.
You can do it using interference colors (like the
colors you see on an oil slick). Don't even need
any measurement equipment except your eyeball
since the reflected color varies with the
thickness. Color blind people not allowed.
I bought one of the second type when the borg lowered the price from
$45 to $15. I've gotten $5 worth of entertainment from it.
I consider the other $10 the cost of 'lessons learned'.
Because they use sonar, the texture of a surface throws it off. How
squarely you hit the surface also affects it- often in some weird
ways. Playing with it, I think there are more variables, but
all in all I wouldn't ever trust a critical measurement to it.
Never used the other type.
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