A couple things:
1. The wavelengths used do not pass through glass and largely do not pass
through quartz.
2. IR non-contact have a minimum source size requirement to read
accurately. Objects less than 10-15 mm wide may not come up accurately.
There are spot focused units available. The chiropracty <sp>
profession uses (years ago) them to examine spinal column temps on a one
inch focus unit, calibrated from ice to just over human body temperature,
about 30° to 120° F. It has a cone shaped nose, and about a half inch
opening, and usually has a little arm with a rolling ball on the end that
is used to maintain a one inch distance from a surface (like flesh or a
vertebral 'high spot').
However, an electric hot plate should push an IR non-contact thermometer
past reading 500 F, since they get to about 1400-1500 F.
Depends. If it is a coil shaped heater, just like on a stove, then
yes. If it is a "pan" or inverted pan, which is heated from beneath (by
a coil shaped heater), it usually passes through a heat spreader of sorts
between the heater and the pan. This makes for a much more uniform
temperature, but it would also be a bit lower. They usually do not get
to the "red hot" temp., so... what... maybe 1000° F? Better to check it
with a higher range device.
A nice, hot teflon skillet is pretty damned evenly heated right after it
comes of the stove (not the sides).
Pretty weird how the bottom does not radiate much heat toward your
cheek, but the teflon side does... a lot. Pretty good lesson in
emissivity.