[RWebb]

Yes, sort of...

They the Stefan-Boltzmann law of radiation: Qt=σeAT4 Q t = σ e A T^4
σ = 5.67 × 10−8 J/s · m2
K4 is the Stefan-Boltzmann constant
A is the surface area of the object
T is its absolute temperature in Kelvin

the surface also has an emissivity, which is about 0.94 for everything that is non-metallic (white people have black skin in the IR)

Sources of error are mainly due to the operator, such as using the wrong angle, not compensating for a polished metal (or metallic painted) surface, or pointing the beam at the edge.

For use on humans they will measure the T forehead or whatever, which is not T core and can vary if the person recently exercised (as the forehead is a "thermal window" used for cooling - you can see this effect clearly by backcountry skiing with attractive Scandihoovean women, which makes a xlnt research project)...

Be happy 'cuzin the 1950s USAF had to use 2 semi-trucks to cart the equipment around... Col. James H. Veghte used it to measure surface T of birds, jack rabbit ears, etc.