Infra-red radiometers can be used to measure the surface temperature of an object, e.g. a leaf. Assuming that the emissivity of the surface equals one, the radiance can be converted to a temperature (Stefan-Boltzmann equation, Note 2.C). These hand-held devices ('IR guns') must be held close to the object of interest, because of their broad field of view, and the contamination by air between the object and the sensor. To minimize the contamination, IR guns often operate in narrow bands of the electromagnetic spectrum only (away from absorption bands of water vapour, carbon dioxide etc).
To retain an accurate relation between measured radiation and skin temperature, IR guns can be pointed at surfaces of known temperature, e.g. a bath of water and ice, or a surface whose temperature is monitored independently. IR guns are used to study skin temperature variations on plants or animals, and to measure sea surface temperature from a ship, for instance.
Even after careful calibration of an IR gun for SST measurements (1), the 'skin' SST usually is distinctly different from the 'traditional' SST (i.e. the temperature of water in a bucket sampled near the ocean surface). At night, the skin SST is often lower, due to evaporative cooling, and during the daytime, it is higher due to the absorption, at the skin, of near IR from the Sun (2).
(1) Rapier, C.B. and K.J. Michael 1996. The calibration of a small, low-cost thermal infra-red radiometer. Remote Sensing Environ. 56, 97-103.
(2) Rapier, C.B. and K.J. Michael 1997. On the skin-bulk temperature difference in the Southern Ocean between Australia and Antarctica. Remote Sensing Environ. (in the press).