Human body temperature to give the most accurate assessment of core body temperature, particularly in hypothermia. In the early 2000s, ingestible thermistors in capsule form were produced, allowing the temperature inside the digestive tract to be transmitted to an external receiver; one study found that these were comparable in accuracy to rectal temperature measurement. In the 19th century, most books quoted "blood heat" as 98 °F, until a study published the mean (but not the variance) of a large sample as . Subsequently that mean was widely quoted as "37 °C or 98.4 °F" until editors realised 37 °C is closer to 98.6
Brightness temperature black body; formula_7 is Planck's constant; formula_1 is frequency; formula_9 is the speed of light; and formula_10 is Boltzmann's constant. For a grey body the spectral radiance is a portion of the black body radiance, determined by the emissivity formula_11. That makes the reciprocal of the brightness temperature: At low frequency and high temperatures, when formula_13, we can use the Rayleigh–Jeans law: so that the brightness temperature can be simply written as: In general, the brightness temperature is a function of formula_1, and only in the case of blackbody radiation it is the same at all frequencies. The brightness temperature