Electronic – How does a temperature sensor work

The MLX90614 actually can work over large distances. Since it is an infrared sensor, it detects the radiation from the object you want to measure, and this does not really change with the distance. The limiting factor for the distance is the angle which the sensor covers. When you look at the data sheet you see that the sensor comes in different variants, from 90 degree down to 10 degree sensor angle. The sensor is most accurate when the object to measure fully covers its field-of-view. For the 10 degree version, the object should be larger than 17cm (at a distance of 1m). If it is smaller, the sensor will report the average temperature of what it sees.

Another option would be the TMP006 from TI, which has an I2C interface. But since it has a larger FOV (nearly 180 degree), at a distance of 1m the object should have a diameter of about 4m. Nonetheless, the TMP006 userguide makes a good read, since it explains the basic of infrared measurements, and all the calculation much better than the Melexis data sheet.

One thing to note, though, is that IR sensors will measure the skin temperature, which can vary significantly from the body temperature.

Kelvins are exactly the same size as degrees Celsius. The Kelvin temperature scale is merely offset by 273.15.

A difference of 1K is exactly the same as a difference of 1°C.

So if you take the 8-bit number (which would have a range of -128 to +127 if it's 2's complement) as 0.5K per LSB you'd have a range from -41C to +86.5C.

Kelvins are often used in place of °C when differences (such as temperature rise) are involved, probably to keep separate the temperature and the difference-in-temperature units. In most electronics contexts (not all!) it's obvious you're not talking about temperatures near absolute zero.