Mozo B. wrote:
> This sensor is I guess the easiest to understand because
> heat emits some kind of infrared and the point here is just to detect
> how much of this light is emitted.

That ist correct, but Sensors which register midrange infrared radiation 
are often based on thermoelements (Seebeck-effect), whereas contact 
thermometers are based on the temperature dependency of the threshold 
voltage of a silicon diode.
Thermoelements most often are made from other materials than silicon and 
therefore are not easily integrated on a silicon die.

That makes the infrared sensitive variety more expensive.

But there are more differences.
IR thermometers measure the radiant power of the emitting surface wheras 
contact sensors (NTCs, Diodes) measure the true temperature of the 
object.
Therefore IR-thermometers in most cases are faster, but they can produce 
large errors when measuring a surface with low emissivity e.g. a shiny 
metal surface.

P.S.:
Her ist a microphotograph of the Melexis IR-Sensor:
https://www.eevblog.com/forum/projects/unknown-digital-infrared-thermometer-chips/?action=dlattach;attach=143314;image

In the middle you can see the thermopile wich consists of a large number 
of thermoelements.
Because the output voltage of a thermoelement is very very small, many 
of them are electrically connected in series in order to give a greater 
voltage to the silicon based microchip which is located above the 
thermopile.
That silicon die does all the voltage to digital conversion and also 
handles the serial protocol with the outside world.

What one cannot see:
There also is a thermometer based on diode voltage drop integrated in 
the silicon die.
This is necessary because thermoelements produce a voltage proportional 
to the temperature difference of their "hot" and "cold" junction.
Therefore the integrated diode thermometer is necessary to compensate 
for temperature fluctuations of the whole device.