EVB90615 Datasheet, PDF (27/30 Pages) Melexis Microelectronic Systems – Infra Red Thermometer

MLX90615
Infra Red Thermometer
When a hot (cold) air stream hits my MLX90615 some error adds to the measured temperature I read.
What is it?
IR sensors are inherently sensitive to difference in temperatures between the sensitive element and
everything incident to that element. As a matter of fact, this element is not the sensor package, but the sensor
die inside. Therefore, a thermal gradient over the sensor package will inevitably result in additional IR flux
between the sensor package and the sensor die. This is real optical signal that can not be segregated from
the target IR signal and will add errors to the measured temperature.
Thermal gradients with impact of that kind are likely to appear during transient conditions. The sensor used is
developed with care about sensitivity to this kind of lateral phenomena, but their nature demands some care
when choosing place to use the MLX90615 in order to make them negligible.
I measure human body temperature and I often get measurements that significantly differ from the
+37°C I expect.
IR measurements are true surface temperature measurements. In many applications this means that the
actual temperature measured by an IR thermometer will be temperature of the clothing and not the skin
temperature. Emissivity (explained first in this section) is another issue with clothes that has to be considered.
There is also the simple chance that the measured temperature is adequate – for example, in a cold winter
human hand can appear at temperatures not too close to the well known +37°C.
I consider using MLX90615 to measure temperature within car compartment, but I am embarrassed
about the Sun light that may hit the module. Is it a significant issue?
Special care is taken to cut off the visible light spectra as well as the NIR (near IR) before it reaches the
sensitive sensor die. Even more, the glass (in most cases) is not transparent to the IR radiation used by the
MLX90615. Glass has temperature and really high emissivity in most cases – it is “black” for IR of interest.
Overall, Sun behind a window is most likely to introduce relatively small errors. Why is it not completely
eliminated after all? Even visible light partially absorbed in the filter of the sensor has some heating potential
– and there is no way that the sensor die will be “blind” for that heating right in front of it.
3901090615
Rev 001
Page 27 of 30
Data Sheet
28/Aug/2008