MLX90616 Datasheet, PDF (34/36 Pages) Melexis Microelectronic Systems – Infra Red Thermometer in TO-39 for high temperature thermometer guns

MLX90616ESF-HCA
Infra Red Thermometer in TO-39
for high temperature thermometer guns
When a body with low emissivity (such as aluminum) is involved in this heat transfer, the portion of the
radiation incident to the sensor element that really comes from the object of interest decreases – and the
reflected environmental IR emissions take place. (This is all for bodies with zero transparency in the IR band.)
The IR thermometer is calibrated to stay within specified accuracy – but it has no way to separate the
incoming IR radiation into real object and reflected environmental part. Therefore, measuring objects with low
emissivity is a very sophisticated issue and infra-red measurements of such materials are a specialized field.
What can be done to solve that problem? Look at paintings – for example, oil paints are likely to have
emissivity of 0.85…0.95 – but keep in mind that the stability of the paint emissivity has inevitable impact on
measurements.
It is also a good point to keep in mind that not everything that looks black is “black” also for IR. For example,
even heavily oxidized aluminum has still emissivity as low as 0.30.
How high is enough? Not an easy question – but, in all cases the closer you need to get to the real object
temperature the higher the needed emissivity will be, of course.
With the real life emissivity values the environmental IR comes into play via the reflectivity of the object (the
sum of Emissivity, Reflectivity and Absorptivity gives 1.00 for any material). The larger the difference between
environmental and object temperature is at given reflectivity (with an opaque for IR material reflectivity equals
1.00 minus emissivity) the bigger errors it produces.
After I put the MLX90616 in the dashboard I start getting errors larger than specified despite that the
module was working properly before that. Why?
Any object present in the FOV of the module provides IR signal. It is actually possible to introduce error in the
measurements if the module is attached to the dashboard with an opening that enters the FOV. In that case
the portion of the dashboard opening in the FOV will introduce IR signal in conjunction with constraining the
effective FOV and thus compromising specified accuracy. Relevant opening that takes in account the FOV is
a must for accurate measurements. Note that the basic FOV specification takes 50% of IR signal as threshold
(in order to define the area, where the measurements are relevant), while the entire FOV at lower level is
capable of introducing lateral IR signal under many conditions.
When a hot (cold) air stream hits my MLX90616 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 for its sensitivity to this kind of lateral phenomena, but the nature of the phenomena
demands some care when choosing the place to use the MLX90616 in order to make the effects 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 accurate – for example, in a cold winter the
human hand can appear at temperatures quite different from the well known +37°C.
I consider using MLX90616AAA 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
MLX90616. 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 to make the sensor chip “blind” for that heating right in front of it.
3901090616
Rev 002
Page 34 of 36
Data Sheet
August 20, 2012