MLX90614_15 Datasheet, PDF (45/51 Pages) Melexis Microelectronic Systems – Single and Dual Zone Infra Red Thermometer in TO-39

MLX90614 family
Single and Dual Zone
Infra Red Thermometer in TO-39
15 FAQ
When I measure aluminum and plastic parts settled at the same conditions I get significant
errors on aluminum. Why?
Different materials have different emissivity. A typical value for aluminum (roughly polished) is 0.18 and for
plastics values of 0.84…0.95 are typical. IR thermometers use the radiation flux between the sensitive element
in the sensor and the object of interest, given by the equation
( ) ( ) q = ε₁ ×α₁ × T₁₄ ×σ × A₁ × F_a−b − ε₂ × T₂₄ ×σ × A₂ ,
Where:
ε₁ and ε₂ are the emissivities of the two objects,
α₁ is the absorptivity of the sensor (in this case),
σ is the Stefan-Boltzmann constant,
A₁ and A₂ are the surface areas involved in the radiation heat transfer,
F_a-b is the shape factor,
T₁ and T₂ are known temperature of the sensor die (measured with specially integrated and calibrated element)
and the object temperature that we need.
Note that these are all in Kelvin, heat exchange knows only physics.
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 MLX90614 in the dashboard I start getting errors larger than specified in spite 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
portion of the dashboard opening 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 MLX90614 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.
3901090614
Rev 009
Page 45 of 52
Data Sheet
June 29, 2015