EVB90615 Melexis Inc, EVB90615 Datasheet - Page 26

• Manufacturer Part Number: EVB90615
• Description: EVAL BOARD FOR MLX90615
• Manufacturer: Melexis Inc

Specifications of EVB90615

• Sensor Type: Temperature
• Sensing Range: -40°C ~ 85°C
• Interface: SMBus (2-Wire/I²C)
• Embedded: Yes, MCU, 8-Bit
• Utilized Ic / Part: MLX90615
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Voltage - Supply: -
• Sensitivity: -
• Other names: Q4435148

Electronic semiconductor products are sensitive to Electro Static Discharge (ESD).
Always observe Electro Static Discharge control procedures whenever handling semiconductor products.
When I measure aluminium and plastic parts settled at the same conditions I get significant errors on
aluminium. Why?
Different materials have different emissivity. A typical value for aluminium (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
where
ε
α
σ is the Stefan-Boltzmann constant,
A
F
T
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 aluminium) 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 is a specialised 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 aluminium 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 MLX90615 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.
3901090615
Rev 001
q
1
1
a-b
1
14 ESD Precautions
15 FAQ
1
=
and ε
is the absorptivity of the sensor (in this case),
and T
and A
is the shape factor,
ε
1
.
α
2
2
1
2
are the emissivities of the two objects,
.
are known temperature of the sensor die (measured with specially integrated and calibrated
are the surface areas involved in the radiation heat transfer,
( )
T
1
4
.
σ
.
A
1
.
F
a
−
b
−
ε
2
.
( )
T
2
4
.
σ
.
A
2
,
Page 26 of 30
Infra Red Thermometer
MLX90615
Data Sheet
28/Aug/2008