© Thermoteknix Systems Ltd 2020
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Skin temperature measurement
The detector in the thermal imager is sensitive to infrared radiation. Everything
radiates infrared according to natural laws and in proportion to temperature and
emissivity. The emissivity is a measure of a body's radiation efficiency and is strongly
influenced by surface texture. The detector measures the radiation after it has
passed through the atmosphere and the IR the FevIR Cam
’s objective lens. Both of
these attenuate the signal and affect the efficiency of detecting the radiation. This
affects the system's ability to measure temperature remotely.
Your thermal imager is based on the latest microbolometer technology. All thermal
imagers in this class exhibit a degree of “pixel to pixel drift”. This
must be corrected
using an internal paddle with an even (uniform) temperature which is situated in
front of the detector. This is referred to as
“Non Uniformity Correction” or “NUC”.
Periodically the uniform paddle, which is at a known temperature, is moved into its
calibration position in front of the detector for a second or two and the image will
freeze during this period. No thermal imaging will take place while this occurs. The
data obtained during the NUC operation is used to correct for non-uniformities
caused by thermal drift due to the warming of components inside the FevIR Cam and
substantially improves the image quality and is essential for accurate temperature
measurement. You may notice that periodically the image freezes and you may hear
a clicking sound. This is the NUC process operating automatically. When the imager
is first turned on the NUC process is frequent but as the imager approaches thermal
equilibrium after 20 or 30 minutes then the period between NUCs extends.
The Calibration Reference Source supplied with the FevIR Cam system has a known
emissivity and temperature setting. This is then used to remove calibration
uncertainties and inaccuracies to provide excellent temperature measurement
capabilities.
The emissivity of human skin has little person to person variation and is not affected
by skin colour. The same is not true for variations in core body temperature either
from one person to another or indeed for the same person at different times of the
day or month or in different environmental conditions. Normal personal body
temperature can vary by up to 2°C over the circadian rhythm and skin temperatures
can vary by 8 or 10°C.
Febrile conditions lead to an elevated core temperature but the extent of this
elevation on the skin, if any, may not correlate with the severity of the fever.
Furthermore, elevated core temperature in one person may correspond to normal
body temperature in another. It should also be noted that cosmetics affect skin
temperature measurement as does sweating which both lower the measured
temperature value.
