Applications Guide
Modline 5
147
the Sensor Models 5R have somewhat looser restrictions on filling the field of view, it is still affected by
background interference if significant sources of background radiation are present.
Significant background radiation comes from comparable temperature or hotter objects in the target's
surroundings. This background radiation may be reflected off, or transmitted through, the target adding
to the radiant energy detected by the instrument related to the target temperature. This results in
measured temperature error. For all Modline 5 Sensor Series, except the Series 5R, this extra radiation,
if detected, causes a temperature indication that is higher than the target's true temperature.
For Series 5R Sensors, background interference that is slightly cooler than the target results in
temperature indications that are too low. When the background temperature is much lower than that
of the target, however, the indicated temperature is unaffected. If the interfering background sources
are hotter than the target, the indicated temperature is too high. There is no error in the indicated
temperature, with a Series 5R Sensor, when the background temperature matches the target
temperature.
Figure 86: Viewing Angle
When the environment includes unfiltered background light or heat sources, select a viewing
arrangement that minimizes these problems. In general, the higher the target's Emissivity, the less
susceptible the measurement is to errors.
Emissivity (E), Reflectance (R), and Transmission (T) are related as follows:
E
= 1 - T
- R
Maximum accuracy is possible when E = 1.0 (blackbody condition). In this condition, there is no
reflection and no transmission of background energy to cause measurement errors.
As emissivity decreases it is harder to get accurate readings because reflectance and/or transmittance
become more pronounced. Use caution when attempting to measure materials with emissivity values
known to be 0.2 or less.
8.8.3
Transmission Effects
If the target has some transmission at the operating wavelength, it can act as a window for infrared
emission from objects behind it (e.g. an oven wall or heating element). Such problems can sometimes
be corrected by changing the viewing angle so that the background source is not directly behind the
Sensor Lens
Max = 45
Target
