Sensor Installation
Modline 5
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3.5
Measuring Temperature with Brightness and Ratio Sensors
3.5.1
Resolving Targets with Brightness Sensors
When using single color Series 52, 56 and 5G brightness Sensors, best results are obtained when the
resolved portion of the target is two times the diameter of the reticle.
The Sensor measures the average temperature of the target or object seen in the reticle. Targets smaller
than the reticle will result in low temperature readings if the background (object within the cone of
vision beyond the target) is lower that the target temperature. If the background temperature is higher
than the target, the averaged temperature indication will be higher.
Low temperature indications can also be caused by other factors. An out of focus cool object cutting the
cone of vision anywhere between the target and Sensor, or a small target may shift position and move
partially out of the measurement cone.
3.5.2
Obstructions in Cone of Vision
Errors can be caused by objects positioned between the Sensor and target. Evaluate the Sensor’s “Cone
of Vision” and if there is any possibility that an intervening object is within the cone, select a different
viewing position. An object inside the Cone of Vision will be visible to the detector and may cause an
error in the temperature measurement. Obstructions in the cone of vision, whose temperatures are
lower than the target temperature, will generally cause lower temperature readings when using single
color brightness units. The obstruction may not be seen clearly in the instrument sight if it is some
distance away from the target and out of focus. Such an obstruction is illustrated below.
High temperatures can be caused by an out of focus hot object cutting the cone of vision anywhere
between the target and Sensor.
Two color ratio units are usually not affected by obstructions that are significantly lower in temperature
than the target. Ratio units are affected by hot objects in the cone of vision that are much higher in
temperature than the target. If too high, the hot targets can dominate the measurement. If the object
temperature is the same, the measured temperature is not affected.
Figure 13: Cone of Vision Obstruction
3.5.3
Resolving Targets with Two Color Ratio Sensors
Modline 5 Series 5R Ratio Sensors utilize a dual detector assembly that measures temperature by
comparing infrared radiation levels in two wavelength bands (0.85 to 1.05 microns and 1.0 to 1.1
microns). Temperature readings are based on the ratio of the two signals in these bands.
Obstruction
Target
Cone of Vision
Sensor
