Basics
Theory for 2-Color Measurements
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3.3.3 Targets Smaller Than Field of View
When a target is not large enough to fill the field of view, or if the target is moving within the field of view, radiated
energies are equally reduced, but the ratio of the energies is unaffected and measured temperatures remain
accurate. This remains true if the background temperature is much lower than the target. The following examples
show where 2-color sensors can be used when targets are smaller than the field of view:
•
Measuring wire or rod
— often too narrow for field of view or moving or vibrating unpredictably. It is much
easier to obtain accurate results because sighting is less critical with two-color sensors.
•
Measuring molten glass streams
— often narrow and difficult to sight consistently with single-wavelength
sensors.
3.3.4 Slope
The slope is the quotient of the emissivities based on the narrow and the wide spectral range (first and second
wavelength). The factory default preset slope is 1.000.
For information on determining an unknown slope, and for sample slopes, refer to section 15.5
Note
The slope is the most important parameter for measurements in 2-color mode! The emissivity affects only
measurements in 1-color mode.
3.3.5 Attenuation
The Attenuation parameter indicates the degree of reduction of the input signal. Attenuation is applicable for
sensors in 2-color mode only. Three causes may contribute to a loss of infrared signal from the target:
•
Low target emissivity
•
Target is too small to fill the measured spot size
•
The optical path is partially obstructed
– as with smoke, steam, dust, a dirty window, or solid obstructions
The total reduction in signal is the sum of the losses from all three causes. The specified attenuation
is how much
reduction in signal the instrument can handle and still achieve an accurate temperature measurement.
Example: An instrument has a specification of 95% for the signal attenuation.
Assume a target with Emissivity = 0.45 equivalent to 45% signal and corresponding to 55% signal loss (100%
- 45% = 55%)
Transmissivity = 0.0
→
Another 40% maximum can be lost due to an unresolved target or obstructions in the field of view.
For the allowed attenuation over the measurement temperature range, see section 15.6
