Endurance
®
Series
Users Manual, Rev. 2.0, Jun 2020
28
3.3 Theory for 2-Color Measurements
The 2-color ratio technology allows accurate and repe
atable temperature measurements, which don’t depend on
absolute radiated energy values. In use, a 2-color sensor determines temperature from the ratio of the radiated
energies in two separate wavelength bands (colors).
The benefits of 2-color sensors are that accurate measurements can be made under the following conditions:
•
When the field of view to the target is partially blocked or obscured.
•
When the target is smaller than the sensor’s field of view.
•
When the target emissivity is low or changing by the same factor in both wavelength bands.
Another benefit is that 2-color sensors measure closer to the highest temperature within the measured spot (spatial
peak picking) instead of an average temperature. A 2-color sensor can be mounted farther away, even if the target
does not fill the resulting spot size. The convenience is that you are not forced to install the sensor at some specific
distance based upon target size and the sensor’s optical resolution.
3.3.1 Low or Changing Emissivities
If the emissivities in both wavelengths (colors) were the same, as they would be for any blackbody (emissivity = 1.0)
or graybody (emissivity < 1.0 but constant), then their ratio would be 1, and target emissivity would not be an
influence. However, in nature there is no such thing as a graybody. The emissivity of all real objects changes with
wavelength and temperature, at varying degrees, depending on the material.
When emissivity is uncertain or changing, a 2-color sensor can be more accurate than a 1-color instrument if the
emissivity changes by the same factor in both wavelength bands. Note, however, that accurate measurement
results are dependent on the application and the type of material being measured. To determine how to use 2-
color sensors with your application when uncertain or changing emissivities are a factor, please contact your sales
representative.
Note
Any dirt (dust, fingerprints) on the optical lens or vision window with unknown spectral characteristics can
influence the measurement result in 2-color mode. Unpredictable temperature readings may result in such a
case!
3.3.2 Partially Obscured Targets
The radiated energy from a target is, in most cases, equally reduced when objects or atmospheric materials block
some portion of the optical field of view. It follows that the ratio of the energies is unaffected, and thus the measured
temperatures remain accurate. A 2-color sensor is better than a 1-color sensor in the following conditions:
•
Sighting paths are partially blocked (either intermittently or permanently).
•
Dirt, smoke, or steam is in the atmosphere between the sensor and target.
•
Measurements are made through items or areas that reduce emitted energy, such as grills, screens, small
openings, or channels.
•
Measurements are made through a viewing window that has unpredictable and changing infrared
transmission due to accumulating dirt and/or moisture on the window surface.
•
The sensor itself is subject to dirt and/or moisture accumulating on the lens surface.
Note
1-color sensors see polluted atmosphere and dirty windows and lenses as a reduction in energy and give much
lower than actual temperature readings!
Note
For accurate temperature readings in the 2 color mode, an attenuation in the sighting path must be equal in both
spectral bands!
