Partially Obscured Targets; Targets Smaller Than Field Of View; Low Or Changing Emissivities - RayTek MARATHON FA Operating Instructions Manual

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.
2.1.1 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.
1‐color sensors see polluted atmosphere and dirty windows and lenses as a reduction in
energy and give much lower than actual temperature readings!
2.1.2 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 as long as the background temperature is much
lower than the target's. 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.
2.1.3 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 greybody. 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 as long as 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.
Marathon Series FA/FR
Product Description
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