Product Description
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 | 3 |