Product Description
Marathon MR Rev. E2 04/2011 9
2.1 Theory of Operation for 2-Color Sensors
Two-color ratio technology makes possible accurate and repeatable temperature measurements that
are free from dependence 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 target emissivities are 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.
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.
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.
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