In production as well as in quality control and maintenance, temperature represents an important indicator of product quality or equipment conditions. Advances in detector and optical technology of infrared thermometers are helping to expand the types of applications where IR can be used effectively and yield a better return on investment. Difficult Applications involving high temperatures and environments with smoke and dust are now being overcome using the principles of two-colour ratio measurement.
A two-colour instrument uses two detectors at two wavelengths looking at the same hot target. The graph (Figure 1) shows the blackbody curve for a target at 1500°C and an emissivity of 1,0. Detector 1 puts out 500 units of energy at that temperature and detector 2 puts out 1000 units. This is called a ratio thermometer; dividing the signal of 2 by that of 1 we get a ratio of 2. Therefore, the number 2 indicates the target is at 1500°C.
Figure 1
Now let us introduce factors to degrade the measurement. Let the lens get dirty, do not fill the optical spot, dust in the way and eventually as much as 90% of the signal can be lost.
Figure 2 is the same graph with a second curve added that shows the 1500°C target but with the emissivity reduced by 90% ie, it is now 0,1. The signal from detector 1 is 50 units and from detector 2 it is 100 units. The ratio is also 2 and the instrument says the temperature is still 1500°C even though 90% of the signal has been lost. This is the advantage of two-colour measurement in this application.
Figure 2
R&C Instrumentation invites interested readers to contact them to get the brochure ‘Principles of Infrared Temperature Measurement’ which covers the theory of infrared temperature measurement including single and two-colour ratio measurement.
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