Transmission Stability and Infrared Windows:
The Effects of Transmissivity on Data Accuracy
by: Joe DeMonte ASNT/PdM Level III Thermographer
Abstract As the saying goes, “garbage in, garbage out.” This truism is every bit as applicable in thermography as it is in computer data-mining. The difference is that the inaccurate data which leads a thermographer to a false-negative conclusion could result in a multi-million dollar catastrophic failure of a company’s electrical distribution system. In fact, the implications to personnel safety, plant assets and production downtime make the results of transmissivity errors more like toxic waste than mere “garbage.” When using infrared (IR) windows or sightglasses, it is imperative to understand the accurate transmission rate of the optic used in the infrared window. As this paper will explore, failure to accurately compensate for actual transmission attenuation can lead to significant errors in data. The magnitude of the error is based on the exponential effect that target surface temperature has on radiated infrared energy. In short, temperature differences (T) will appear to be minimized if the effects of transmission attenuation are not considered, or if not accurately compensated for. Such errors in T may thereby lead thermographers to underestimate the magnitude of many serious electrical faults.
Transmission Stability & IR Windows © 2009; Joe DeMonte
Background As an instructor for both ITC and for TEGG Corporation, I have noticed that many Calcium Fluoride windows, even in controlled environments, have lost significant transmission rate within just a two to three year timeframe. In fact, in 2003 I came across a Calcium Fluoride infrared sightglass (shown in Figure 1) which had lost all transmissivity in the infrared and visual spectrums. It was being used in a motor termination box in an electrical generation plant in Figure 1 – Calcium Fluoride Sightglass Tennessee. This is an extreme case to...