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Know your IR window transmittance

13 October 2011

When taking radiometric thermal images it is important to know what variables in the external environment, such as reflection and emissivity, can lead to possible measurement errors. But when thermographers use IR windows, improper window transmission compensation can easily affect apparent temperature and delta T calculations by 30% or more and that must be factored in too. IRISS tells you how to avoid the pitfalls in part four of its Ten Things To Know About IR Windows.

Maintenance professionals can use infrared (IR) cameras to take both qualitative (image only) and quantitative (temperature measurement) images. To ensure that quantitative images are accurate, it is important to understand what other variables in the external environment can lead to possible measurement errors.

In addition to reflection and emissivity, distance, humidity and camera angle can all play an important role in accurate temperature measurement. But when thermographers use IR windows, improper window transmission compensation can easily affect apparent temperature and apparent ?T calculations by 30% and more.

Infrared inspection windows have become an industry standard way to facilitate infrared electrical inspections while increasing both the safety and the efficiency of the inspection process. While window optics can be made of a variety of different materials, they generally fall into one of two categories: crystal or polymer. It is important for the thermographer to know the transmission rate of the window being used. However, some crystal windows that look similar might actually be made from different materials, causing the transmission rates of two apparently similar windows to be very different. Even the same crystal material from the same manufacturer may be cut to different thicknesses. This difference will cause the thicker crystal to have a lower transmission rate than a thinner lens of the same material.

These potential sources of variation underscore the importance of understanding the characteristics of the inspection window being used.

Transmission rates of different materials will change across the IR spectrum. Some materials will be unsuitable for use with a LW camera (as is the case with sapphire, Al2O3). Other materials, such as Zinc Selenide (ZnSe) are well suited for use with both MW and LW – these materials also tend to be more expensive for that reason.

Given the transmission variability across different wavelengths, one needs to define the transmission rate at a specific wavelength. Our research shows that this “PdM” wavelength is at about 9µm in the LW band and about 4µm in the MW (SW) band.
For accurate temperature calculations, it is irrelevant whether the window’s transmissivity coefficient is 90%, or 50%... or something in between. What is important is that the thermographer knows the exact transmission rate. Then when the thermographer enters the correct coefficient into the camera or software, the final temperature calculation will be accurate and reliable. If, however, the thermographer is not aware of the actual transmission rate, or does not adjust for it, the errors can be significant.

Kirchhoff’s Law and Infrared Windows
As discussed in Chapter 3 of this series, Kirchhoff’s Law shows us that the total infrared radiation a camera receives is actually composed of the total emitted, reflected and transmitted radiation from an object. With this in mind, the ideal window would therefore be one that allows 100% transmission, i.e. zero loss to emittance and reflection. To maximize transmittance, and minimize emittance and reflectance values, manufacturers carefully select optic materials and window coatings for their performance in specific or general wave bands. Unfortunately, as Chart 1 depicts, with the materials presently available, 99% is the highest transmission rate available (coated ZnSe in the LW portion of the spectrum).

Note that calculated temperatures can be adversely affected when a thermographer improperly compensates for transmission.
 
All window optics will filter, or “attenuate” the amount of infrared radiation that ultimately reaches the imager. Our research shows that for calcium fluoride windows the attenuation reduces the temperature about 5% for every millimeter of window thickness (temperatures in the range of 60°-120°C). In addition, tilting the camera angle 30° to either side of 90° (perpendicular) reduces the temperature by an additional 2-3%.

Compensate for Transmission
A simple way to check transmission rate is the “Coffee Cup Test”. After determining the proper transmission rate of a window, make a note of that value and adjust the camera’s transmission rate (or emissivity setting) accordingly each time that window is used for an inspection.

The majority of infrared cameras do not have the ability to compensate for transmission losses from an IR window directly; but almost all software packages do. To make a quick adjustment in the field, simply multiply the transmission coefficient by the target emissivity to arrive at a “calculated emissivity value,” and adjust the camera’s emissivity setting to this value. For example, an IR window with a 0.55 transmittance rate and a target with an emissivity of 0.95 (electrical tape) would require a camera emissivity setting of 0.49 (0.90 x 0.55 = 0.495) to properly compensate for both transmission and emissivity.

Degradation
Once the transmissivity coefficient has been properly determined for an application, it is important to realize that coefficients of some materials can change over time. Crystals, in particular, are susceptible to mechanical stress from high frequency noise and vibration. Some crystals, such as those in the Fluoride family are also hydroscopic; meaning that they will absorb moisture, humidity or industrial solvents (despite the fact that they are coated with materials to slow the inevitable degradation process). These mechanical stresses and absorption characteristics of crystals will degrade a crystal window’s transmission rate over time.

A recent paper on the topic of transmission degradation (“Transmission Stability and Infrared Windows” by Joe DeMonte) illustrates how this degradation can affect the data accuracy. The research compared a Calcium Fluoride infrared window to a polymer-based window. Both windows were shown to have a 50% transmission rate when new. After just two years in a non-aggressive environment, the transmission rate of the calcium fluoride window had dropped significantly.
   
The white paper 
was published in the April 2009 edition of Uptime Magazine.

Recalibration of Window Transmittance
The “Coffee Cup Test,” or some similar form of verification, should be performed prior to installation – and again prior to any important measurement or inspection if using an optic that is known to degrade over time. Furthermore, because each crystal is unique, transmittance must be tested for each crystal since each one will have a slightly different transmittance rate when new. Similarly each crystal’s rate of degradation will be unique as well.

However, if using an engineered polymer, testing a single window when it is new should be sufficient since all windows will be virtually identical in transmittance and since the material will not degrade over time.

Maintaining IR windows
During maintenance shutdowns, firms should check gaskets and screws for seal integrity, and windows should be thoroughly cleaned with a compatible cleaning agent. If using crystal IR windows, this is also a great time check transmittance. 
 


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