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Quantitative Thermography vs Qualitative Thermography?

emissivity, Qualitative, Technical, thermal scanning,

July 30 , 2021

There are two main types of thermal screening analysis that can be completed.  These are quantitative and qualitative, where quantitative aims to find the exact temperature of a component and qualitative compares similar components to find faults.  Most thermal surveys will be completed in a qualitative manner due to the additional technical hurdles involving emissivity which increase the associated costs. Certain fields of thermography lend themselves to qualitative inspections; especially electrical thermal scanning and mechanical thermal imaging, as there are similar components to compare each one to.  These technical hurdles include knowing the materials true emissivity, atmospheric conditions and utilising angles of incidence.

Is Qualitative accurate?

The simple answer is yes!  This is because when completing a thermal scan in a qualitative manner components are being compared to one another and the difference in operating temperatures is what is being considered.  Another way to consider this is that when looking at components in an electrical board (such as the photo below) the same components will all have the same emissivity and be under the same atmospheric conditions such that their baseline temperatures are all factored by the same amount.

Above is a qualitative analysis and as can be seen there is a very noticeable difference in temperature between the same components.  Showing that there is a fault on the centre component.

Why is Quantitative analysis so much more difficult?

When attempting to gather accurate temperature data, the exact atmospheric conditions present must be known such as; humidity, ambient temperature and reflected temperatures.  In addition the distance from the imager to the components must be known.  These are able to be gathered accurately through the assistance of other equipment such as anemometers (pictured right).  

However, the most important and hardest to account for is the materials emissivity.  This is because the emissivity of an object changes depending on various factors such as; what the component is made of, the components surface texture and even the exact temperature it is at.

How can thermographers account for all this?

Depending on the accuracy requirements thermographers can utilise emissivity tables which will give estimates for materials, (such as this one https://www.sika.net/images/Documents/Table_of_Emissivity.pdf).

As can be seen in the table, even aluminum can vary from .02 -> .40.

Luckily thermographers have another way to overcome these limitations, we can utilise other materials such as specialised high-emissivity tape or sprays which will bring the emissivity close to a more controlled value that is known. However this is usually a length process as the thermographer has to either apply the tape or spray to a component (which must be turned off!) or to an adjoining location and calculate the relative emissivity.