How easily a thermal image can be misinterpreted?

Advancements in technology have seen the cost of rather new thermal imaging technology reduce significantly where now some manufactures are able to produce thermal imaging cameras for under £1000.00. Whilst this is great, I expect that thermal imaging cameras will become common technology affordable to everyone in the industry and therefore subject to the same criticism as other tools such as; the electrical moisture meter.

As I’ve already mentioned in my previous article “Thermal Imaging for damp inspections” understanding the use of thermal imaging cameras is not that dis-similar to electrical moisture meters. We’ve all heard about cowboy damp proofing companies using moisture meters as the only source of diagnostic equipment undertaking remedial repairs where ever high readings or a red light is observed. As professionals however, we know and understand the limitations of the moisture meter and that results displayed need to interpreted and analysed either with further investigations or judgement on a professional level.

This knowledge and understanding of the equipment comes initially from education and developed further with experience.

It is essential that if you are using thermal imaging technology in your investigations that you understand its limitations as in the hand of an idiot any equipment can lend to miss identification and diagnosis.

So how can a thermal image can be miss interpreted?

Thermal imaging cameras basically see temperatures emitted from the surfaces of materials (infrared radiation). All materials above absolute zero temperature (-273.15 degrees C or 0 Kelvin) will emit radiation in proportion to their temperature. The camera is able to measure this radiation across a particular field of view through a detector and display the results as a picture (thermal image). A colour spectrum is used to display the variance in radiation within the image, with warmer temperatures represented by white/yellow colours and cooler temperatures by black / blue.

Unlike visible light where the colours we see are simply a reflection of light from the objects surface. The big difference in thermography, is that infra red images express’s both reflective and emitted temperatures. All materials will emit and reflect infrared radiation differently, in order to achieve accurate temperature measurements the values of emission and reflection must be known by the operator and programmed into the camera to avoid misleading results.

The texture, decay and oxidisation, shape and degree of viewing angle of the surface will all affect image displayed and must be known and understood by the user. The weather, wind speed, humidity and time of day will also affect image results.

See below, this is an image of my toaster which had just been turned on, Its construction is plastic trim edges with a stainless steel front.

Looking at the image and the colour variations you would assume the middle of the toaster (with exception to the orange band) is much cooler than the surrounding plastic edges as the colour changes from purple in the centre to yellow/white at the edges. This visual variation in temperature can be seen by looking at the scale on the righ.

In fact however, the colour variation is merely an illusion caused by a change in material. The darker purple front is a reflective stainless steel material reflecting radiation from the surrounding surfaces which in this image happens to be the nights sky and myself.

The accurate temperature of the stainless steel fronted toaster is only achieved by reading from the non reflective material which I have adhered to the face stainless steel plate, seen in the image as the orange band.  The heat emitted from the toaster is transferred through the non reflective material allowing an accurate temperature measurement to be recorded.

Sometimes it is the very small detail noted by the surveyor that can make an image outstanding.

As we can now see by the two temperature measurements plotted on the image there is actually no difference in temperature between the stainless steel front and the specific area of plastic trim – illusion resolved.

Obviously my reflection into the camera as a person is noticeable however, could easily be miss interpreted as another object or surface reflection during a building survey. For the enthusiastic thermographers amongst you, If you look close you will notice a bottle in my hand, a keen eye will also be able to see the level of liquid in the bottle.

This is an image which to the untrained and inexperienced could easily be misinterpreted. Unfortunately for a thermographer buildings and the materials used in construction are complex with varying degrees of emissivity. In addition, as surveyors we also have to comprehend with challenging conditions such as solar gain, night sky radiation, wind and rain making the task of interpreting images even more difficult. It is therefore essential that if your using thermography as a building survey tool that your trained in the cameras use.

I’ve been using my high end Flir infrared thermal imaging camera for almost three years now and am trained and qualified in its use. Its presence on my surveys have proved invaluable, helping me identify problems which weren’t apparent to the naked eye without time consuming and disruptive investigations.  I have however had the benefit of training to be able to use my thermal imaging camera correctly and to the best of both its and my abilities.

If you suspect a problem or need specialist advice call the “Qualified, Trained and Experienced Experts” Dryfix Preservation Ltd “Yorkshires Leading Damp and Timber Specialists”

www.dryfix.net
01904 791388

Russell Rafton C.S.R.T / A.Inst.SSE
Dryfix Preservation Ltd – Surveyor