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Infrared Spectrum

Infrared covers four regions of the spectrum; (a) Near infrared, 0.7 -1 micron, this is nearest to the visible wavelength. (b) Short wave, 1.0 - 2.5 microns. Both of these wavelengths regions rely on reflected solar radiation and can only be used in daylight or illuminated conditions. (c) the Medium or short wave thermal infrared, 3 to 5 microns, detects radiation emitted from objects and can be used in total darkness or daylight. This wavelength is often used for high temperatures such as boilers, kilns etc. (d) the Long wave thermal infrared, 8 -14 microns, is most commonly used in industry since the detectors are efficient at environmental temperatures and can also be used for high temperature operations with appropriate filtering.

Thermal Imaging Optics

Most materials are opaque to medium and long wave infrared including glass and water. Optical materials such as germanium and some other exotic materials such as zinc sulphide, zinc selenide, magnesium fluoride and sapphire are used since they are mostly transparent to the thermal wavelength. These materials are very expensive. Some low grade commercial thermal imagers utilise composite materials to lower the costs but there is no compromise where quality is required. Most military specification thermal imagers use coated germanium and optical magnification rather than digital magnification.

Industrial Thermal Imagers

Over the past few years there has been an incredible advancement in thermal imaging technology. Miniature electronics has allowed for cameras to become very much smaller and far more efficient however the optics and detectors are still very expensive.

Industrial applications can be broken down into two basic categories; Ground and Aerial. Although it is not quite this simple, most ground applications can be accommodated using good quality hand-held equipment. Aerial surveys however require high thermal and spatial resolution to provide quality data at long range. This very specialised equipment is usually mil-spec or high end commercial and few companies in the UK have invested at this level.

The most common types of thermal imaging equipment used in industry are: Pyro-Electric Vidicon, (PEV), Focal Plane Array (FPA) and the SPRITE (Signal Processing In The Element). The PEV is the lower end of the price scale and image quality but is perfectly acceptable for Electrical Condition Monitoring and close proximity surveys. The FPA is usually of mid range and is used for general survey applications. The SRITE detector is generally used in top grade military equipment. Each type has advantages and disadvantages and more detail can be given on request.

Interpretation of Thermal Images

Aerial or Ground infrared, also known as Thermal Imaging or Thermography, is best suited to give qualitative rather than quantitative data. Infrared non-contact quantitative systems need accurate information of surface emmissivities if radiant energy is to accurately relate to surface temperature.

Thermal images recorded in 8 -14 micron wavelength (Long Wave) have no visible content and no natural colour. Because of the long wavelength, thermal or infrared images cannot compare in spatial resolution to visible photographs that are recorded in 0.4 -0.8 micron band. Thermal resolution and spatial resolution are inter-dependant to produce good quality thermal images, therefore if a house roof is close to or the same temperature as the nearby road, for example, the image will appear dull grey with little detail. Temperature contrast will produce picture or image contrast and high detail.

Thermal imagers detect and record Infra-red radiation emitted from the surface of any subject being viewed. The imager does not have the ability to see below the surface. However, the radiation from the surface is often influenced by sub-surface detail, which effects the thermal characteristics of adjoining material(s).

When looking at a large area, the emissivity of various surfaces must be considered. Most materials found on the surface of buildings will have a relatively high emissivity but there will still be noticeable differences in the perceived image due to a change in surface material. This can be overcome by a detailed knowledge of the building under investigation. Some metals and glass can reflect infra-red radiation and apparent 'hot spots' can be a reflection from a hot object nearby.

Infra-red aerial surveys provide a 'global' visualisation of heat radiation from building surfaces. It is useful data for determining areas of concern or for determining work priority. We can also provide detailed ground level thermal imaging surveys in support of aerial data.

Infra-red surveys of heated buildings are always conducted during the evenings of the Winter/Autumn months of the year.

Aerial thermal Imaging for surveying buildings for heat loss and moisture operate in 8-14 micron wavelength and detect heat only, visible light is not detected at all. Heat energy from daytime sunshine can be absorbed in brickwork and therefore a survey is conducted well after sunset to ensure that all effects of solar energy have dissipated. 

Infra-red - just like visible light - absorbs, reflects and re-radiates from materials in amounts depending upon their colour and structure. Brown building bricks for example absorb and retain heat energy more readily than lighter coloured building materials, this must be considered when analysing data since they may effectively appear at different temperatures simply due to their emissivity. Water bodies such as rivers or lakes retain heat and are slow to change with ambient temperature changes, whereas ground surface temperatures can change rapidly. This is why water often appears warmer than its surroundings.

A thermal Imager detects and displays surface temperatures only. The surface temperature under normal conditions is the result of heat energy conduction through the walls from a heated internal room. Moisture is an excellent conductor of heat and when insulation is damp it can become a conductor rather than an insulator. 


When analysing thermal data, monochrome images are normally preferred because of the wide range of grey tones of temperature that can be differentiated by computer. There are of course no natural colours in the long wave infra-red wavelength so we apply a palette of colours to the grey tones. Colour images are very much easier for the naked eye to interpret so both colour and monochrome are supplied.

To assist in analysing your aerial data a temperature palette is provided for assessing temperature differences.


Emissivity

A measure of the ability of a surface to radiate energy as measured by the ratio of the radiant flux per unit area to that radiated by a black body at the same temperature. 

Example:- Black body = 1.00
Red Rough House Brick = 0.93
Polished Aluminium = 0.095
A comprehensive emissivity list is available on request
 

Seeing Heat Energy

The human eye is designed to see visible light and colours as we know them but below red or infra-red is beyond our capabilities. Infra-red light is not visible but can be sensed and felt as warmth. An infra-red camera or Thermal Imager detects heat energy and converts it to an electrical signal which is then processed into an analogue representation of the subject. Since there are no colours in the infra-red spectrum, 
(8-14 microns) colour palettes are assigned to the grey tones to represent temperature bands. 

Atoms and molecules are affected by magnetic and electrical components of light.
Different materials absorb and reflect thermal infra-red at different wavelengths depending on the composition of each material. A heat 'signature' can therefore be assigned to each mineral. Visible light is also absorbed and reflected off materials at different wavelengths and we see these as colours.

Flat Roofs - Example Application

Flat roofs are usually thermally imaged for heat loss during darkness but can also be conducted during daylight under certain conditions. In general, the main reason to thermally image a flat roof is to detect the presence of moisture ingress beneath the roofing felt. 

Moisture is an efficient conductor of heat energy. Wet or damp insulation renders the insulation useless and it becomes a conductor which is worse than having no insulation. 

There are two methods of detecting the moisture;
1. to thermally image the roof surface for temperature anomalies at night or on an overcast cold day to record conduction of heat from within a heated building , or,
2. to record heat 'sink' into the roof on a warm day using solar energy as the source of heat. Both method s are very effective.

 

For Archived Projects and Information visit  www.thermal-imaging-survey.co.uk

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