Thermal Imaging is the conversion of radiated or reflected
heat into real-time pictures or images. A thermal image is an analogue
pictorial representation or visualisation of temperature differences.
All objects above absolute zero (-273 degrees) emit radiation, some of which
is infra-red. Depending on temperature and emissivity, most objects in the
world can be thermally imaged.
Horton Levi specialises in all forms of thermal imaging surveys; electrical
condition monitoring, heat loss surveys, cavity wall inspections, damp
tracing, sub surface pipes etc. Our Aerial survey work includes heat loss
mapping of cities, land fill sites, tyre dumps, pipelines and electrical
power lines. Apart from survey work we are involved with a number of EC
research and development projects.
To find out how thermal imaging can assist you,
here to contact us.
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
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
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
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.
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
Atoms and molecules are affected by magnetic and electrical components of
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
Centre for Alternative Technology.
Information about alternative/renewable technologies and energy
Energy Saving Trust.
Energy efficiency advice for domestic and small/medium enterprises.
Energy Action Grants Agency.
Information about the government grants scheme for energy efficiency.
Leeds Council - HECA page.
An explanation of the HECA act and the councils energy efficiency