- Aerial thermal imaging
- Sample imagery
- Important points to note
- Further information
Thermal imaging involves the use of a special thermal camera, which operates within the infra red area of the spectrum to measure the amount of heat that an object is emitting. Infra red covers four regions of the spectrum: near infra red (0.7 - 1.0 micron); short wave (1.0 - 2.5 microns); medium or short wave thermal infra red (3 - 5 microns) and long wave thermal infra red (8 - 14 microns). All objects above absolute zero will emit radiation and so can be thermally imaged.
The ability of an object to emit infra red radiation is termed 'emissivity' which is a relative value given to all objects measured in comparison to a black body at the same temperature.
Sample Emissivity Values
Black body = 1.00
House brick = 0.93
Aluminium = 0.095
As evident from the above list the 'shinier' the surface the lower the emissivity value - hence a brand new slate roof may appear to be emitting less heat than an older concrete tiled roof and buildings constructed of dark materials will also absorb and retain heat far more readily than those constructed of lighter materials.
Thermal imaging has numerous uses
- Animal censuses
- Flat roof surveys
- Cold store surveys
- Pipeline surveys
- Building heat loss maps
- Damp mapping of buildings
- Detection of moisture ingress
- Power line surveys
For more information on possible applications of thermal imaging please see the link to the Thermal Imaging Surveys website in the external links section at the bottom of the page.
An aerial thermal survey involves the use of modified military specification equipment to find the amount of infra red radiation being emitted from the surface of any object in view - roofs, cars, lakes etc. Most materials will have a fairly high emissivity, however differences will be noticeable between different surfaces. Some metals and glass will actually reflect infra red radiation and hence a falsely high reading can be generated by a reflection from a hot object nearby. Large bodies of water such as rivers or lakes will retain heat better than the surrounding earth and hence water often appears warmer than its surroundings.
The aeroplane typically flies at a height of 2000 feet and makes a series of overlapping passes over the area to be surveyed. The aircraft is fitted with satellite navigation equipment to prevent drifting, which would lead to 'holes' appearing in the map where areas have been missed. Aerial thermography can only be undertaken on a cold, clear, cloudless night during the winter months and at least two hours after sunset to minimise the effects of the daytime sunshine and to ensure that most properties are heated to a comfortable level.
The thermal imager will detect and display the surface temperature only, which is the result of heat energy lost from the building below. Monochrome images are generated (there are no natural colours in the long wave infra-red spectrum) and hence a colour palette is applied to the grey tones to make them more interpretable to the naked eye.
For a much more detailed explanation of thermal imaging and the equipment and techniques employed please visit the website of aerial thermal imaging specialists Horton Levi, by clicking on the link provided in the external links section at the bottom of the page.
The photograph below shows an original image as obtained in flight:
The image below shows the above greyscale image after processing and colourisation:
Further processing is necessary to generate the image shown below, which has been overlaid with Ordnance Survey data:
For a much more detailed explanation of this process please follow the link to the website of Hot Mapping, which is provided in the external links section at the bottom of the page.
Thermal imaging can be fantastically useful, providing information in moments that would otherwise be very difficult to obtain. However it is important to bear in mind the following points when looking at an aerial thermal image:
- The image is simply a snapshot at one moment in time of how much heat a property was losing.
- The colour of the property does not necessarily indicate how energy efficient or well insulated it is.
The colour of the property will be affected by the following factors:
- How many people were at home on the night of the survey
- Whether the heating was turned on on the night of the survey
- The building materials used in constructing the property
This means a well insulated property may appear to be losing a lot of heat because it has a new roof and the occupants were holding a party that night, whereas a poorly insulated property may appear not to be losing much heat because the occupants were on holiday!
However, Haringey Council's intention in publishing the data online was to stimulate interest in the subject of home heat loss and to motivate residents to adequately insulate their properties, and we are hopeful that this will be the outcome.
It is hoped that your questions will have been answered by reading the information on this page and following the links provided to additional sources of information, however should you have any further queries then please do not hesitate to submit them to firstname.lastname@example.org