Infra red is effectively heat images. When water is present in brickwork/masonry or the structure is under stress the heating and cooling of the brickwork changes because of the changed specific heat of bricks making up the wall.
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The walls of a property show significantly lower specific heat when dry and much higher specific heats when saturated with water.
Depending upon the time of IR examination water areas appear brighter (when the surrounding walls have cooled quickly) or darker where the surrounding walls have warmed up more quickly.
Wet walls take longer to heat up and longer to cool down then dry walls.
The images are take with a specially modified Digital SLR camera, then futher processed improve contrast, especially when images are being produced inside (IR dark Space – such as inside a building) - the colour that appear in the image are purely the result of the settling I choose to apply. This system gives an superior image for this purpose rather than the red/orange/green produced with other systems where the operator has settings to apply colours at specific temperatures.