Infrared photography is a newer and very useful technique in the study of vegetation because the molecules of pigments in plants do not absorb infrared wavelengths. Instead the infrared is either transmitted through the leaf or is reflected by the cell walls.
Cells of one species of plant have a different reflectivity than those of another, and normal cells have a different reflectivity than abnormal ones. Therefore, infrared photography has been used to distinguish species and to detect unhealthy plants.
It is also used to detect differences in environmental temperatures. Such pictures or thermographs, in which differences in temperature appear as contrasting bands of colour, permit the detection of thermal pollution.
One of the most sophisticated remote sensing technique, called multispectral scanning, several or many spectral bands from ultraviolet to infrared recorded. The visual data are gathered by telemetry or are recorded on film or magnetic tape. The image data are fed into a multichannel led sensor, which sorts out the various displays or images.
Because all the channels were recorded simultaneously, each image is in perfect register. This enables the interpreter to pick out and study an object of interest with different types of images taken at the same instant.