5.3 Radiometric

Radiometric resolution in remotely sensed data is defined as the amount of energy required to increase a pixel value by one quantisation level or 'count'. The radiometric extent is the dynamic range or the maximum number of quantisation levels that may be recorded by a particular sensing system. Most remotely sensed imagery is recorded with quantisation levels in the range 0­p;255, that is, the minimum 'detectable' radiation level is recorded as 0 while the 'maximum' radiation is recorded as 255. This range is also referred to as 8 bit resolution since all values in the range may be represented by 8 bits (binary digits) in a computer. Radiometric resolution in digital imagery is comparable to the number of tones in a photographic image ­p; both measures being related to image contrast.

Quantisation levels are frequently given in terms of the number of bits rather than the number or range of levels. These values are related by:

In image processing, quantisation levels are usually referred to as Digital Numbers (DN).

The effect of changes in radiometric resolution on image feature contrast is illustrated in Figure 37. As discussed in Section 1.4, the human eye can only perceive 20­p;30 different grey levels so the additional resolution provided by images with more than about 30 levels is not visually discernible. This sequence of images emphasises the value of interpreting imagery using digital techniques to derive maximum discrimination from the available radiometric resolution.

a) b)
c) d)
e) f)

Figure 37: Effect of changes in radiometric resolution on image feature contrast.
Number of quantisation levels used:
(a) 2
(b) 4
(c) 8
(d) 16
(e) 32
(f) 64

Since the radiometric resolution defines the maximum number of quantisation levels detectable by a sensor, it is most unlikely that a single remotely sensed image would actually contain data values covering the entire range. For Landsat MSS data, most imagery would only contain a maximum range of 50­p;100 DN (or less) in each channel. This situation is usually simply due to the range of objects being imaged, since the detector sensitivities are generally selected to cope with the brightest and darkest objects of interest and a single image would rarely contain both. Some sensing systems (such as CZCS or Geoscan Mark 2 scanner) have adjustable sensitivities (or gains) for imaging over different surface features, the principal difference being between land and water. Atmospheric scattering and absorption effects decrease the radiometric extent of an image by reducing the discrimination between different radiation levels, especially at shorter wavelengths. Methods for improving contrast within an image for presentation and interpretation purposes are discussed in Part TWO, Introduction to Image Processing. It should be noted however, that these techniques can not improve the radiometric resolution of the data itself, as this is dependent on the scanning instrument, but only alter the visual contrast in an image.

Landsat MSS imagery, while distributed as 8 bit data, is actually recorded as 6 bit data with bands 4, 5 and 6 being recorded non-linearly to have an effective data range of 7 bits. Since the imagery is formed using 6 different detectors (see Section 2.2.1), the final conversion to 8 bit format is combined with a rescaling process which accounts for calibration differences between sensors. By expanding the data range during this process, fractional differences between sensors in terms of the original range can be represented more accurately (that is, with fewer rounding errors) in the final expanded range.

microBRIAN Version 2 handles all image data as 8 bit (that is data values in the range 0­p;255; Version 3 will process both 8 bit and 16 bit image data). In cases where the data were recorded with greater radiometric resolution, such as AVHRR (10 bit), the sub-range of values actually contained in the image is converted to fill the range 0­p;255. For example, the data extent of the AVHRR thermal infrared channel has been designed so that value 0 records a brightness temperature of approximately -273°C and value 1023 relates to about 50°C. This range allows recording of detailed information from targets covering a wide range of temperature such as clouds, oceans and earth surface features. The latter are only ever recorded as a portion of the full data range, so the sub-range rescaling process used during conversion to 8 bit data preserves the resolution of the original data as much as possible.