6.3 Radiometric Characteristics of Image Data

6.3 Radiometric Characteristics of Image Data

Electromagnetic energy incident on a detector is converted to an electric signal and then digitized. In this quantization process, the relationship between the input signal and the output signal is generally represented as shown in Figure 6.3.1. In this curve the left part corresponds to the insensitive area, with less response, while the right part is the saturated area with almost constant output regardless of the input intensity.

In the central part, there is almost a linear relationship between the input and the output. The approximation to a linear relationship is called linearity. The range of the linear part or the ratio of maximum input to minimum input is called the dynamic range, which is usually expressed in dB (see 2.2).

One should be careful of the noise level in the case of quantization, as explained in 6.1. The ratio of effective input signal S to the noise level N is called the S/N ratio (signal to noise ratio), which is given as follows.

S / N ratio = 20 log10 (S/N) [dB]

In conclusion, quantization is specified by the dynamic range and the S/N ratio. Information contained in digitized image data are expressed by bit (binary digit) per pixel per channel.

A bit is a binary number, that is 0 or 1. Let the quantization level be n, then the information in terms of bits is given by the following formula.

log2 n (bit)

In remote sensing, the quantization level is normally 6, 8 or 10 bits as shown in Table 6.3.1. For computer processing, the unit of byte (1 byte = 8 bits;integer value 0-255 ; 256 gray levels) is much more convenient. Therefore remote sensing data will be treated as one or two byte data.

The total data volume of multi-channel data per scene is computed as follows.

Data Volume(byte) = (line number) x (pixel number) x (channel number) x (bits) /8

Output data usually corresponds to the observed radiance detected by the sensor. The absolute radiance is converted by a linear formula from the observed radiance (see 9.1). The parameters are usually listed in the User's Manual for the particular remote sensing system.