Colour concordance theory and control techniques of LANDSAT image maps
Shi Junmei,Zhang Shengkai
Institute of Remote Sensing Applications,Chinese Academy of Sciences, Beijing 100101, China
Abstract :
During the compilation of large-sized image maps the colour concordance theory and control techniques are studies. Firstly, main factors influencing the colour concordance of image map are analyzed, and the density measurement and control in composite processing are studied. Then the general evaluation of colour concordance is present. Finally, the control techniques are generalized.
Introduction
Since 1970s Landsat MSS images have been used for map compilation in China, experiencing for single band black-and-white mosaic to large-sized image maps. The Landsat image maps play an unique role in geoscience study since they display the natural scenes in a large scale compared with a single image scene. However, the technique are complicated to mosaics hundreds of scenes into a large – sized Landsat image map. One of the key problem is color concordance. During the last 10 years, we have compiled Landsat image maps with various regional scales, and finally the Landsat image map of China. Colour concordance theory and control techniques are studied during the compilation and positive results have been achieved.
Color concordance theory and maine effect factors
1 Environmental Factor
The objective of Remote Sensing is the ground surface, which is a multi-factor, dynamic physical geographic and environmental system. The spectral characteristics of objectives will changes due to not only spatial but also temporal varieties. These changes will be recorded as film’s density, resulting in different tones on the colour composite as shown in Table 1.
2 Relations among Film Density, Special Reflectance (p) and Exposure (t).
In colour composite process black-and-white transparent films of black-and-white transparent films of each band are put into the optical path, and the receptor is colour print.
Table 1: Colour Print Density Comparison of Various Objective with Different Imaging Time
Path No.
158-30
|
Imaging date |
Colour
Print
Density |
Farmland |
Grassland |
Forest |
Gobi |
Snow |
Rock |
| Aug. 13, 1978 |
Yellow
Magnetic
Cyanic |
0.11
0.37
1.25 |
0.13
0.38
1.25 |
0.12
0.38
1.29 |
1.42
1.44
1.43 |
0.76
0.85
0.42 |
0.04
0.42
0.67 |
| Sep. 22, 1972 |
Yellow
Magnetic2 Cyanic |
0.37
0.67
0.95 |
0.31
0.63
0.84 |
0.19
0.44
0.90 |
1.25
0.94
0.17 |
0.80
0.94
0.75 |
0.06
0.05
0.20 |
| Oct.16, 1976 |
Yellow
Magnetic
Cyanic |
0.64
0.97
1.11 |
0.45
0.7821.01 |
0.12
0.49
0.98 |
1.39
1.45
1.43 |
1.24
1.21
1.14 |
0.05
0.14
0.08 |
The modulation of transparent film in a colour print system is called print density. There are 3 print densities of colour print, acting on red----, green---- and blue--- sensitive layers, respectively. Since primary filters are used in color composite process, the balance of three layers are adjusted by exposure time, which is different form color print using complementry filters. The characteristic relation between print using complementry filters. The characteristic relations between pring density and exposure time are shown as below:
Where E
l is the spectral radiancy of light source:
tl is the spectral transparency of filters;
al is the spectral transparency of optical projection system
rl is film transparency of each spectral band.
In a given composite system, E
l,
tl and
al is taken as a constant K. Equation (1) is then simplified as :
This means that the density (D
p) of each layer of colour print is correlated with the film transparency and exposure time. the former is the intrinsic factor of tone variety, while the latter is a means for colour concordance in colour composite process.
3 General Evaluation of Colour concordance
The varieties of objectives on a colour composite are displayed by means of their hue, intensity and saturation. In general, colour evaluation is made by visual impression of naked eyes, which is affected by biophysical and psychological factors. These make the evaluation more complex, especially for false colour composite, which records information in not only visible but also invisible spectrum. The main problem is how to evaluate colour balance of false colour composite . The “mid-grey optical wedge” method was put forward. However, this is only suitable for a single composite, and is impossible for larger-sized image maps. This is because that : a) in a large region, the temporal and – spatial effect will influence colour concordance; b) the tone of a certain scene will be changed in order to concord with tones of neighbouring scenes, which will destroy colour balance conditions; c) some films are copied several times non-standardly, which are unsuitable for mid-grey optical wedge composite. Considering the mentioned factors, we should focus on macroscopic colour effect during the general design of image maps. That is, there should be natural transition between neighbouring scenes, and general colour harmony in a large region, while each scene has much information and is interpretable.
