The "multi-spectral model" is commonly utilized in conventional" normal or true" colour images and false colour image. The "multi-temporal model" is another common model. All multi-temporal images are false colour images. As indicated above, a false colour multi-temporal image is the composite of a a single spectral band collected at three different time periods (S1T1; S1T2; S1T3). Each of the three time periods of information are assigned colours just as in the production of a multi-spectral colour photograph. In this case, however, the colours represent change in brightness values or tone (spectral reflectance or thermal emittance) over time. The change recorded may be due to a variety of variable that can affect reflectance or emittance (sun angle, atmospheric conditions, land cover changes, etc.). The challenge for the interpreter is to make sense of the colours and to isolate the parameters affecting the change. The specific colours on the multi-temporal false colour image are also ependent on the colours assigned to each of the three time period. For example, the scenario of additive colours used in a colour monitor presentation will determine - along with actual change in reflectance - the colours on the final product of a multi temporal false colour image. if the colours blue, green and red are used for the temporal sequence temporal false colour image.If the colours blue, green and red are used for the temporal sequence T1, T2 and T3, respectively, a change from high (white) to low (black) brightness values between T1, and T2 and remaining low (black) in T3 would result in the colour blue; whereas, if the colour for sequence red, green and blue were used for the same time sequence, T1, T2 and T3 the colour for the same brightness value change would be red. By standardizing the colours assigned to the time sequence to T1, (blue) T2 (green) and T3 (red) the meaning of the basic colours on a multi-temporal false colour image can be summarized (See Table 1 and 2). Only chromatic colours (blue, green, red, cyan, magenta and yellow) indicate change (Table 1); achromatic colours (white, black and grey) indicate no change in reflectance over time (Table 2).
Table 1. Reflectance change over time indicated by chromatic colours on multi-temporal false colour image.
| T1-blue | T2-green | T3-red | Resultant chromatic colour |
| White | Black | Black | Blue |
| Black | White | Black | Green |
| Black | Black | White | Red |
| White | White | Black | Cyan |
| White | Black | White | Magenta |
| Black | White | White | Yellow |
With knowledge of the spectral bands selected for the multi-temporal false colour composite and an understanding of the spectral reflectance and /or thermal emittance characteristics of the target, the image analyst may be able to decipher the meaning of the colours and determine the reason (s) for the tonal change over time. For example, if three dates of thermal imagery were composited using the colour scenario presented in Table 1, a feature coloured blue would be interpreted as being hot in T
1 and cold in T
2 and T
3. If near-infrared imagery (for example, Landsat TM, Band 4)
were used to generate a three data multi-temporal false colour composite of a coastal or wetland area, the colour blue could indicate a change from land to water (land loss) during T
3 and T
2 and remaining as water in T
3. However, if the area imaged was dominated by agricultural by agricultural patterns, the same tonal change and colour sequencing scenario could indicate a healthy crop in the field during T
1, and the field in fallow during T
2 and T
3.
Table 2. Achromatic colours on multi-temporal false colour image indicating no reflectance change over Time
| T1-blue | T2-green | T3-red | Resultant chromatic colour |
| White | White | White | White |
| Black | Black | Black | Black |
| Grey | Grey | Grey | Grey |
Colour Radar
Colour radar has been around for more than twenty years; however, until recently the image were "colourised radar and not " true" colour radar. Early "colourised" radar images were single band radar image that were level sliced, and colours were assigned to selected ranges of gray scale values (digital numbers (DNs) or brightness values (BVs(). Colour composite radar images, on the other hand, use multi-dimensional radar data sets and are generated by assigning a primary colour (blur, green or red) to the three single dimensional data sets being combined. If registerable, any two of three different sets of multi-dimensional data sets can be combined in his way to generate colour composite radar images or, perhaps more accurately, a false colour multi-polarized, multiple incident angle, multiple look direction, and multi-squint angle radar data. Examples of false colour multi-dimensional radar images are found in a variety of NASA publications (Ford et al., 1989; NASA 1986; 1989) and ESA publication, such as esa bulltetin. False colour multi-dimensional radar images are also available through the NASA/JPL home page (southport.jpl.nasa.gov) and other home pages on the Internet.
Although radar or active microwave data can be used to generate multi-dimensional false colour images, the interpretation of the colours is not as "conventional" as with visible and infrared images because of the differences in the signal to target interaction. Tonal (brightness) changes on radar imagery are related to system and target related parameters defined in the radar equation as well as to the interplay of these system and target parameters (Table 3).
