Adapting an existing RGB composite for a new imager


How are RGB schemes inherited from other imagers migrated to a new sensor? In this case we already have a recipe, and we would like to adapt it for the new imager.

Step 1

The purpose remains the same.


Step 2

Which channels or channel combinations can be used in the adapted RGB scheme?

  • If we have the “corresponding” channels in the new imager, then we can adapt the RGB scheme. The channel of the new imager does not have to be exactly the same as the corresponding channel of the old imager. The central wavelengths and spectral response functions can be somewhat different. The main point is that the channel of the new imager should contain the same information content as the corresponding channel of the older imager.
  • If more than one channel in the new imager corresponds to a channel used from the older imager, one has to decide which new channel to use, by testing which channel is better for the actual purpose. For example, Himawari AHI imagers measure at 10.4 and 11.2 µm and both these channels “correspond” to the SEVIRI IR10.8 channel (similar features, characteristics). Tests were performed to determine when to use which channel to obtain optimal results.
  • It can also happen that a channel is missing in the new imager but the creators simulate it, as was done in the GOES ABI True Color RGB. GOES/ABI does not measure in the green spectral range, but the missing data was simulated using other solar channels and Himawari/AHI green channel data.

Step 3

The channels of a new imager are usually different in some way from the corresponding channel of the old imager, such as with a shifted central wavelength and/or a different spectral response function. The main issue is whether they contain the same information content. Small differences can be corrected by tuning (using modified ranges and/or gamma parameters).

When adapting an RGB recipe originally tuned for an older imager, one can

  • use the same parameters, or
  • tune the parameters using RTM model calculations (physical method), or
  • tune the parameters using statistical methods.

Japanese meteorologists developed a statistical method to tune the ranges and gamma parameters in order to produce as similar results as possible when they adapted the SEVIRI RGBs to Himawari/AHI images (H. Murata and A. Shimizu, 2017). E. Berndt et al. (2018) also presented methods for adjusting RGB recipes.

Step 4

The color assignment should be the same.


Step 5

The adapted RGB composite must be reviewed for appearance and effectiveness.