using colorbar() fixing a limit value

Hi everyone,

I want to know if its possible to fix a value in the colorbar() which would change the level of transparency of every value under the limit value fixe in the colorbar.

For exemple, if i want to show all values > 50, i can fix the limit value to 50 on the colorbar and plot is refresh to show only values upper than 50.

Thanks for any answer.
Philippe Collet