thank you rfox,
It worked well!
To avoid the loss of IES
data from the file I've found this method useful:
create a diffuse/emission/emission texture/graysacle image/file.ies
material and press "Apply" button (as you said).
save the file, close all the octane viewports, save the file again and close Sketchup.
When you'll re-open the file the IES
data are preserved.
Sometimes the sources of light aren't visible below a certain angle of view and I don't know why...