Hi Gordon,
Thanks for the files. I loaded up ignoring the request for the HDR image, and the render proceeded as per your image with the spectrum very clearly visible. Wonderful! However, I then loaded another HDR image, hit the reset and redraw buttons and .....nothing! The beam didn't build up and the spectrum was nowhere to be seen. Perplexed, I restarted, cleared the scene and reloaded from fresh default extractions from your files, changing nothing. Still no joy. The LW image cache file is empty, but are there perhaps other hidden caches that I should be looking at?
So, what worked first doesn't work now, aside from loading the HDR I didn't change anything. Bizarre! Sorry to be such a trouble Gordon! David