Thank you for the tip about Cycles and Blender. I know zero about Cycles! Hborre pointed me to the blackbody node. And I'm using it. And I understand the concept of blackbody, but otherwise -- no clue about Cycles. Just copying what I find or someone shows me, which is how I got started with Fortran, ages ago, and assembly language for a 360 slightly later. Sooner or later I manage to figure it out! But a little reading does seem to help from time to time.