I have used a cheap craft store mirror cut to size in the 138/S45. I was getting some dark banding from the standard Durst mirror when using a halogen point source bulb. The craft store mirror actually worked better in this case. I doubt that you would see any difference with an opal bulb. BTW, I glued strips of 3/16" masonite to the back of the mirror to hold it in place after closing the door -- otherwise it wants to slide down slightly and won't remain at a 45 degree angle.