I agree with you 500 but if you use a tube just lock the front with a screw into the length of the tube. Then remove it to clean with Windex, Carb cleaner or other spray. Dry then reinstall the screw. If the inside of the tube stays bright maybe it will bounce back some lost light. Maybe there is little loss of light. A diamond in brilliant cut seems to be a great multiple prism. Carpetsahib, do those stones lose light behind them if cut right? TIA Rustyzipper