This is the correct 6¼" x 6½" heat absorbing glass for the Omega DV variable condenser lamphouse as fitted to the D-2, D-3 and D-5 enlargers. It is simply inserted into the appropriate slot in the lamphouse. Heat absorbing glass eliminates much of the infrared radiation from the light produced by the enlarging lamp, thus helping to protect negatives and filters from heat damage and minimizing negative "popping" which affects focus.
Note: This glass is not suitable for use in the D-6 Zoom Condenser lamphouse. For the Zoom lamphouse, please see the listing for 473-105 heat absorbing glass.