Upgrade 4 inch fl Lenses for TDC Stereo Slde Projectors - Matched Pair - vintageUpgrade 4 inch fl Lenses for TDC Stereo Slde Projectors Matched Pair of Lenses vintage Excellent Condition Optional upgrade offered at the time of purchase by Bell & Howell for their TDC Stereo Slide Projectors Brightness of image on the stereo screen is always the concern as the projector's polarized filters and the viewers' polarized glasses reduce brightness by as much as 30%. Therefore, the optional 4 inch fl lenses were a distinct improvement