The glass must be without optical faults, neither as far as geometry nor as far as colorimetry are concerned. The use of glass whose surface and density are treated is recommended. You have to use an extra white glass with anti-reflecting coating; this avoids any losses due to absorption of 10 % of the luminous flux. Be careful: some glasses may cause problems with 3D passive polarized light. Portholes that are installed in booths serving several auditoria have to meet the security standards,.
