Sony DXC 1200P Trinicon TV Camera - Infrared Filter

The infrared filter ( the bluish piece of glass )removed from the circular plastic housing.

On the left is the supposed clear plastic disc that sits in front of the infrared filter. A possible reason that a clear plastic disc is included is because the infrared filter is made of a thin piece of glass, possibly to reduce distortion of the light passing through to the trinicon, the plastic adds some mechanical strength to the infra red filter assembly.

Even then it seems to be quite a complicated way to create a infra red filter ( must be difficult engineering problem given the level of materials science and manufacturing expertise available in the 1970s ).

The plastic and glass is possibly fastened together around the edges using some form of silicon adhesive. Over time or maybe because of adverse storage conditions, the adhesive have migrated over to the whole plastic disc, creating a cataract like effect over the infra red filter.