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It looks suspiciously similar to what I had - a partially collapsed light tunnel. I think only DLP projectors have light tunnels, and they prevent light loss between the colour wheel and the lens/mirror/DMD chip assembly.
The light tunnel (on my projector) is just 4 mirrors that form a rectangular tunnel, and the intense heat can cause the glue (or whatever it is that holds them in place) to fail. I've read of people that have repaired it themselves using a heat resistant cement/glue, but mine was particularly tricky to get to, so I sent it off to NEC and they replaced the part for £20 (including collection and return).
Just type 'light tunnel collapse' into google to find lots of examples from different manufacturers.