A dewar assembly is presented for use in an optical IR detection system defining a light collecting region. The dewar assembly comprises a warm shield unit configured as an enclosure for optically enclosing the light collection region and having an optical window through which incident light enters the dewar. The warm shield defines a reflective inner surface configured such that light portions of the incident light propagating through said optical window onto said inner surface are reflected by the inner surface towards regions outside said light collecting region.