The present invention provides a sensor or detector device (100). The sensor or detector device comprises a flat lens (110) and a housing assembly (102) comprising a front housing (104) with a wall (106) defining an opening (108), and being adapted for removably receiving the flat lens (110), and a back housing (112) removably attached to the front housing (104). The front housing (104) comprises a reinforcement structure (116, 116A, 116B, 400, 400A, 400B) for protecting the flat lens (110) from being damaged. The reinforcement structure (116, 116A, 116B, 400, 400A, 400B) is attached to an inner periphery (118) of the wall (106) of the front housing (104). The reinforcement structure (116, 116A, 116B, 400, 400A, 400B) and the flat lens (110) are attached to the front housing (104) such that the reinforcement structure (116, 116A, 116B, 400, 400A, 400B) abuts with the inner surface (120) of the flat lens (110).flat lens