Optical beam scanning device and image forming apparatus having window for polygon mirror cover

An image forming apparatus, comprising:

• an optical beam scanning device including:
• a light deflecting device, at least a polygon mirror portion of which is covered by a cover having a cover window portion for keeping a passage for an entering/emitting light;
• a pre-deflection optical system for entering an optical beam from a light source into said light deflecting device;
• and
• a post-deflection optical system for focusing a reflection light from said light deflecting device on a surface to be scanned,
• wherein constituent elements of said light deflecting device and said post-deflection optical system and said cover window portion are disposed, so that the pretreated light-shielding reflection light from said cover window portion, avoids from striking upon a flange portion and an edge portion of one or plurality of said optical element parts within said post-deflection optical system, with respect to said entering light;
• and
• a photosensitive body having a surface to be scanned, on which a latent image is formed upon basis of an optical beam from said optical beams scanning device,
• the optical systems do not match, with the optical axis of the post-deflection optical system, the direction when the optical axis of entering light is projected on a plane including a central optical axis of the post-deflection optical system and a main scanning direction axis, and
• the optical systems are configured to partially overlap an incident light and a reflection light in an effective area of a cover glass;
• wherein the constituent elements of said light deflecting device and said post-deflection optical system and said cover window portion are disposed, so that the pretreated light-shielding reflection light from said cover window portion passes through an outside of a certain optical element part;
• and wherein a light shielding structure is provided in vicinity of the certain optical element part, through which said reflection light passes in the outside thereof.