Patent ID: 7791632

Claim:
An optical scanning device, comprising: two scanning units each having an input optical system configured, in a sub-scan section. to make a light beam from a light source means be incident at a finite angle on a deflecting surface of deflecting means which is arranged to scanningly deflect a light beam from said light source means, and an imaging optical system configured to image, on a scan surface to be scanned, the light beam scanningly deflected by the deflecting surface of said deflecting means, said two scanning units being disposed opposed to each other with said deflecting means intervening therebetween; wherein one scanning unit of said two scanning units has at least two imaging optical elements including a first imaging optical element and a second imaging optical element disposed. in this order from said deflecting means, wherein the other scanning unit of said two scanning units has at least two imaging optical elements including a third imaging optical element and a fourth imaging optical element disposed in this order from said deflecting means, wherein said one scanning unit includes a first light blocking member configured to intercept unwanted light reflected at an optical surface of said fourth imaging optical element and directed to the scan surface at one scanning unit side, wherein said third imaging optical element of said other scanning unit is provided at a light path between said fourth imaging optical element and said first light blocking member, wherein an optical surface of said third imaging optical element through which unwanted light reflected at the optical surface of said fourth imaging optical element and directed to the scan surface at the one scanning unit side passes has a positive refracting rower in the sub-scan section, wherein, in the sub-scan section, said first light blocking member is disposed at the light path between said deflecting means and said first imaging optical element, and wherein, when a combined refracting power in the sub-scan section of said third imaging optical element is φs, a distance from the optical surface of said fourth imaging optical element where the unnecessary light is produced to a rear principal plane of said third imaging optical element in the sub-scan section is S 1 , a distance from a front principal plane of said third imaging optical element in the sub-scan section to said first light blocking member is L, a condition −( S 1 +L )/2 <S 1 −L ( S 1 ·φs −1)<( S 1 +L )/2 is satisfied.