US Patent 7130096 Light beam scanning device and image forming apparatus

An effective scanning range of laser light in a scanning direction is configured so that a center position of the effective scanning range is displaced by a distance β along the scanning direction from an optical axis f an fθ lens. The incidence angle α between a center of luminous flux of the laser light on a deflection surface of polygon mirror and the optical axis of the fθ lens can beset to satisfy a relational expression (4π/N)−(W/D)≦|α|<(4π/N)−{(W−β)/D}.