Multi-beam scanning optical system and image forming apparatus using the same

A multi-beam scanning optical system comprising entrance optical means for introducing, to deflecting means, a plurality of light beams emitted from light source means having a plurality of light emitting portions arranged apart from each other at least in a main scan direction;

• scanning optical means for focusing the plurality of light beams deflected by said deflecting means on a surface to be scanned;
• and synchronous detection optical means for converging, by a synchronous detection lens unit, parts of the plurality of light beams deflected by said deflecting means on a plane of a slit, introducing the parts of the plurality of light beams to a synchronous detection sensor, and controlling timing of scan start positions on said surface to be scanned with respect to the plurality of light beams by using signals from said synchronous detection sensor,
• wherein said multi-beam scanning optical system further comprises compensating means for compensating a dot position shift of each scanning line on said surface to be scanned, the dot position shift being caused by the fact that, assuming that a focal distance of said scanning optical means is Ff, a focal distance of said synchronous detection lens unit is Fb, a focus position shift amount of each light beam introduced to said synchronous detection sensor in a main scan section is δM1 looking from said slit, and a focus position shift amount of each image height is δX looking from said surface to be scanned, Ff≠Fb holds and |(Fb/Ff)×δX−(Ff/Fb)×δM1| has a certain value.

A multi-beam scanning optical system comprising entrance optical means for introducing, to deflecting means, a plurality of light beams emitted from light source means having a plurality of light emitting portions arranged apart from each other at least in a main scan direction;

• scanning optical means for focusing the plurality of light beams deflected by said deflecting means on a surface to be scanned;
• and synchronous detection optical means for introducing, by a synchronous detection lens unit, parts of the plurality of light beams deflected by said deflecting means to a synchronous detection sensor and controlling timing of scan start positions on said surface to be scanned with respect to the plurality of light beams by using signals from said synchronous detection sensor,
• wherein, assuming that a focal distance of said scanning optical means is Ff, a focal distance of said synchronous detection lens unit is Fb, a focus position shift amount of each light beam introduced to said synchronous detection sensor in a main scan section is δM2 looking from a light receiving surface of said synchronous detection sensor, a focus position shift amount of each image height is δX looking from said surface to be scanned, an allowable dot position shift amount of each scanning line is δYmax, and a maximum angle difference between incident angles of the light beams upon the light receiving surface of said synchronous detection sensor is θmax, Ff≠Fb holds and the following conditional formula is satisfied:
• |(Fb/Ff)×δX−(Ff/Fb)×δM2|≦δYmax/θmax

A multi-beam scanning optical system comprising entrance optical means for introducing, to deflecting means, a plurality of light beams emitted from light source means having a plurality of light emitting portions arranged apart from each other at least in a main scan direction;

• scanning optical means for focusing the plurality of light beams deflected by said deflecting means on a surface to be scanned;
• and synchronous detection optical means for introducing, by a synchronous detection lens unit, parts of the plurality of light beams deflected by said deflecting means to a synchronous detection sensor and controlling timing of scan start positions on said surface to be scanned with respect to the plurality of light beams by using signals from said synchronous detection sensor,
• wherein said multi-beam scanning optical system further comprises compensating means for compensating a dot position shift of each scanning line on said surface to be scanned, the dot position shift being caused by the fact that, assuming that a focal distance of said scanning optical means is Ff, a focal distance of said synchronous detection lens unit is Fb, a focus position shift amount of each light beam introduced to said synchronous detection sensor in a main scan section is δM2 looking from a light receiving surface of said synchronous detection sensor, and a focus position shift amount of each image height is δX looking from said surface to be scanned, Ff≠Fb holds and |(Fb/Ff)×δX−(Ff/Fb)×δM2| has a certain value.

A multi-beam scanning optical system comprising entrance optical means for introducing, to deflecting means, a plurality of light beams emitted from light source means having a plurality of light emitting portions arranged apart from each other at least in a main scan direction;

• scanning optical means for focusing the plurality of light beams deflected by said deflecting means on a surface to be scanned;
• and synchronous detection optical means for converging, by a synchronous detection lens unit, parts of the plurality of light beams deflected by said deflecting means on a plane of a slit, introducing the parts of the plurality of light beams to a synchronous detection sensor, and controlling timing of scan start positions on said surface to be scanned with respect to the plurality of light beams by using signals from said synchronous detection sensor,
• wherein an amount of dot position shift of each scanning line on said surface to be scanned is not larger than ½ of resolution in a sub-scan direction, the dot position shift being caused by the fact that, assuming that a focal distance of said scanning optical means is Ff, a focal distance of said synchronous detection lens unit is Fb, a focus position shift amount of each light beam introduced to said synchronous detection sensor in a main scan section is δM1 looking from said slit, and a focus position shift amount of each image height is δX looking from said surface to be scanned, Ff≠Fb holds and |(Fb/Ff)×δX−(Ff/Fb)×δM1| has a certain value.

A multi-beam scanning optical system comprising entrance optical means for introducing, to deflecting means, a plurality of light beams emitted from light source means having a plurality of light emitting portions arranged apart from each other at least in a main scan direction;

• scanning optical means for focusing the plurality of light beams deflected by said deflecting means on a surface to be scanned;
• and synchronous detection optical means for introducing, by a synchronous detection lens unit, parts of the plurality of light beams deflected by said deflecting means to a synchronous detection sensor and controlling timing of scan start positions on said surface to be scanned with respect to the plurality of light beams by using signals from said synchronous detection sensor,
• wherein an amount of dot position shift of each scanning line on said surface to be scanned is not larger than ½ of resolution in a sub-scan direction, the dot position shift being caused by the fact that, assuming that a focal distance of said scanning optical means is Ff, a focal distance of said synchronous detection lens unit is Fb, a focus position shift amount of each light beam introduced to said synchronous detection sensor in a main scan section is δM2 looking from a light receiving surface of said synchronous detection sensor, and a focus position shift amount of each image height is δX looking from said surface to be scanned, Ff≠Fb holds and |(Fb/Ff)×δX−(Ff/Fb)×δM2| has a certain value.