Optical scanning device and image forming apparatus

An optical scanning device comprising:

• a light source including a plurality of light emitting parts emitting a light beam, the light emitting parts arranged in a predetermined direction at fixed intervals in a sub-scanning direction;
• a deflector formed into a polygonal prism rotating around an axis and having deflection surfaces formed in each side surface of the polygonal prism, the deflection surface reflecting the light beam to scan a circumferential surface of an image carrier rotating around an axis with the light beam in a main scanning direction;
• a random number generator which sequentially generates a plurality of pseudo random number sequence with an initial random number sequence having a predetermined bit length as a seed, the pseudo random number sequence having a same bit length as the bit length of the initial random number sequence;
• a selection part configured to select the two light emitting parts as a set of target emitting parts among the light emitting parts, the target emitting parts arranged adjacently to each other in the predetermined direction and capable of emitting the light beams at the same time to one of the deflection surfaces in order to form one pixel of an electrostatic latent image to be formed on the circumferential surface by the two light beams, and to change a combination of the light emitting parts constituting the set of target light emitting parts for each position of the pixel of the electrostatic latent image in the sub-scanning direction;
• a random number assignment part configured to assign the random number sequence selected from the initial random number sequence and the pseudo random number sequences individually to each light emitting part constituting the set of target light emitting parts as an index for specifying a timing at which a light emitting time at the light beam emitting of the set of target light emitting parts is set to a correction value different from a predetermined reference value, and to update the assignment of the random number sequence at a random number update period specified for each light emitting part constituting the set of target light emitting parts;
• and
• an exposure control part configured to output a light emitting control signal containing a pulse individually for each light emitting part constituting the set of target light emitting parts and to cause each light emitting part constituting the set of target light emitting parts to emit the two light beams, the pulse serving as an index of a timing for causing each light emitting part constituting the set of target light emitting parts to emit the light beam and having a pulse width which defines the light emitting time and is different between the width corresponding to the reference value and the width corresponding to the correction value, wherein
• the random number update period specified for each light emitting part constituting the set of target light emitting parts coincides with a scanning period specified for each light emitting part constituting the set of target light emitting parts, the scanning period indicating a scanning period in which the light beam emitted from each light emitting part constituting the set of target light emitting parts scans the circumferential surface in the main scanning direction.