Image forming apparatus including multibeam exposure unit having surface emitting laser array

1. An image forming apparatus comprising:

an electrophotographic photoreceptor having a conductive substrate and a photosensitive layer provided on the conductive substrate;

a charging unit that charges the electrophotographic photoreceptor;

an exposure unit that exposes the charged electrophotographic photoreceptor to light thereby forming an electrostatic latent image;

a developing unit that developes the electrostatic latent image with toner thereby forming a toner image; and

a transfer unit that transfers the toner image from the electrophotographic photoreceptor to a transferred image-receiving medium,

wherein said exposure unit is a multi beam exposure unit which has a surface emitting laser array and which carries out the electrostatic latent image formation by scanning the electrophotographic photoreceptor with eight or more light beams,

wherein said electrophotographic photoreceptor gives a quantum efficiency of 0.3 or higher when the electrophotographic photoreceptor is charged to a charged potential absolute value of 500 V and then irradiated with a monochromatic light of the same wavelength as that of said light beams to decay the charged potential absolute value to 250 V, and

wherein the light beams emitted by the exposure unit have a wavelength of 780 nm.

2. The image forming apparatus according to claim 1 , wherein said photosensitive layer comprises at least one charge generating material selected from the group consisting of hydroxygallium phthalocyanine, chlorogallium phthalocyanine, oxytitanium phthalocyanine and a trisazo pigment.

3. The image forming apparatus according to claim 1 , wherein said photosensitive layer includes at least one member selected from the group consisting of:

hydroxygallium phthalocyanine having diffraction peaks at least at 7.6° and 28.2° in terms of the Bragg angle (2θ±0.2°) of an X-ray diffraction spectrum using CuKα radiation;

chlorogallium phthalocyanine having diffraction peaks at least at 7.4°, 16.6°, 25.5° and 28.3° in terms of the Bragg angle (2θ±0.2°) of an X-ray diffraction spectrum using CuKα radiation; and

a trisazo pigment represented by either one of following general formulas (1) to (4):

wherein, in the formulas (1) to (4), R represents a hydrogen atom, a halogen atom, an alkyl group, an alkoxy group or a cyano group; and Ar represents a coupler residue.

4. The image forming apparatus according to claim 1 , having a resolution of 1200 dpi or higher.

5. The image forming apparatus according to claim 1 , having a resolution of 2400 dpi or higher.

6. The image forming apparatus according to claim 1 , wherein said surface emitting laser array has light emitting points arranged two-dimensionally.

7. The image forming apparatus according to claim 1 , wherein the exposure unit gives a scanning interval between adjacent light beams scanning on the electrophotographic photoreceptor of 0.15 mm or larger.

8. The image forming apparatus according to claim 1 , wherein the exposure unit gives a scanning interval between adjacent light beams scanning on the electrophotographic photoreceptor of 0.2 mm or larger.

9. The image forming apparatus according to claim 1 , wherein the exposure unit gives a scanning interval between adjacent light beams scanning on the electrophotographic photoreceptor of 0.3 mm or larger.

10. The image forming apparatus according to claim 1 , which gives half decay exposure amounts, as determined by charging the electrophotographic photoreceptor to a charged potential absolute value of 500 V and then irradiating with a monochromatic light of the same wavelength as that of said light beams to decay the charged potential absolute value to 250 V, satisfying the relationship represented by the following expression (A):

E L /E M ≦1.15  (A)

wherein E L represents a half decay exposure amount under the conditions of 10° C. and 15% RH, and E M represents a half decay exposure amount under the conditions of 22° C. and 50% RH.