Patent ID: 8204412

Claim:
An image forming apparatus comprising: an image bearing member that bears an electrostatic image thereon; a charging device that charges said image bearing member; an exposure device that forms said electrostatic image by exposing a surface of said image bearing member, which has been charged to a dark potential V D by said charging device, thereby to change the image bearing member surface into a bright potential V L ; a developing device that has a developer carrying member on which a developer including a toner and a magnetic carrier is carried; and a power supply that applies a developing bias on said developer carrying member; wherein said developing bias is an oscillating voltage having a first peak voltage V 1 generating an electrostatic force in a first direction to cause said toner to move in a direction from said developer carrying member toward said image bearing member, and a second peak voltage V 2 generating an electrostatic force in a second direction to cause said toner to move in a direction from said image bearing member toward said developer carrying member, said first and second peak voltages being applied on said developer carrying member in an alternate manner; a duty ratio Du (%), denoted by (T 2 /(T 1 +T 2 ))×100, is in the range of 60≦Du≦80, where T 1 is a phase time in said first direction, and T 2 is a phase time in said second direction; said magnetic carrier has a characteristic that: said magnetic carrier has a resistivity ρ which decreases in accordance with an increasing electric field strength, and a relative dielectric constant ∈ which increases in accordance with an increasing electric field strength; a product of a time constant ∈ 0 ∈ρ(s) of electric charge decay, which is denoted by a dielectric constant of a vacuum ∈ 0 , the relative dielectric constant ∈ of said magnetic carrier, and said resistivity ρ in an electric field strength E 2D decided by said second peak voltage V 2 and said dark potential V D , and said electric field strength E 2D satisfies a relation of 20≦∈ 0 ∈ρ E 2D (s·V/cm); and said time constant ∈ 0 ∈ρ(s) and said relative dielectric constant ∈ in an electric field strength E 1L , which is decided by said first peak voltage V 1 and said bright potential V L , satisfy the following relations: ∈ 0 ∈ρ(s)≦6.0×10 −4 , and 30≦∈.