Development apparatus and image forming apparatus

1. A development apparatus comprising:

an electrostatic latent image carrier configured to hold an electrostatic latent image on a surface thereof while rotating;

a developer carrier configured to develop the electrostatic latent image using a developer while rotating;

a developer accommodation section configured to accommodate the developer; and

a first and second developer supply members each configured to supply the developer accommodated in the developer accommodation section to the developer carrier while rotating, wherein

the first developer supply member and the second developer supply member are each arranged at positions opposite to the developer carrier so as to come into press-contact with the developer carrier,

the first developer supply member is arranged on a downstream side of the developer carrier in a rotation direction than the second developer supply member,

an elastic layer is formed on an outer peripheral surface of each of the first developer supply member and the second developer supply member, and

Hs1, Δ1, t1, Hs2, Δ2, and t2 satisfy a following formula (A),

10≦ Hs 1×(Δ1/ t 1)+ Hs 2×(Δ2/ t 2)≦50  (A)

where

Hs1 is a hardness of the outer peripheral surface of the first developer supply member,

t1 is a layer thickness of the elastic layer of the first developer supply member,

Δ1 is an NIP amount that is determined when the first developer supply member is in press-contact with the developer carrier,

Hs2 is a hardness of the outer peripheral surface of the second developer supply member,

t2 is a layer thickness of the elastic layer of the second developer supply member, and

Δ2 is an NIP amount that is determined when the second developer supply member is in press-contact with the developer carrier.

2. The development apparatus according to claim 1 , wherein

Hs1, Δ1, t1, Hs2, Δ2, and t2 further satisfy following formulas (B) and (C):

5 ≦Hs 1×(Δ1/ t 1)≦25  (B); and

5≦ Hs 2×(Δ2/ t 2)≦25  (C).

3. The development apparatus according to claim 2 , wherein

Hs1, Δ1, t1, Hs2, Δ2, and t2 further satisfy following formula (D):

Hs 1×(Δ1/ t 1)≦ Hs 2×(Δ2/ t 2)  (D).

4. An image forming apparatus comprising:

an electrostatic latent image carrier for holding an electrostatic latent image on a surface thereof while rotating; and

a development apparatus for developing the electrostatic latent image of the electrostatic latent image carrier, wherein

the development apparatus is a development apparatus according to claim 1 .