Anti-reflection coating, optical member, exchange lens unit and imaging device

1. An anti-reflection coating comprising first to seventh layers formed on a substrate in this order, said first layer being an alumina-based layer, said seventh layer being a porous, silica-based layer, and in a wavelength range of 400-700 nm,

said substrate having a refractive index of 1.45-1.72,

said first layer having an optical thickness of 25.0-250.0 nm,

said second layer having a refractive index of 1.95-2.23 and an optical thickness of 27.5-52.5 nm,

said third layer having a refractive index of 1.33-1.50 and an optical thickness of 37.5-54.0 nm,

said fourth layer having a refractive index of 2.04-2.24 and an optical thickness of 45.0-62.5 nm,

said fifth layer having a refractive index of 1.33-1.50 and an optical thickness of 77.5-102.5 nm,

said sixth layer having a refractive index of 1.85-2.40 and an optical thickness of 16.0-26.5 nm, and

said seventh layer having an optical thickness of 112.5-162.5 nm.

2. The anti-reflection coating according to claim 1 , wherein said second layer, said fourth layer and said sixth layer are made of (a) at least one material selected from the group consisting of Ta 2 O 5 , TiO 2 , Nb 2 O 5 , ZrO 2 , HfO 2 , CeO 2 , SnO 2 , In 2 O 3 , ZnO and Pr 6 O 11 , or (b) a combination of at least one material selected from the group consisting of Ta 2 O 5 , TiO 2 , Nb 2 O 5 , ZrO 2 , HfO 2 , CeO 2 , SnO 2 , In 2 O 3 , ZnO and Pr 6 O 11 , and Y 2 O 3 , and said third layer and said fifth layer are made of (c) MgF 2 and/or SiO 2 , or (d) a combination of MgF 2 and/or SiO 2 , and Al 2 O 3 .

3. The anti-reflection coating according to claim 1 , wherein said first layer has a refractive index of 1.58-1.71.

4. The anti-reflection coating according to claim 1 , wherein said seventh layer has a refractive index of 1.09-1.19.

5. The anti-reflection coating according to claim 1 , wherein said seventh layer is a silica aerogel layer.

6. The anti-reflection coating according to claim 1 , which has reflectance of 0.3% or less to incident light at 0° in a wavelength range of 450-600 nm.

7. The anti-reflection coating according to claim 1 , wherein a fluororesin layer as thick as 0.4-100 nm having water repellency or water/oil repellency is formed on said seventh layer.

8. The anti-reflection coating according to claim 1 , wherein said first to sixth layers are formed by a physical film-forming method, and said seventh layer is formed by a wet method.

9. The anti-reflection coating according to claim 1 , wherein said physical film-forming method is a vacuum vapor deposition method, and said wet method is a sol-gel method.

10. An optical member having the anti-reflection coating recited in claim 1 .

11. An exchange lens unit comprising the optical member recited in claim 10 .

12. An imaging device comprising the optical member recited in claim 10 .