Optical film, polarizing plate, image display device, and 3D image display system

An optical film comprising a transparent film having a thickness of 10 μm to 150 μm, an alignment layer which is formed on one surface of the transparent film, and a retardation layer comprising a liquid-crystalline compound, in this order, on the transparent film, and comprising at least one hard coat layer on the other side of the transparent film,

• wherein values of in-plain mean refractive indices thereof satisfy the formula:
• the retardation layer comprising a liquid-crystalline compound>the alignment layer>the transparent film;
• the difference in in-plane mean refractive indices between the alignment layer and the retardation layer comprising a liquid-crystalline compound and the difference in in-plane mean refractive indices between the alignment layer and the transparent film are each 0.02 or more;
• and
• the optical thickness D of the alignment layer satisfies the formula:
• 270×N−150-75 nm≦D≦270×N−150+75 nm(1≦N≦8 where N is a natural number).