Optical film and backlight module using the same

An optical film for disposing on a transmittance element, the optical film comprising:

• a basic layer having a first surface and a second surface;
• a plurality of periodically arranged reflective convex parts disposed on the first surface of the basic layer, wherein each of the reflective convex-parts comprises:
• at least one reflective side surface, wherein an included angle between the reflective side surface and the first surface is from 20 to 80 degrees;
• and
• an incident bottom surface substantially parallel to the first surface to contact the transmittance element;
• and
• a plurality of periodically arranged collimating parts disposed on the second surface of the basic layer, and the reflective convex-parts are respectively corresponded to the collimating parts,
• wherein, in each corresponding pair of the reflective convex-part and the collimating part, the reflective convex-part has a central axis substantially coinciding with a central axis of the collimating part.