The present invention provides a planar light guide film for a backlight unit having at least one point light source, the light guide film comprising a light input surface for receiving light from the point light source, a light redirecting surface for redirecting light received from the light input surface and a light output surface for outputting at least the light redirected from the light redirecting surface. The light input surface further comprises a composite lens structure having a first and second circular tip segment each circular tip segment with a first contact angle, and a first and second circular base segment each circular base segment with a top and bottom contact angle, the contact angles of the circular base segments being less than the contact angle of the circular tip segments. Further, the circular tip segment satisfies the following equations:
y1=a1+√{square root over ((r12−x2))}
y2=a2+√{square root over ((r22−x2))}
and the circular base segments satisfies the following equations:
y3=b3−√{square root over ((r32−(x+a3)2))}
y4=b4−√{square root over ((r42−(x−a4)2))}
and each of the composite lens structures is randomly disposed along the light input surface.