White polyester film and surface light source therewith

A white polyester film comprising a white polyester layer (W layer) which includes a polyester resin component (A) and a component (B) incompatible with the resin component (A);

• wherein the component (B) comprises a cyclic-olefin resin (b) having a glass transition temperature of 110° C. or more and a content Z in the range from 5 to 50% by weight based on the weight of the W layer;
• wherein polyester resin component (A) comprises a crystalline polyester resin (a1) and an amorphous polyester resin (a2) produced by copolymerization;
• wherein the crystalline polyester resin (a1) is present in an amount from 47.5 to 90% by weight based on the weight of the W layer;
• wherein a weight ratio of the amorphous polyester resin (a2) to the cyclic olefin resin (b) is in the range selected from the group consisting of 0.01 to 0.85 and 0.95 to 1.75;
• wherein the film is free of polybutylene terephthalate, having voids inside and a reflectivity of 96.1 to 99.1%.