Patent ID: 6995907

Claim:
A diffusion sheet for use in a transmission-type screen, the diffusion sheet comprising: a main diffusion layer having an incidence surface and an emergence surface that are parallel to each other; and an assisting diffusion layer provided at least on an emergence surface-side of the main diffusion layer; wherein: the main diffusion layer is provided with a plurality of substantially parallel groove channels in the emergence surface extending along the main diffusion layer in a vertical direction; each of the groove channels has a substantially V-shaped cross-section defined by a first side wall and a second side wall that extend into the emergence surface and intersect within the main diffusion layer, each of the first side walls and the second side walls having a shape selected from planes, curved surfaces, or bent planes; the main diffusion layer is provided with a plurality ribs separating the plurality of groove channels; each of the ribs has a substantially trapezoidal cross-section defined by the emergence surface, the first side wall of an adjacent groove channel, and the second side wall of another adjacent groove channel; light substantially perpendicularly incident on the incidence surface is reflected by the first and second side walls so that the light is diffused in a diffusion direction; the assisting diffusion layer comprises a light diffusion component that diffuses light at least in the diffusion direction; and the light diffusion component of the assisting diffusion layer is selected so that a gain curve for light emergent from the diffusion sheet at horizontal viewing angles has no minimal point.