which is centered on the axis and extends in a flat circular configuration almost to the most remote dimension of the diffuser from the axis the collector wall 14 is a toroidal continuation of the image wall and curves sharply away from the plane of the image wall until it approaches the axis of the diffuser at an angle of about 85 u00b0 it terminates in a cylindrical input face 15 at a normal distance from the axis of about 3 8 inches its maximum distance from the axis being about 11 8 inches to form a conelike toroidal structure both walls are composed of a light conducting material such as lucite and are preferably constructed in one place the wall thickness is approximately 3 8 inches the outer surface of the collector wall is covered with a light reflecting coating 16 such as a white gloss paint the inner surface is covered by a similar coating 17 on both the collector and the image walls the generally cylindrical input surface 15 of the collector wall is not coated but is polished to improve light transmission a transparent non reflective coating e g u03bb4 can be used on this surface if greater efficiency is required the inner surface of the image wall is additionally covered with a black felt 18 or similar material to block direct transmission of light into the wall through this surface the outer surface 19 of this same wall is provided with a fine grain either by removing or etching transparent material therefrom one method calling 34 hydrohoning 34 involves passing a liquid such as water containing 600 abrasive grit over the plastic surface the grain must of.