Method of manufacturing a transparent light conducting element of synthetic resin having refractive index gradient

A method of manufacturing a transparent light conducting element of synthetic resin in which a continuous change takes place in the refractive index progressively from the surface of the element to its interior, said method comprising, in combination, the steps of

• 1. submitting a monomer A to a prepolymerization reaction, said monomer A being one that can form a transparent network polymer by the polymerization reaction, and before completion of the polymerization reaction and at a stage wherein 2 - 80% by weight of a solvent-soluble component is contained, interrupting the polymerization reaction to form a prepolymer;
• 2. imparting a desired form to the prepolymer to obtain an article thereof having a uniform refractive index;
• 3. contacting for a predetermined time said prepolymeric article on at least a part of the surface thereof with a monomer B, said monomer B being one which forms a transparent completely polymerized polymer having a refractive index different from that of the completely polymerized polymer formed from said monomer A, said predetermined time being selected such that a gradient in the concentration of the monomer B diffused within said article is set up, said concentration gradient being one which gradually decreases from the contact surface to the interior of the article, wherein when a part or all of the diffused monomer has been polymerized, the amount of the monomer B corresponding to that which has been polymerized being included in calculating the concentration;
• and
• 4. carrying out a postpolymerization step consisting of polymerizing the monomer B present in the article and completing the polymerization of the prepolymer, whereby said gradient in the concentration of the monomer B forms said continuous change of the refractive index without causing objectionable turbidity, concurrently with step (3).