Two companies that specialize in laser-based rapid prototyping technology have announced plans to merge this year.3D Systems Corp. of Valencia, Calif., has agreed to buy all of the outstanding shares of DTM Corp. of Austin, Texas, for $5.80 per share, or a total of about $45 million.Under the agreement, employee stock options would be converted to a right to receive cash, and DTM would become a wholly owned subsidiary of 3D Systems.The companies reported about $150 million in pro forma revenues in 2000.3D Systems manufactures stereolithography equipment that uses lasers to selectively harden a photopolymer to rapidly create prototypes of parts. DTM uses a similar technology, selective laser sintering, to fuse plastic, metal or ceramic powder into prototype parts.