Described are various embodiments of a length adjustable LED assembly, optical system using same, and method of assembly therefor. In one embodiment, an optical system comprises a first LED and second LED board each comprising a plurality of LEDs operatively mounted thereon along their respective lengths in accordance with a designated array. The second LED board is adjustably fixable in lengthwise overlapping relationship to the first LED board to provide a length adjustable extension thereto and substantially continuously maintain an optical system output over a combined length of the first LED board and the second LED board.