Modular electric lighting systems comprising lighting fixtures and parts thereof, lighting units, light bulbs, lighting tracks, track lighting fixtures, track lighting adapters, retrofit lighting fixtures, screw on lighting fixtures, lighting fixtures with changeable optical systems comprising luminaire refractors, lenses, light filters, louvers, and color filters, recessed lighting fixtures, specialty lighting fixtures, recessed lighting fixture upgrade kits comprising housing units, trim, light hangers, electronic ballasts and light sockets all sold as a unit