Fluorescent tubes have long been the mainstay of lighting for many premises, due to relatively high efficiency and long life. LED efficiencies are set to exceed those of fluorescent tubes, with dimming and controllable colour rendering readily achievable. Two multichip LED lamps are presented, mounted in a four foot fluorescent fitting, with ballast replaced by driver electronics. The first lamp employs white LEDs with optional cyan and red/orange spectral improvement; the second lamp is based on primary colours. Spatial distribution, intensity and spectrum of light output are compared to those produced by a fluorescent tube with the same power input. Lighting systems are sensitive to power system voltage disturbances resulting in visible light flicker; the sensitivity of the white LED lamp is compared with that of fluorescent tubes under voltage modulation and transient dips and swells.