In a lighting fitting provided with at least two low-pressure mercury vapor discharge lamps whose radiated light has a different spectral composition. The lighting fitting is provided with apparatus to control the lumen ratio of the two lamps without varying the lumen level of the fitting. According...http://www.google.com.au/patents/US4045664?utm_source=gb-gplus-sharePatent US4045664 - Lighting fitting provided with at least two-low-pressure mercury vapor discharge lamps
