Gas discharge lamp

A mercury discharge lamp includes an amalgam for controlling the mercury vapour pressure within the lamp, the amalgam being selected and positioned within the lamp to provide a substantially optimum vapour pressure of mercury at the temperature of the amalgam corresponding to the maximum rated operating power of the lamp. The optimum vapour pressure is close to a minimum (B) at the temperature corresponding to maximum operating power, and adjacent a region (D) of steeply rising mercury vapour pressure with falling temperature. By operating the lamp in this region, a reduction in the operating power of the lamp causes a fall in temperature and hence an increase in the vapour pressure of the mercury to effect dimming of the lamp. The lamp may be electroded or electrodeless.