The emittance of new and aged amalgam low-pressure lamps used by a drinking water ultraviolet (UV) disinfection reactor was measured along the length and about the circumference. The new lamp had a non-uniform emittance along the length of the lamp with local maxima between the electrodes and the amalgam deposits. The aged lamps showed a reduction in emittance and a non-uniform output both along the length and about the circumference. The reduction and non-uniformity increased with lamp age and were most notable at the lamp ends, and the position of the maximum emittance moves to the lamp centre. Computer simulations of the UV reactor show that the position of the UV sensor that indicates UV dose delivery is crucial. The UV dose will be overpredicted if the UV sensor views a location that ages the least but underpredicted if the UV sensor views a location that ages the most.