The light bulb in the AlphaBright QTH is a G4 type 6V, 20W Quartz Tungsten Halogen type light bulb. We have selected the light bulb specifically for this item and to work with the power supply, collimating lens system and set up in this instrument. The light bulb is rated for 2,000 hours operation by the manufacturer but if the AlphaBright QTH is being used for spectroscopic applications we suggest changing the light bulb more frequently as the filament ages over time and the spectral output will therefore vary with both the number of times the unit is switched on and the total burn time of the light bulb. It is generally recommended that the light bulb is changed after 500 hours of operation for spectroscopic applications. Further QTH light bulb information.