Sources for Visible Region:

In the starting of development of instruments for colorimetric work, sunlight was used as source for measurements within the visible range.  The modern instruments use a tungsten filament lamp as the radiation source. This consists of a thin, coiled tungsten wire which is sealed in an evacuated glass bulb. This provides radiations within the range of 350-2200 nm. As the results depend on the voltage, the tungsten lamp is energised through a 6 or 12 volt storage battery or through the output of a constant voltage transformer.

Now a day's a few instruments use tungsten-halogen lamps which hold a small amount of iodine in the quartz bulb housing the tungsten filament. The presence of iodine extends the result wavelength range of the lamp from 240 -2500 nm.