It may be define as “ the ratio of illumination under normal working condition to the illumination when ever thing is clean and new.

D. F = illumination under normal working conditions/ illumination when everything is clean.

The depreciation factor based on how often the light are cleaned and replace. It take into account such factor as decrease efficiency with age , accumulation of dust within the fitting itself and depreciation of reflection as wall and ceiling age.

It is usually give as three option

Good= 0.70

Medium= 0.65

Poor = 0.55