See the economics section for the details of group vs. spot relamping analysis and other economic considerations. Airborne dirt deposited on luminaire surfaces affects the amount of light leaving the luminaire and the intensity distribution of the luminaire. The amount of light leaving the luminaire decreases for the simple reason that dirty surfaces absorb more light than clean ones. The distribution is affected because specular surfaces become more diffuse when dirty, a principle that becomes obvious when you consider your image in a dusty mirror as compared with the image in a clean mirror. The cleanliness of the environment in which a lighting system is operated is, of course, a major factor in luminaire dirt depreciation.
More detailed information is available in the lumen method section .
In addition to the amount and kinds of dirt and impurities present in the air, dirt depreciation also depends on some aspects of luminaire design. Luminaire surface finishes vary in the ease with which dirt can attach itself and in the ease with which an electrostatic charge can form. Ventilated luminaires tend to collect dirt less rapidly than those with closed tops. This is due to the chimney effect by which the temperature difference between the lamp and the ambient room air cause convective air motion that helps keep dust from settling on the luminaire surfaces. On the other hand, completely closed units, if properly designed, exclude almost all dirt.