Which of the following statements are correct about the maintenance factor?

I) It is the ratio of illumination under normal working condition to the total lumen given out by the lamp.

II) It is the ratio of illumination under normal working condition to the illumination when the things are perfectly clean.

III) The accumulation of dust, dirt and smoke on the lamp decreases the maintenance factor.

IV) It is the ratio of illumination when the things are perfectly clean to the illumination under normal working condition.

Maintenance factor

• It is the ratio of illumination under normal working conditions to the illumination when the things are perfectly clean.
• The accumulation of dust, dirt, and smoke on the lamp decreases the maintenance factor.
• Its value is always less than 1, and it will be around 0.8. This is due to the accumulation of dust, dirt, and smoke on the lamps due to which they emit less light than that they emit when they are clean. Frequent cleaning of lamps will improve the maintenance factor.
   

Therefore, Statements II and III are correct.

Additional Information

• Depreciation factor: It is defined as the ratio of initial illumination to the ultimate maintained illumination on the working plane. 
• Coefficient of utilization or utilization factor: It is defined as the ratio of the total number of lumens reaching the working plane to the total number of lumens emitting from the source.
• Space to height ratio: In illumination, it is defined as the ratio of the distance between adjacent luminaires (center to center), to their height above the working plane. It should be around 1.2.