The ratio of illumination under actual condition to illumination when everything is perfectly clean is known as:

Ratio of Illumination under Actual Condition to Illumination

• The ratio of illumination during typical working conditions to illumination when everything is clean is known as the maintenance factor.
• Its value is typically 0.8 and is never greater than 1. This is because the lamps' accumulation of dust, filth, and smoke causes them to emit less light than when they are clean. Regular lamp cleaning will increase the maintenance factor.
• The ratio of initial illumination to final maintained illumination on the working plane is known as the depreciation factor.
• Maintenance factor / Depreciation factor
• It always has a value greater than 1.
• Utilization factor or coefficient of utilisation It is defined as the proportion of all lumens that reach the working plane to all lumens that are emitted from the source.
• The term "space to height ratio" refers to the relationship between the height of adjacent luminaires above the working plane and the distance between them (centre to centre). It ought to be close to 1.2.

Therefore, the maintenance factor is the ratio of illumination in current conditions to illumination in a spotless environment.

Summary:

The ratio of illumination under actual condition to illumination when everything is perfectly clean is known as: (a) Maintenance factor (b) Utilisation factor (c) Reduction factor (d) Space-to-height ratio

The maintenance factor is the ratio of illumination in actual conditions to illumination in spotless conditions. The maintenance factor is also called the loss factor which refers to the loss of light over time.