Apron lighting uniformity is calculated by comparing the average illuminance across an aircraft stand with the lowest illuminance measured within the same area.
In simple terms, it tells you how evenly the light is distributed. A stand can have a good average lux level but still contain poorly lit areas. Uniformity identifies those differences.
For airport apron lighting, uniformity is commonly expressed as an average-to-minimum ratio:
Uniformity ratio = Average illuminance ÷ Minimum illuminance
Or
Eavg ÷ Emin
For example, if an aircraft stand has:
The calculation is: 24 ÷ 6 = 4
So the uniformity ratio is: 4:1
Current aerodrome guidance referencing the ICAO Aerodrome Design Manual specifies 20 lux average horizontal illuminance on aircraft stands with an average-to-minimum uniformity ratio of no more than 4:1.
During the lighting design, the aircraft stand is divided into a calculation grid.
Lighting software such as DIALux calculates the illuminance at each point across that grid.
The process is:
That gives the average-to-minimum uniformity ratio.
The same principle can also be used when testing an existing installation using physical lux measurements across a defined grid.
There is another way you may see uniformity expressed in lighting reports:
U0 = Emin ÷ Eavg
This is effectively the inverse of the average-to-minimum calculation.
For example: 6 lux ÷ 24 lux = 0.25
Therefore: U0 = 0.25
A 4:1 average-to-minimum ratio is equivalent to a U0 of 0.25.
This can cause confusion when comparing lighting specifications because:
So a report showing U0 = 0.50 has better uniformity than one showing U0 = 0.25.
Uniformity matters because airport operations do not take place at the average calculation point.
Ground crews, vehicles and aircraft move across the entire stand.
Large variations in illuminance can create bright and dark patches, making it more difficult to see equipment, markings, objects and activity around the aircraft.
ICAO guidance specifically identifies uniform illumination of the pavement within the aircraft stand and the elimination of glare as major requirements for apron floodlighting.
Good apron lighting therefore needs to consider more than the headline lux figure. Average illuminance, minimum illuminance, uniformity, vertical illuminance, glare and shadows all need to work together.
Uniformity is influenced by the entire lighting design, including mast positions, mounting heights, floodlight optics, aiming angles and the interaction between different luminaires.
ICAO referenced aerodrome guidance also recommends that aircraft stands receive light from two or more directions to minimise shadows.
This is why replacing existing HID or sodium floodlights with LED on a simple one-for-one basis does not automatically guarantee a good result. The photometric performance of the complete installation needs to be considered.
Apron lighting uniformity is calculated using:
Average illuminance ÷ Minimum illuminance = Uniformity ratio
So if an aircraft stand achieves an average of 24 lux and a minimum of 6 lux:
24 ÷ 6 = 4:1
The key point is that average lux alone does not tell you whether an aircraft stand is well lit. Uniformity shows how consistently that light is distributed across the working area, which is why it should always be assessed alongside the overall illuminance level.