Light levels, uniformity and spacing

A lighting calculation puts a grid of points on a surface: a floor, a road, a path or a desk height. It calculates the illuminance at each point.

Illuminance is the light that lands on each square metre of a surface. Its unit is the lux (lx). The isolux diagram shows it as lines on the floor.

Four figures sum up the grid:

• Average. The mean of all points. It tells you how much light the surface gets.
• Minimum. The darkest point. A dark patch on a path or a desk starts here.
• Maximum. The brightest point, usually below a luminaire.
• Uniformity. The minimum divided by the average. EN 12464-1 calls it U₀. A value of 1 means that every point gets the same light. A value near 0 means a dark patch.

The minimum and the maximum come from the grid points. A finer grid can find a darker or a brighter point between two points.

The table shows one row of seven round downlights at a mounting height of 3 m. The figures cover the line below the row, between the two middle luminaires. The values are invented.

As the spacing grows, the minimum falls much faster than the average. The uniformity shows the dark patch before the average does.

In this example, a spacing equal to the mounting height keeps the uniformity above 0.5. Some data sheets give a spacing criterion: the largest spacing divided by the mounting height that keeps the light even.

Straight below one downlight, the illuminance is 500 lx at 2 m and 125 lx at 4 m. At double the height, the value falls to one quarter.

A higher luminaire also spreads its light over a larger area. The beams of neighbouring luminaires overlap more. The table keeps the spacing at 4 m and changes the height.

From 3 m to 6 m, the average falls from 120 lx to 59 lx. The uniformity rises from 0.28 to 0.91. A higher luminaire gives less light, but more even light.

The calculation plane is the surface where people work. It is often at desk height, not on the floor. ANSI/IES LS-1-25 gives 0.76 m as the usual height.

A room layout has two targets: an average illuminance and a uniformity. More luminaires raise both figures, but each one adds cost and power. The aim is the smallest layout that meets both targets.

In Lighting Scenes, "Find an even layout" tests one to six rows and columns of one product, up to 36 luminaires. It gives one of three answers:

• Layout found. The smallest tested layout that meets both targets.
• No tested layout meets both targets. The page shows the closest layout and the target that it misses.
• Search incomplete. The time limit or the grid stopped a sure answer. The page can show a provisional layout.

In the middle of the row, each point gets light from luminaires on both sides. Near an end wall, the light comes from one side only.

So check the end areas apart from the whole floor. A good average for the whole corridor can hide a dark end.

Lighting Scenes gives separate figures for each end area: from the end wall to the nearest luminaire. To raise them, move the first and the last luminaire closer to the end walls.

• Reflections. Walls, ceilings and floors reflect light. In a room, the real light levels are usually higher.
• Obstructions. Furniture, shelves, people and trees block light and cast shadows.
• Light loss over time. Dirt and LED ageing lower the light output. A design applies a light loss factor, which EN standards call a maintenance factor.
• Size of the luminaire. The calculation treats the luminaire as a point. The error stays below 1% when the distance is at least five times the largest size of the luminous area. Lighting Scenes warns you when a surface is closer.

Use these figures to compare products and to choose a spacing or a height. They do not show that a design meets a lighting standard. A full lighting calculation gives the final answer.

Try it with your own file: Lighting Scenes.

• EN 12464-1:2021, Lighting of indoor work places. Illuminance uniformity U0: the minimum divided by the average. Catalogue page.
• ANSI/IES LS-1-25, section 3.3.1, illuminance. The light that arrives on each unit of area at a point.
• ANSI/IES LS-1-25, section 9.1.1, inverse-square law. Illuminance falls with the square of the distance. The error stays below 1% at five times the largest source size.
• ANSI/IES LS-1-25, section 9.2, work plane. The plane where people work. The usual height is 0.76 m (30 in).
• ANSI/IES LS-1-25, section 9.4.9, light loss factor. The drop in light output over time. Formerly named maintenance factor.
• ANSI/IES LS-1-25, section 10.3.1.3, luminaire spacing criterion. The largest spacing divided by the mounting height that keeps the light even.
• CIE e-ILV 17-25-104, photometric distance law. Illuminance from intensity, distance, and angle. Exact for a point source only.