How to read an isolux diagram

A luminaire is a complete light fitting. An isolux diagram looks down at the floor from above. The luminaire hangs over the centre.

Each line joins the points that receive the same illuminance.

Illuminance is the light that lands on each square metre of a surface. Its unit is the lux (lx).

The lines work like the height lines on a map. Close lines mean that the light falls off fast.

The CIE vocabulary calls the line an iso-illuminance curve. Most data sheets still say isolux.

A luminaire that is not round

The downlight gives circles, because its light is the same in every direction.

The linear pendant gives ovals. The ovals are long in the direction of the pendant. In an LDT file that direction is normally C90–C270.

The pendant also sends light up to the ceiling. That light does not show here. The diagram counts direct light only.

The illuminance at a point

Find the point on the axes and look at the lines around it.

One metre from the centre, the downlight point lies between the 50 lx line and the 100 lx line. The calculated value is 81 lx.

Straight below the luminaire the value is 125 lx.

The spacing between two luminaires

Two luminaires add their light. Halfway between them, each one gives half of the total.

1. Choose the illuminance that you want between the luminaires. This example uses 40 lx.
2. Take half of it: 20 lx.
3. Read the distance from the centre to the 20 lx line. It is 2.18 m.
4. Double it. A spacing of 4.36 m gives 40 lx at the halfway point.

This method is for two luminaires, or for one row. Below each luminaire the value stays high, about 125 lx here.

In a square grid, the point between four luminaires is farther from each one. It gets about 19 lx here.

This is a first estimate. Walls and ceilings reflect light and add to it. A lighting calculation program gives the final answer.

Intensity is the strength of the light in one direction, in candela (cd). For each floor point, the program finds the direction from the luminaire to the point. It reads the intensity in that direction.

Then it uses this law: illuminance = intensity × cos³(gamma) ÷ height². Gamma is the angle from straight down.

The law is exact for a small light source. Near a long luminaire the result is approximate.

• Mounting height. At double the height, the centre value falls to one quarter. High-value lines shrink or vanish. Low-value lines move outward. Always check the height first.
• Line values. One vendor draws 200, 100, and 50 lx. Another draws 300, 150, and 75 lx. The lines then sit in different places.
• Lux or percent. Some diagrams label the lines as a percentage of the maximum, not in lux.
• Area. A large area makes the pattern look small. A small area cuts the outer lines.
• Reflections. Most isolux diagrams count direct light only. A room calculation adds reflected light and gives higher values.
• Orientation. IES and LDT files normally put the luminaire length on different axes. The same pendant can look turned by 90°.

• CIE e-ILV 17-29-078, iso-illuminance curve. The definition of the line, and the note that the term isolux curve is deprecated.
• CIE e-ILV 17-25-104, photometric distance law. Illuminance from intensity, distance, and angle. Exact for a point source only.
• Cornell University, DEA 3500, Lighting lecture notes 1. The cosine-cubed form of the law, with the mounting height.
• CIE e-ILV 17-21-060, illuminance. The definition of illuminance.
• CIE e-ILV 17-25-090, (C, γ) coordinate system. The C-planes and the gamma angle.
• CIE 121:1996, The Photometry and Goniophotometry of Luminaires. The LDT convention: the length of the luminaire lies along C90–C270.
• Viso Systems, Dimensions of lighting fixtures in .IES and .LDT files. Drawings of the two orientations, and the 90° rotation between the formats.
• EN 13032-2:2017, Presentation of data for indoor and outdoor work places. The European standard for the presentation of photometric data. Catalogue page.