How to read a cone diagram

What a cone diagram is

A luminaire is a complete light fitting. A cone diagram shows its beam from the side. The beam opens like a cone below the luminaire.

The diagram gives two numbers for each mounting height. They are the diameter of the light spot and the illuminance at its centre.

The mounting height is the distance from the luminaire to the lit surface.

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

The round downlight. The values are invented.

The numbers on the diagram match the notes beside it.

The same diagram with five markers.

The luminaire is at the top. It aims straight down.
The beam angle is the full opening angle of the cone. Intensity is the strength of the light in one direction, in candela (cd). At the edge of the cone it is 50% of the peak.
Each horizontal line is one mounting height, in metres.
The diameter is the width of the cone at that height. The light does not stop at this edge. It fades.
The centre value is the illuminance straight below the luminaire, in lux.

A cone diagram assumes a round beam. One beam angle then describes the full light spot.

The linear pendant has two beam angles: 65.6° across it and 101.9° along it.

A C-plane is a vertical slice through the luminaire. The pendant has a different beam angle in each one, so its light spot is an oval.

A data sheet then shows two cones, or it shows an isolux diagram.

You can calculate each row yourself. You need the peak intensity and the beam angle.

Divide the intensity straight down by the square of the height. This is the inverse-square law.

2000 cd ÷ (4 m × 4 m) = 125 lx.

At double the height, the illuminance falls to one quarter.

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

Multiply the height by two, then by the tangent of half the beam angle.

2 × 4 m × tan(20°) = 2.91 m.

At double the height, the diameter doubles.

The file holds no beam angle and no mounting height. A program calculates both results from the intensity values.

Part of the diagram	IES file	LDT file
The peak intensity and the 50% points	Candela values at the vertical angles	Intensity values at the gamma angles
Candela from the stored values	The candela multiplier and the ballast factor	The total luminous flux, because the values are in cd/klm
The mounting heights	Not in the file. The person who makes the diagram chooses them.

Beam angle or field angle. The beam angle uses 50% of the peak. The field angle uses 10%, so its cone is much wider. For this downlight the field angle is 71.3°.
Peak or centre. The CIE definition uses the intensity at the centre of the beam. Many programs use the peak. The results differ when the peak is off centre.
Heights. One vendor lists 1 m to 5 m. Another lists 2 m to 10 m, or uses feet.
Centre or average. Some diagrams give the illuminance at the centre. Others give the average across the spot, which is lower.
One plane or all planes. A program can measure the beam angle in one C-plane, or across all C-planes. The two methods give different angles for a luminaire that is not round.
Units. Some diagrams use lux and metres. Others use footcandles and feet. One footcandle is 10.764 lux.

The data is the same in all four. Only the style changes. Read about diagram styles.

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CIE e-ILV 17-27-077, beam angle. The 50% rule, and that the beam angle is a full angle.
CIE e-ILV 17-25-105, photometric inverse square law. Illuminance from intensity and distance, for a surface square to the light.
CIE e-ILV 17-25-104, photometric distance law. The general law, and the note that it is exact for a point source only.
CIE e-ILV 17-21-060, illuminance. The definition of illuminance.
CIE e-ILV 17-21-085, lux. The lux, and the footcandle conversion.
AGI32 Photometric Toolbox, IES Flood Report Summary. Beam at 50% and field at 10% of the maximum candela.
Cornell University, DEA 3500, Lighting lecture notes 1. The inverse-square law and the cosine law in plain formulas.