How to interpret a light distribution curve?

How to interpret a light distribution curve?

By Kristof

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Did you just have a spontaneous flashback to maths tests in sixth form when you saw this diagram? Well, you are not alone because Light Distribution Curves can be quite a headache for most people. In this article we will try to explain these graphs in a simple way.

So what is a Light Distribution Curve?

As its name points out, a light distribution curve is a visual representation of the light diffused by a luminaire. This graph tries to transpose a three-dimensional concept (the light diffusion of a lamp or fixture in a space) onto a two-dimensional medium (a sheet of paper or a computer screen).

Interpreting a light distribution curve

At first glance, a light distribution curve may look quite complicated. But appearances can be deceiving because they are (relatively) simple to read as soon as you understand the various components.

Symmetrical light distribution

Let’s start in the middle of the diagram (see below). This marks the lamp‘s position. Usually you will see two lines radiate from the centre, a solid line and a dotted line. These lines indicate the light distribution and intensity from various angles.

The solid line indicates the frontal view (C0/180), the dotted line the side view (C90/270). The shape of both lines is usually about the same for most lamps. In the example below the two curves overlap. You can see why in the 3D chart. The light distribution is the same, in the frontal view and in the side view. The two curves overlap on the right side of the chart, which is why the dotted line is invisible.

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Asymmetrical light distribution

In the case of an elongated pendant light with two separate TL lamps like in the example below, the two curves will have a different shape.

We will illustrate this with the example below. If you look straight at the luminaire, meaning if you position yourself along the 0-180° axis, you can see that the upward beam is spherical (top right). The downward beam, however, is split into two spherical planes, because of the inbuilt reflector. The light is blocked by the reflector in the centre.

If you look at the luminaire from the side, meaning if you position yourself along the 90-270° axis, then you can see a spherical plane upwards and downwards. This means that the luminaire distributes the light evenly. The luminaire’s elongated shape does not mean the curve is flat or elongated. The light distribution is measured from the central point of the luminaire.

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PS: sometimes there are three or four lines visible, e.g. at 0°, 30°, 60° and 90°. This allows you to show even more detail (all around the lamp). Each perspective has a different colour to clearly distinguish between them.

Up and/or downlighter

The lines of the curve show how and where the light is distributed. Is the curve completely located under the 90° axis? Then this is a downlighter, the light only shines down. In the case of an up/downlighter like this lamp, the curve is located both above and below the 90° axis.

The light distribution curve values

In addition, we can deduce the light intensity in the various corners to which the light radiates. This is always measured starting from the centre point of the light source. The light intensity is expressed in Candela (cd) and is indicated on the circles in the graph. The larger the circle, the higher the candela value. This will probably become clearer if we look at the following example.

Point A in the diagram below tells us that the light intensity at 30° is 400 candela. In point B, the angle is 20°, which yields a reading of 800 candela.

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Why light distribution curves are so useful

These light distribution curves are invaluable when drawing up a light plan for a space. They allow designers to choose the right lighting for the right room and application.

Imagine you are devising a light plan for an office building. You have an open plan office, with various desks next to each other, as in the photo below:

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Once you know how much light is required for every workstation (for the highest productivity), you can easily select the right luminaires and choose how best to install them based on the light distribution curve.

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