How to interpret a light distribution curve?
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. Fortunately you can always rely on dmlights for assistance. 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.
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.
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.
To make certain that you really understand the concept we will discuss a few more examples below.
In the case of the DeltaLight Reo the two curves of the frontal and side view overlap. The beam is the same, regardless of the direction from where you are looking. This spotlight has a normal downward beam.
The Flos Glo-ball is a spherical pendant light which lights up and down. You can see this in the graph, as the curve radiates above and below the 90° axis. Here again the two curves overlap. The Glo-Ball’s shape and light distribution is completely symmetrical.
Modular Lighting Duell
The Modular Lighting Duell has a rather interesting light distribution curve. If you look straight at this wall light, you can make out an upward and downward beam, which features a small indent just above and below the luminaire and then fans out. In the curve this is reflected in the solid red line, which is somewhat arrow-shaped.
The red dotted line indicates the light distribution as seen from the side. The beam undulates a little, casting a nice reflection on the wall. The top and bottom curves are always identical, meaning the Duell wall light has the same light distribution, both upwards and downwards.
Flos Miss K
The Flos Miss K table lamp has a symmetrical shape and the light distribution looks the same from every angle. The curves completely overlap. The beam is split into two separate planes, both upwards and downwards. The downward split is due to the fact that the light source is positioned on top of the luminaire meaning the lamp cannot radiate any light directly below it. Hence the indent in the middle.
Flos Cicatrices de luxe
The Flos Cicatrices de luxe pendant light is truly a special case. The glass vases distribute the light in an extraordinary way. All these reflections are represented by the star shape in the light distribution curve.
We end with a rather suggestive curve. The Flos Gatto table lamp has a symmetric light distribution. Here too the two curves overlap. The lamp radiates light upwards and downwards. The Gatto table lamp however has a round opening at the top, which explains the upward bulge in the curve.
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:
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.