How to understand photometric polar diagrams
If you are working in the lighting industry sooner or later you will come across photometric diagrams and you must know how to interpret them. This web page is quick introduction on how to look at a photometric diagram and get important information from it. There are more details in chapter 3 of my book "Candelas Lumens and Lux":
First we'll start off with the photometric diagram called “C-Gamma”. Here is a C-Gamma diagram with some of the luminous intensity “rays of light” left in:
This makes the diagram more confusing than it needs to be and photometric diagrams always leave out those “rays” to give you a simpler diagram as shown below:
The point to remember is that the distance from the center of the diagram to one of the points on the “outline” corresponds to a luminous intensity value, often in candelas, in the given direction.
These diagrams tell you immediately if most of the flux (the lumens, the “flow of light”) goes upwards downwards or sideways. In the example opposite all the light flows in a downard direction.
With C-Gamma photometries the gamma is the “elevation angle” and gamma=0.0 corresponds to a ray of light pointing downwards. The C angle, the angle of the “C-Plane” is usually represented as C=0 going off to the right along the positive x axis, and C=90 going along the positive y axis.
The luminaire whose polar diagram is shown below therefore, shoots most of its flux “out to the left” and is symmetrical in the C90 - C270 plane.
A concrete example might explain better the concept of C-Plane. If mounted inside a room, you could put the C0 plane pointing north, then the C180 would point south, and so on. The 3D view below should help you orient yourself. Remember though that this is the default positioning, the luminaire can be rotated and tilted in real life.
"Candelas Lumens and Lux" gives a longer explanation with more examples:
The images below show you two different sorts of luminaires in a polar C-Gamma diagram. The first luminaire shoots all of its flux upwards, presumably it is used for indoor indirect lighting, when the light is first reflected by the ceiling before arriving at the worksurface. All the light is in the gamma = 90 to 180 degrees. The second luminaire shoots some of its flux upwards and some downwards, a “direct-indirect” method of lighting an indoor environment.
Sometimes, if the luminaire distributes light very unevenly or assymetrically it is useful to see a complete “photometric solid.” An example is given below:
The bigger the bulge the greater the intensity of the light along the the "bulge".
Another photometric diagram is the VH diagram. This is used for floodlights, for luminaires that have to illuminate a large area. As in CGamma two angles are used as the coordinate system, V and H.
The immediate difference you see with this system is that it only covers half of a sphere. It is assumed that floodlights will never emit light away from the main beam.
The VH system is generally shown “pointing horizontally”, but in practice the luminaires will often be directed down onto a sports field (or carpark) from a high pole. There is a complete explanation in "Candelas Lumens and Lux".
You may or may not have noticed in the previous diagrams that the mostly the units are "cd" (candelas) but sometimes the units are "cd/klm", or “candelas per kilolumen”. These cd/klm units are often used for multiple luminaires which are identical "in shape" except for their flux. Maybe the a luminaire can have bulbs with more or less flux (lumens) inserted into it. The photometric curves considered as shapes only, are identical even with the different bulbs. This means that a single diagram can represent several different luminaires if we scale everything to candelas per kilolumen. This is explained, with more examples, in chapter 3 of "Candelas Lumens And Lux" ($19.99):
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