Often photometric diagrams use the C-Gamma system. Gamma=0 points downwards towards the floor or road. Gamma=180 points upwards to the ceiling or sky.
Here is a C-Gamma diagram with some of the luminous intensity “rays of light” left in:
The "rays" make 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 above all the light "flows" in a downward 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 (the dotted line):
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.
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:
These polar diagrams are sometimes called C-Gamma diagrams and are normally used for roads and interior lighting. Click here for floodlights and VH diagrams.