The opening angle (also called angle of aperture or radiation angle) is the extent to which the light of a light source is spread. The opening angle is therefore expressed in terms of degrees. An opening angle is always measured on a light source, and not on the lighting fixture (the appliance in which the light source is installed). With LED lighting the opening angle is often directly expressed on the lighting fixture, since the light source is mostly fixed into the fixture. The opening angle you require mainly depends on the purpose the lighting needs to serve.
Small or large opening angle dependent on the lighting purpose
A little opening angle with a narrow light beam is rather suited for accent lighting. With a small opening angle you are able to efficiently illuminate compact surfaces without a lot of “light pollution” in the rest of the room. For example if you want to put a certain ornament, statue or plant in the spotlight with an accentuating spotlight.
Larger opening angles on the other hand are usually meant for general lighting applications. To supply a room with sufficient general lighting, you need evenly distributed lighting without too much lighting contrast. General lighting is the light necessary to enable all basic functionalities of the room.
How is the opening angle measured?
The opening angle is measured from the point where the light source emits its maximum light output (the center) to the point where the light source emits 50% of its light output. This means that a light spot with an opening angle of 30°, still emits a certain amount of light outside this opening angle. We call this additional amount of light “stray light” or “scattered light”.
LED emits a tightly focused light beam
You need to take into account that the visible light beam varies per type of lighting source. Light beams of for example LED lights are more targeted and more delineated. LED almost exclusively radiates light inside its lighting beam. There is almost no stray light being emitted (the light outside the actual lighting beam or opening angle).
More stray light with halogen and energy saving light bulbs
With halogen lights there is more stray light. Finally energy saving and fluorescent lights have the most diffuse type of lighting. Outside their lighting beam, they emit a big amount of light that falls just below 50% light output. This light is very diffuse resulting in less sharp and vaguely delineated shadows. Less sharp shadow generation makes energy saving and fluorescent lights ideal as task lighting. Most office spaces and warehouses are lit by these type of light sources.
So the opening angle needs to be chosen based on the lighting purpose. A halogen spot with a 30° opening angle does not radiate the same light as a 30° LED spot light, which is illustrated in the picture above. Don’t hesitate to leave a question if you want to know more!