Technical Lighting Help

What’s A Beam Angle?

Last week, I delivered a webinar to over 450 designers. The most popular request was for clarification of beam angles. If smart professionals need a refresher, then perhaps others do, as well.

Directional vs. Omnidirectional Light

Light from a light source is delivered to us in one of two ways, directionally, or omnidirectional. A typical 60 watt incandescent lamp is an omnidirectional source. Light exits the lamp in all direction and around the full circumference of the glass envelop.

In this typical incandescent lamp, light is delivered 360 degrees around the circumference of the glass envelope and about 280 degrees from the screwshell to the top and back down to the screwshell on the other side.

A directional light source is different. Light is delivered in only one direction. Think of a flashlight. A reflector built inside influence the delivery of light and prevent it from traveling backwards. Luminaires can be directional as well. Consider undercabinet lights or a recessed can. Light is emitted in a single direction aimed toward a task below.

Directional light is delivered from the lamp in only one direction. Light rays, which might have illuminated the screwshell are instead re-directed out the front with engineered reflectors that cover the side.

Beam Angle

The delivery of light from a directional source can be broad or narrow. Those triangles of light are defined by a geometric angle and known as a beam angle. Previously given a name (spot, flood) or a number, with the introduction of better optics and LED technology, beam angle are now much more specific and an angle is now much more common than a name.

When designing a grid of recessed cans for a kitchen ceiling, or determining the correct accent light in a landscape lighting design, angle is crucial to a successful job. Failure to heed the importance of beam angle will result in a poorly illuminated kitchen and incorrectly lit trees and buildings.

Understanding Beam Angle

In a directional light source, the most intense light is measured at the centerline. This is called the center beam candlepower (CBCP.) Intensity dissipates as it moves away from the center. When that intensity is reduced by 50%, the beam angle is established. All remaining light, outside of the beam angle is called spill. Some light sources create a large amount of spill, others are engineered to drop off to black very quickly. In some applications, such as landscape lighting, you want a tight, defined angle. In others, spill can be an advantage. In heavily regulated outdoor environments, spill can impact plants and wildlife, so it is deemed detrimental. It is important to understand what light you want, how it is used and how it will impact the lighting design.

In this fictitious luminaire, we imagine the CBCP to measure 2000. When that measurement drops 50% to 1000, the angle is established. Everything outside the beam angle is considered spill. The amount of spill varies according to luminaire, lamp and reflector design.

Information From Manufacturers

It is very common for manufactures to provide photometric data that helps in the selection of a product. This will appear in the form of a rectangle with rays and quarter-circles. Through the center will be an irregular shaped arc. At first glance, they may appear confusing. Use these helpful descriptions.

  • The Center Beam runs along the left side of the rectangle
  • The quarter-circles represent different amounts of candlepower
  • The rays represent angles
  • The Center Beam Candlepower (CBCP) is reflected in the point where the arc intersects the left side of the rectangle
  • The arc represents the light delivery for that particular light source. As it moves from its maximum delivery of light at the lower-left corner to zero at the upper-left corner, it defines the quantity of light at various angles.

To determine beam angle, if not provided by the supplier, simply divide the CBCP in half and find that point on the chart. Move right and find that point on the arc. Remember, this chart represents  “half” of the light, so if 50% of CBCP intersects with the arc at the 30° ray, then the beam angle is 60° (30° + 30° = 60°). On the example, this manufacturer calls out the CBCP and the beam angle, but that is not always the case.

This rectangular image is a common way for manufacturers to tell users about their product photometry. (My notes inserted.) The arc that runs through the quarter-circles and rays represents the light for that particular product. The intersection points provide the needed data.
Here we see a very similar rectangle, from the same company (DMF) but with a luminaire that is delivering a lot more light in a much smaller beam angle. Regardless, the same chart layout is used.

Beam angle is an important part of lighting design. Knowing what is meant and how to find it will make all your design work easier and better, to say nothing of the improved light quality!

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