Is Beam Spread Confusing?: Field Angle vs. Beam Angle in Outdoor Lighting

First, we need to define Beam Spread. The term is a bit of a catch all phrase but commonly refers to the angle/width of the light emitted by a luminaire without defining intensity or distance.  Usually when you're in the field and someone speaks about beam spread they are just talking about how wide the light reaches edge to edge. However, there isn't a specific definition for beam spread by IES.  What is defined is Field Angle and Beam Angle.  

Let's look at the definitions:

Field Angle:

[8.6.4.2] The angle between the two directions for which the intensity is 10% of the maximum intensity as measured in a plane through the nominal beam centerline. For beams that do not possess rotational symmetry, the field angle is generally given for two planes at 90 degrees, typically the maximum and minimum angles. Note that in certain fields of application the angle between the 10%-of-maximum directions was formerly called beam angle.

 

Beam Angle:

[8.6.4.1] The angle between the two directions for which the intensity is 50% of the maximum intensity as measured in a plane through the nominal beam centerline. For beams that do not possess rotational symmetry, the beam angle is generally given for two planes at 90 degrees, typically the maximum and minimum angles. Note that in certain fields of application, beam angle was formerly measured to 10% of maximum intensity.

At first look, these definitions may seem a little tough to decipher. Let's put these definitions in layman's terms.

Field Angle is how wide the light spreads from the source in degrees measured from the center of the source out to 10% of the maximum intensity.

Beam Angle is the width of the main, intense beam in degrees measured from the center of the source out to 50% of the maximum intensity.    

Think of the field angle as the width of a flashlight beam. If you point the flashlight straight ahead, the field angle tells you how wide the visible beam of light gets as it travels away from the flashlight. So, if the field angle is 100° then 50° to the right of the source and 50° to the left of the source. Think of beam angle as only measuring the portion of the flashlight beam that is most intense. Ultimately, field angle is going to be the wider measure in degrees and beam angle will be the more narrow measurement in degrees with the most intensity of light. 

When you're looking at a Sterling Lighting specification sheet, we list the field angle because that is what most designers and installers are interested in.

IES is a great reference for definitive terms used in the lighting industry which can help you find what you need. Hopefully this is helpful for when you're talking about the spread of light, look for the field angle of the luminaire.  Keep in mind when you see "beam spread" used, it is sometimes referring to beam angle, field angle or is just a generalization. This can get tricky if say you were looking for a 14° narrow spot and you saw a beam angle that matched, but if the field angle was 100° it wouldn't do what you wanted it to do.  

Hopefully this help clear up any confusion when it comes to beam spread.

Keep an eye out for our next post on Volts Ampere (VA) in outdoor lighting.