What is beam angle?
The angle between the two directions, either side of the beam axis, where intensity falls to half of its peak. One number that sums up how wide the beam is.
Beam angle is the angle between the two directions, one on each side of the beam axis, where a fixture’s intensity has fallen to a stated fraction of its peak, usually half. It is the full angle across the beam: a beam that falls to half intensity 10° either side of the axis has a beam angle of 20°. Some data sheets quote the half angle instead, so check which one is given before using it.
It is useful because it turns the beam into simple geometry. The beam is a cone, so its width grows with distance: w = 2 × d × tan(θ/2). A wider beam angle covers more width at the same distance, while the illuminance on a surface still falls with the square of the distance. The angle is quoted separately in the horizontal and vertical planes.
A single angle has limits. Two beams with the same 50% angle can have very different tails, which is why a field angle at a lower level, often 10%, is sometimes quoted as well. AGL requirements are written as intensity at defined angles in the isocandela diagrams, so beam angle is a summary, not an acceptance test. For the whole shape, see Beam Pattern.
Frequently Asked Questions
Q1What is beam angle in airfield ground lighting?
The angle between the two directions, one on each side of the beam axis, where a fixture's intensity falls to a stated fraction of its peak, usually 50 percent. It sums up how wide the beam is in a single number.
Q2How is beam angle measured?
In a plane through the beam axis, as the angle between the two points where intensity has fallen to the stated fraction of the peak. Horizontal and vertical angles are quoted separately, and the fraction used should be checked in the source you are working from.
Q3Is beam angle the full angle or the half angle?
Beam angle is normally the full angle across the beam, so a beam that falls to half intensity 10 degrees either side of the axis has a beam angle of 20 degrees. Some data sheets quote the half angle, so confirm which one is given before using the value in a calculation.
Q4How is beam angle different from beam spread?
They describe closely related ideas, the width of the beam at a stated level. Beam angle is the term used in general lighting and in engineering calculations, while beam spread is more common in AGL documents, where it is tied to the intensity levels of the isocandela diagram. Check the level each document uses before comparing values.
Q5What is the difference between beam angle and aiming angle?
Beam angle is how wide the beam is. Aiming angle is where the beam axis points relative to a reference such as the horizontal or the runway. A correct beam angle with the wrong aiming still misses the observer, which is why alignment is checked at commissioning.
Q6Can beam angle show that a fixture meets its requirement?
Not on its own. Requirements are written as intensity at defined angles in the isocandela diagrams, and a fixture can have the expected beam angle and still fall short at another angle. Beam angle is a useful summary, not an acceptance test.
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