What is Beam Angle
Beam angle refers to the angle between the two planes of light where the intensity is at least 50% of the maximum intensity at center beam. The average beam angle on most par lights is 25 degree and will work well for most purposes.
The Beam Lumens are the total luminous flux that is emitted within the region of space, where the luminous intensity is more than half the maximum luminous intensity. With most light source, this region can be described as a cone shaped by all beam angles.
What is Field Angle?
Field angle refers to the angle between the two planes of light where the intensity is 10% (or less) of the maximum intensity at center beam. This is sometimes referred to a ghosting or spill, it is not considered usable light. The type of lens used in the fixture will usually determine the amount of field angle or spill. For example, a lens that has a textured surface or frosted coating will create a wider field angle. This is because the textured surface on the lens (or frosting) will improve color mixing, but also diffuses the light and creates softer edges along the beam creating a wider field angle, and more spill.
The Field Angle is the angle between the two directions opposed to each other over the beam axis for which the luminous intensity is 10% that of the maximum luminous intensity. Note that in certain fields of applications the field angle was formerly called beam angle.
The beam angle is determined by the type of lens used in your LED fixture. Below are 2 examples of common lenses. The smaller lens on the left is typical in RGB fixtures (one color lamp under each lens) and this sample is a 25 degree lens. Beam angles ranging from 10 degree (narrow) to 60 degree (wide) are common. The lens on the right is a larger lens and this type is common on Quad-LED fixtures in which all 4 LED color lamps are located under each lens, hence the reason for the larger size. See this page for more info on Quad-LED. These larger lenses usually have wider beam angles of 25 - 60 degrees. Notice the textured surface, the purpose of this is to produce better color mixing. However, the textured surface will reduce beam intensity by a small percentage.
The luminous intensities are measured in a plane normal to the nominal beam centerline. If the beam is not rotationally symmetric, then the beam angle is usually given in two planes at 90° of each other, possibly the maximum and minimum angles. Other angles (eg. at 45°) may also be given.
The Beam Spread is a general term, describing the angle between the two directions opposed to each other over the beam axis for which the luminous intensity is a certain fraction of that of the maximum luminous intensity. The amount of that fraction needs to be given in each specific case.
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