Beam angle

[JeffM]

I have seen many LED lights that tell their beam angle and field angle. What's the difference and how can I use that information to determine which lights I need to use. I want to use some to just provide color on my stage but not sure on the specs. needed.
Thanks.

[waynehoskins]

Beam angle is the main intensity portion of the beam of light; if I recall, it's where the light falls to half its intensity. Field angle is farther out, where it falls to 5 or 10 percent its intensity.

Of the two, the beam angle is the more useful measurement. If you assume the light stops at the beam angle, you're safe when the field angle goes a bit more. If you assume you have useful light out to the field angle, you'll usually get a nasty surprise when black holes pop up.

[JeffM]

Originally Posted by waynehoskins

Beam angle is the main intensity portion of the beam of light; if I recall, it's where the light falls to half its intensity. Field angle is farther out, where it falls to 5 or 10 percent its intensity.

Of the two, the beam angle is the more useful measurement. If you assume the light stops at the beam angle, you're safe when the field angle goes a bit more. If you assume you have useful light out to the field angle, you'll usually get a nasty surprise when black holes pop up.

Ok. How do I determine the size of area I would would wash with knowing the beam angle and knowing the distance? I want to add color to a stage but not knowing how much of a wash I would get based on a known distance from the stage.

[waynehoskins]

Beam diameter = 2 * Throw distance * tan(Beam angle/2) if I recall my trig correctly.

Don't forget that the throw distance is the actual path in the air, not just the distance on the ground, and that if you're lighting people, the target is about six feet in the air, not on the ground. The actual throw distance is easy enough to come up with using right triangles.

Not all of the LED fixture photometric data is quite correct. In particular, people have said that the measured light output from many of them is less than the datasheet suggests. I would expect a similar discrepancy in beamwidths, that the datasheet figures may be a bit wider than you'd find in reality. Most of the ones I've seen are comparable to a Narrow Spot PAR lamp, that they have a pretty narrow main beam, 20 degrees or less from memory. They're perfectly fine if you expect it, but you might get a surprise if you're expecting them to be a 60-degree wash fixture.

[Steve Vanciel]

If you don't have a trig calculator handy, you can easily draft the angles to scale and measure the spread at a given distance.

For example a 30 degree fixture will provide about a 6 ft. spread at 12 ft. and about a 12 ft. spread at 24 ft.

SteveV
Orlando, FL