# Beam Angle, Field angle, And Ellipsoidal degrees?



## carsonld (Aug 30, 2013)

What are these three things? I have noticed that all ellipsoidals are categorized into degrees, and then when I am looking online a LED fixtures that we are thinking about getting it will say the beam and/or field angle? What do these three things mean?


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## JD (Aug 30, 2013)

Beam angle is what you see when you read Ellipsoidal degrees. 
It is the angle of the primary light coming out of the front.
Beam angle may refer to a non-hard edged light (such as a PAR) and refers to the area in which most of the illumination is supplied.
Field angle usually refers to a wash light and is the angle beyond which the outer field of light drops off.

For example, a beam projector produces a very tight beam angle (1 to 5 degrees when focused tight) but also drops light outside of that area, and depending on the type of baffle used, you may have larger than a 30 degree "field angle."

If you put a gobo template into an ERS, then you can used the "Ellipsoidal degrees" to calculate how large the image will be at varying distances. Without a template, you can use it to calculate the size of the beam at varying distances, which would be the same as the "beam angle."


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## TDN (Aug 30, 2013)

JD said:


> Beam angle is what you see when you read Ellipsoidal degrees.
> It is the angle of the primary light coming out of the front.
> Beam angle may refer to a non-hard edged light (such as a PAR) and refers to the area in which most of the illumination is supplied.
> Field angle usually refers to a wash light and is the angle beyond which the outer field of light drops off.



I don't think that's quite accurate- Beam angle is the area in which the light output is above 50 percent of it's maximum- the center of the beam. The field angle is the total width of the spread of the light. Field angle corresponds to the degree listed on the ellipsoidal. So for example, a 50 degree light will have all of its light spread in a 50 degree cone, but the beam angle inside is where the light is the strongest, and then falls off to the side.


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## xander (Aug 30, 2013)

TDN said:


> I don't think that's quite accurate- Beam angle is the area in which the light output is above 50 percent of it's maximum- the center of the beam. The field angle is the total width of the spread of the light. Field angle corresponds to the degree listed on the ellipsoidal. So for example, a 50 degree light will have all of its light spread in a 50 degree cone, but the beam angle inside is where the light is the strongest, and then falls off to the side.


While this is more accurate than the first, it still isn't entirely accurate. Yes, beam angle refers to the size of the beam from the center out to where it is 50% the brightness of the center. With the same idea, field angle is the size of the beam from the center out to where it is 10% the brightness of the center. And, yes, field angle is what a fixture's name is referring to.

-Tim


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## carsonld (Aug 30, 2013)

So is the degree on a ellipsoidal how far the beam can go with it still looking right?


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## Les (Aug 30, 2013)

Not quite. In simple terms, it refers to how wide or narrow the beam is. While these do have implications as to how short or far your throw can be, one has little to do with the other. 

In the diagram below, each ellipsoidal is a different degree. Notice how each instrument is the same distance from the target, but the pool of light is smaller or larger. This is what the degree designations mean. A narrower degree (say, 19*) will have a smaller pool of light at 20' than a 50* unit would. 




As you can see, the Altman fixtures listed above went by the old system of listing the focal lengths to determine the angle. I wrote in the conversions for you.


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## JD (Aug 30, 2013)

TDN said:


> I don't think that's quite accurate- Beam angle is the area in which the light output is above 50 percent of it's maximum- the center of the beam. The field angle is the total width of the spread of the light. Field angle corresponds to the degree listed on the ellipsoidal. So for example, a 50 degree light will have all of its light spread in a 50 degree cone, but the beam angle inside is where the light is the strongest, and then falls off to the side.



In a properly working ERS that is hard focused, all three are the same!
Your beam should not have more than a 50% variance and spill light outside of your hard cut better be less than 10% our you should clean/bench focus/replace your ERS ! 

Beam/Field ratings are geared to soft focus/no focus fixtures such as fresnels, beam projectors, PARs, etc. where there is a decline in output as you mover further and further from 0 degrees.

(Yes, no matter how good an ERS is, there is always a beam drop variance. Better fixtures have less drop, a perfect fixture would have a perfectly smooth area with no drop. No such thing.)


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## techieman33 (Aug 30, 2013)

JD said:


> In a properly working ERS that is hard focused, all three are the same!
> Your beam should not have more than a 50% variance and spill light outside of your hard cut better be less than 10% our you should clean/bench focus/replace your ERS !
> 
> Beam/Field ratings are geared to soft focus/no focus fixtures such as fresnels, beam projectors, PARs, etc. where there is a decline in output as you mover further and further from 0 degrees.
> ...



An ERS with an LED source could be tuned to have 0 drop or very close to it. If I remember correctly ETC said exactly that, but they tuned it to have a hot spot and fall off at the edges for better blending of fixtures.


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## carsonld (Aug 30, 2013)

Okay, thank you so much!


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## BillConnerFASTC (Aug 31, 2013)

Beam and field angles were a concept long before ellipsoidals were designated in degrees and were known by the focal length of the lens. Also, while beam and field angles expected to be basically the same today, such was not always the case.


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## DELO72 (Sep 3, 2013)

carsonld said:


> What are these three things? I have noticed that all ellipsoidals are categorized into degrees, and then when I am looking online a LED fixtures that we are thinking about getting it will say the beam and/or field angle? What do these three things mean?



This should help. Buy the book!

A Practical Guide to Stage Lighting - Steven Louis Shelley - Google Books


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## derekleffew (Sep 3, 2013)

DELO72;291102[ "This should help. Buy the [autolink said:


> book[/autolink]!
> _ A Practical Guide to Stage Lighting_ by Steven Louis Shelley"


Well, since you brought it up, something about which I've been concerned for some time ...


> "field angle / 2 = beam angle°
> The *slinky formula* states that if the field angle of any instrument is known, dividing that number by two results in a rough number that can be used as an approximate beam angle [for the purpose of constructing a wash]."


The example in the book states that a 36° Source Four has an FA of 36° and a BA of 24. The RevG (most recent) cut sheet from the manufacturer's site states 34 and 27, quite far from the 2:1 ratio Shelley suggests.

My hypothesis is that Shelley's methodology is somewhat dated. Looking at a 1971 cut sheet for a Century 2321 (gray, radial, 6x9) lists BA 24°, FA 40°. I contend that over the years, as fixtures have optically improved, beam angle and field angle have become closer to one another, so much so that the LED Source Four offers a variety of diffusions and beam softening options specially to facilitate blending between fixtures. But most (experienced) designers instinctively know that if they have three fixtures to cover a 40' width, focus one at center and the other two 10'-15' L&R of CL, or some such.

Further, while Shelley's _slinky method_ of overlapping fields but not beams looks good on paper at first glance, it does not hold up under scrutiny or in actual practice. All this geometry reminds me of another quote (ten points to anyone who can cite the source).

> "The Method: (A) Divide the stage into a grid of twelve circular Areas, all neatly labeled with large numerals. ... The Results: (A) The holes between the Areas must be plugged up with added units, it all ending in an arbitrary patchwork.
> ...
> While the pros were laboring on subtle, minor variations inside their scenery boxes and the school children were making circles with their compasses, things were happening in musicals."


.


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