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    Rick36616
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Source 4 beam widths

I was reviewing the Source 4 documentation for Field and Beam widths.  I calculated the fields based on the data sheets and found something I don't understand.  Here's the info I extracted:

Act Degrees Multiplier
Nominal Field Beam Field Beam
5 7 6 0.12 0.10
10 10.8 8 0.19 0.14
14 14.85 12 0.26 0.21
19 18.1 15 0.31 0.26
26 25.8 18 0.45 0.31
36 35.7 27 0.61 0.47
50 55.5 37.9 0.93 0.65
70 90 69.5 1.41 1.14
90 140 110 1.88 1.64

The 5 degree, 70, and 90 don't calculate as expected.  The 5 degree seems to be really 7 degrees.  Ok, that's close enough I guess, about 1/3 bigger: .12 vs. .09 multiplier. 

The 70 degree looks to be more like a 90 degree instrument and the 90 degree seems to be a 140 degree instrument.  All the other data sheets check out with what I expect, but these two don't.  I found the data sheets linked here:  https://www.etcconnect.com/Products/Lighting-Fixtures/Source-Four/Documentation.aspx

The 90 deg data sheet says the field diameter at any distance, multiply by 1.88; for the beam diameter multiply by 1.63.  That works out to 140 degrees and 110 degrees.  The 70 deg data sheet say to multiply by 1.40 and 1.14 respectively.  That works out to be 90 degrees and 69.5 degrees respectively.  The other instruments match my calculations, but not these. 

The math is simple for the 90 degree case:  the beam radius will be the sine of 90/2=45 degrees which is sqrt(2)/2 or .707.  The diameter is double the radius or 1.414 or square root 2 times the distance. 

Are the numbers for these instruments really what is on the data sheet or did I miscalculate the angles?  Why the discrepancy?

  • Hello,

    I have asked our optics team to take a look at this and they have confirmed that the numbers appear to be correct.   When they calculated the values they come up with 69.8 for the 70 degree and 86 for the 90 degree.

    Here is the math they are using for this:

    From the datasheet you take half the field diameter divided by the throw distance and take the arc-tangent of that multiplied by 2(see below).

    Field Angle = 2*Arc-Tangent(Half of Field Diameter/Throw Distance)

    So for the 70 degree at a 4.6m throw (from the Datasheet):   Field Angle = 2*Arc-Tangent(3.2m/4.6m) = 69.6 degrees

    I hope this is helpful. 

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