# Source Four Photometrics

Discussion in 'Lighting and Electrics' started by hemismith, Nov 13, 2010.

1. ### hemismithMember

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Although many years ago I was pretty good at physics, I have not been able to get my arms around all the light terms. But in particular I'm confused by some of the Source Four photometrics. I've attached a table. I don't understand why in one case the Candela multiplying factor, Lumen MF, and ratio of Initial Lumens are all the same, but in most cases they are not. Ultimately I just want to make sure I am calculating the illuminance correctly.

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2. ### shibenWell-Known Member

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Have you looked at the ones posted online and shipped with the instruments? There is a factor on there for multiplying the lumens by a factor times feet or something of the sort to get lumens at any given distance... I think. Is that what your looking at?
3. ### hemismithMember

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Yes, that is where I got the information. Everything in the table except the "Ratio" column, which is the ratio of the Initial Lumens of the given lamp to the lamp that is used for the main specs. The issue I have is the specs don't make a lot of sense to me. For example, the 2nd and 3rd lamps have almost the same Initial Lumens, but they have different multiplying factors and yield quite different illuminence. And I don't understand why the candela and lumen multiplying factors are different (but not consistently different). Now, one thing I understand is that Initial Lumens is measured when the lamp is brand new. Perhaps the other numbers are based on averages over the life of the lamp, and perhaps the high output lamps keep their intensity better than the long life lamps?
Last edited: Nov 13, 2010
4. ### hemismithMember

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I see that on the 19 degree spec sheet, the Lumen and Candela multiplying factors are identical to the 26 degree except for the Candela factor on the 3rd lamp. On the 36 degree, both factors for the 3rd lamp are different still; the others are the same.

Is there a way I can convert my PAR64 specs to compare to this? It's listed at 110000 Cp and 6500 Lumens.
Last edited: Nov 14, 2010
5. ### derekleffewSenior TeamSenior TeamPremium Member

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Cp/candle-power, BCP/Beam Candle Power, and candelas all mean the same thing. For your PAR64 lamp, illumination in FC at a distance d is 110,000/d^2.

You can pretty much ignore lumens, except when it comes to comparing lamps in a given fixture. Then use the lumen MF or initial lumens to find the candela for lamps other than the one listed. Most fixture manufacturers highlight performance with the brightest lamp available, and give a correction factor for other lamps.
6. ### hemismithMember

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Thanks very much. I did some more reading as well and it's starting to become clearer. I thought I had to use some correction factor on the PAR CBCP as the lumen value I saw on another site (not GE's) didn't make sense, but I'll ignore that. I also called ETC and an engineer is looking into the inconsistency of the multiplying factors. So here's what I get (wattage, candela) after using the MFs:

19 degree
Source Four HO 575 206,992
Source Four LL 575 136,371
PAR64 NSP 500 110,000

36 degree
Source Four HO 575 60,893
Source Four LL 575 50,896
Source 4 Jr. HO 575 42,490
Source 4 Jr. LL 575 27,194
PAR64 MFL 500 37,000

They advertise that the 575W lamp is brighter than a 1000W in other fixtures, which certainly doesn't seem accurate, although it's close for the HO lamps in the true S4. But when you consider the PAR bulb is rated at 2000 hours and compare that to the HPL lamp that's also rated at 2000 hours, the S4 Jr. is actually worse. The PAR MFL is actually listed as 35 degree though, so that makes a little difference. So I'm wondering how bright these things will actually be compared to my PARs. I was hoping to use the long life lamps and the Jr. for 26 deg and up, but it looks like the real S4 with the high output lamp is the way to go.