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Conventional Fixtures Conversions- Candela to foot Candles

Discussion in 'Lighting and Electrics' started by Goph704, Jan 22, 2009.

  1. Goph704

    Goph704 Active Member

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    Well, this threw me off a bit.
    I spent this morning working on finding accurate Photometrics for the Strand SL series. The entire stock is Strand SL series conventional fixtures. I'm trying to figure out if it's worth the effort to replace the 19 degrees that are on my second cat walk with 26 degrees, and move the 19's down to my balcony rail. The space I'm working in is traditionally considered dark ( hard to see the actors) by several designers and as the new kid on the block I'm trying to right this wrong.
    Now that being said I found a highly useful web page Entitled the Stage lighting fixture tables version 2.3 which was created in 1999 by Bill Williams. Joy of joys there were all the SL units I needed, with the lamps that I'm using in my fixtures. Beam and field angles were noted and Candelas were given for each instrument. Bill was even nice enough to provide all of the conversions for me, which is lucky cause I haven't had to convert Candelas to foot candles in a while. So I began my conversions using the formula I thought was correct, which used the inverse square law to determine illumination.

    Foot candles= Candelas / distance from target ^2

    However very quickly something seemed wrong. But then I got the great Idea of checking Bill's information against the second edition of the photometrics hand book by Robert C. Mumm. I use his book almost religiously, but regrettably the Strand SL Series is not included in the edition I have. While I could not compare the SL series I could compare the S4rs in that edition, so I did.
    In Bill's sheet the 50 degree S4 had a Candle power of 32,000
    and In the Hand book the 50 degree S4 had a Candle power of 34,866
    At a distance of 20 feet that means that Bill had 80 fc while The photometrics hand book had 88 fc. I assumed that there could have been some rounding done, and what's 8 candles anyway. However i continued and found that the difference between the two 36 degree S4rs was 15, 929 Candelas and a difference of (172.5-212) 39.5 fc. that's a big difference considering that both groups are using the same lamps and shooting from the same distance
    Bill's information (while it is older) apparently came directly from Strand and ETC while I assume the photometrics handbook used a lumen-meter,but could have also used factory data. Regardless this information should be standard right? I've double checked myself a few times on this since I have a form of dyslexia involving numbers. Given all of the information and knowing that most mechanical ( and mathematical ) errors are due to operator error, I have to assume that I'm making a mistake somewhere, But where???
     
  2. xander

    xander Well-Known Member

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    You said that the website was much older than your handbook, manufaturers are constantly upgrading their equipment. Maybe S4s have actually gotten better between the two publishing dates. Better reflectors, better lenses, and wiggle room has to be given for the fact that they may not have been running on exactly the same voltage.

    I dunno,
    -tim
     
  3. derekleffew

    derekleffew Resident Curmudgeon Senior Team Premium Member

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    Be aware that much of the data contained in http://www.mts.net/~william5/library/lxspot11.htm is either outdated, incomplete, or just plain wrong. (Don't get me started on the 0.018 constant used in Table #101.)

    I can't speak about the Strand units, but ETC has published six versions of data for its SourceFour fixtures (current sheets are version F). I would go with the version found at http://www.strandlighting.com/clientuploads/directory/downloads/SL19_26_36_50_Data.pdf.

    The above formula should read:
    Foot candles= Beam Candle Power, expressed in Candelas / distance from target ^2

    Perhaps the Collaborative Article: http://www.controlbooth.com/forums/collaborative-articles/7664-mathematical-formulas-lighting.html may help you.

    What are the throw distances and angles of elevations, from the FOH2 and BalcRail positions to the plaster line at +5'?

    Attached is an Excel workbook which may be of assistance.
     

    Attached Files:

    Last edited: Jan 25, 2009
  4. Goph704

    Goph704 Active Member

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    Thanks Derek, I had hoped you would look at this. I'm looking over the collaborative article but it's something I'm going to really have to go over, but it's great to see that this site provides information like this

    In answer to your question about the catwalks the specs for this space are kind of rough, and for ease of use I've rounded everything to whole numbers, but here goes.

    Our stage is a proscenium five feet above the audience level. The stage pit sticks out a good 15 feet from the plaster line at Center and shrinks down to 10 feet on either side. The height of the catwalks is roughly 45 feet above audience level so i used 40 feet as my standard height for both my second and third Catwalks. The first cat walk is roughly 5 feet in font of the plaster line, the second cat is fifteen feet back (20' from plaster) from that and the third is another fifteen feet ( 35' from plaster) farther back.
    Using the theorem (a^2*B^2=C^2) I found out that my throw distances from each Cat walk at center would be
    First Cat - 40.3 feet Second Cat-45 feet Third Cat- 53 feet
    The first cat is almost unusable except as down light since the Proscenium arch blocks any front light . The second Cat is currently holding a majority of 26 and 36 degree instruments, and the third cat is mostly 19 degree instruments.
    Using the sheet you sent me it looks as though I should leave what's up there and just change gels. The 19 degree is the only instrument that will even reach up to 55 feet. I think I'm going to need at least 4 areas to cover the 50 foot width of the proscenium. Still I'm pushing all of these instruments to the outer limits of their Candle power.
    As for the balcony rail it's 15 feet off of the audience level and there for 10 feet above the deck. The rail is all so 55 feet from away from the plaster line and 40 feet from the front of the Pit. When I reapplied (a^2*B^2=C^2) I found the throw distance to be 56' feet to the plaster line, and 41 feet to the front of the pit. That's actually outside the range of a 19 degree, but I'm afraid that I don't have anything better to put there.
    So with all of this is there something I can be doing to increase the actual amount of light we're putting on the stage? This summer I cleaned and polished both lenses, and reflectors on the entire stock. I would welcome any ideas at this point, since buying new fixtures in this economy is out of the question.
    -Adam
     
  5. derekleffew

    derekleffew Resident Curmudgeon Senior Team Premium Member

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    NOT THAT I AM IN ANY WAY ADVOCATING, RECOMMENDING, OR SUGGESTING THIS, but I wonder what Strand's official stance on using a 750W lamp, such as the GLD, in the SL Coolbeam series is? GLD's 19,000 initial lumens vs. the GLA's 13,000 or GLC's 14,500. (Comparing Thorn to GE to GE.)

    It appears you really need at least ten 14° or even twelve 10° ERS units, or more.:) In the meantime, as you said, pay particular attention to the transmission values of colors you choose.
     
    Goph704 and (deleted member) like this.

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