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???