Manual entry of candlepower distribution data

Maybe trivial question. Would it be possible to build a luminaire in radiance only from data coming from a printed polar diagram (only candle power distribution without lumen or else)?
I see on the manual (pag.323) that for that purpose a .dat file, containing only candlepower data, is called by the .rad file, this last containing information about the material and the geometry of the lamp.
What is not clear to me is the RGB values specified in the light material. Where do those value come from (.0418 .0418 .0418)?

Thank you for your help!

Looks like no one responded to this. The RGB color typically compensates for the size of the source, but the math is a bit involved, and ies2rad.c is difficult to decipher. Charles Ehrlich wrote a helpful chapter in Rendering with Radiance, which has the linked section on converting photometry. Hopefully, you will find this helpful.

Hi Greg, thanks for your reply. While the useful section you mention is more related to conversion, my question was referring to the next section of the book (if I am not wrong), concerning the manual input when one has just the plot of the intensities of a luminaire. Apart from the RGB question, I was wondering if Radiance would need more than the candlepower to describe a luminaire (as I have not lumens or else) as in the section it seems so.

Thank you for your help.

Whoops – yes, it looks like I cut the section short of what you were asking. My apologies. Shall I upload the rest, or do you have a copy? (How else would you know?)

Candlepower is lumens/steradian, so if you divide by 179 lumens/watt to get watts/steradian, all that is left to do (for far-field photometry) is divide by the projected area of the light source.

Hi Greg, no prob it is great to get help so quickly when one is learning :slight_smile:
My problem is that I only have a graphical representation, a plot, of the candle power distribution of the luminaire I need to simulate, nothing else. So I was wondering if this would suffice to build a luminaire in Radiance.

If I got it correctly, ies2rad should be always used. When the ies file is not available, and this is case, it should be sufficient to create a ies file based on the plot of the lamp (as described in pag 98 of Rendering with Radiance).

As for the other values needed to build the ies fle (see pag 97 of Rendering with Radiance), I might miss only -lumens per lamp-. Would this be correct?

So in case I should find a calculator to obtain lumens from the candlepower distribution?

Thank you for your help.

PS: Yes I have a copy of the book :grinning:

If your photometric report is relative to lamp lumens, then yes, you would need to multiply the values you get from the plot by lamp lumens. I have not worked with this kind of data in decades… If your lamp distribution is symmetric about the axis, then you can use a simple 1-dimensional brightdata primitive. The lampcolor script is supposed to help for assigning the RGB value, but again, I have not used it in a very long time, so best to rely on the example in RwR rather than anything I say on the subject.

Thank you for your help.

as I mentioned you, the only thing I have is a plot of the polar curve of luminous intensity. My concern would be how to turn this into a Radiance source of light.

As I got it a ies file should be created. A ies file should look like this:

  Id  Description

  00  IESNA:LM-63-1995
  01  <Keyword 1>
  02  <Keyword 2>
  03  ...
  04  <Keyword n>
  05  TILT=<file-spec> or <INCLUDE> or <NONE>
  06  <lamp-to-luminaire geometry>
  07  <# of pairs of angles and multiplying factors>
  08  <angles>
  09  <multiplying factors>
  10  <# of lamps> <lumens per lamp> <candela multiplier>
      <# of vertical angles> <# of horizontal angles> <photometric type>
      <units type> <width> <length> <height>
  11  <ballast factor> <future use> <input watts>
  12  <vertical angles>
  13  <horizontal angles>
  14  <candela values for all vertical angles at first horizontal angle>
  15  <candela values for all vertical angles at second horizontal angle>
  16  ...
  17  <candela values for all vertical angles at nth horizontal angle>

Now I realize that ‘lumen per lamp’, ‘input watts’ and the geometric dimensions would be all missing. On the other side the source of light for which I have the polar diagrams is a simple flame (candle like). I thus wonder how one could plausibly render the missing data. Do you have any suggestion?
Thank you for your help!

Related to one of Greg’s “watch-it” comments about a photometric report being relative to lamp lumens - this is less common now, with LED data mostly being reported as “absolute” photometry not relative to lamp lumens. This is largely because so many LED products are really lots of tiny LEDs rather than one main traditional lamp.

If your graphical plot shows total candela from the source at each angle (doesn’t need any scaling by lumens), then enter the values into your invented IES file according to the plot, and enter a value of -1 for lumens. That is the marker to indicate it is an absolute test (not relative to lumens).

I don’t think the watts value would impact your Radiance definition. It think that’s only used in software like Dialux or AGI when they have lighting power density functions (maybe in things like Open Studio too?). I’d just put in a value that’s your best guess of the light source input wattage. If it’s an actual candle then I guess it’s not connected to power. I’m not sure but maybe the file format won’t complain if the value is a 0.

Thank you Christopher for your very useful remarks! I was also wondering about the ‘Luminous Opening Dimensions’. How would you define it for a light source as a candle or a similar device using a flame?

By the way, how would one define ‘luminous opening’ ? According to “Parsing The IESNA LM-63 Photometric Data File” by Ian Ashdown, the term ‘luminous opening’ is somewhat ambiguous for many architectural luminaires and other light sources…

Of particular note, for best accuracy you’d use the width length and height just of the light emitting part of the source. For example a candle would just indicate the size of the flame at the top, not the full candlestick height. You could create a separate geometry for the candlestick in your radiance scene if you needed something like that.

For the IES file format, your choices are either a point, flat ring or rectangle, box, sphere, cylinder, but apparently an extruded ellipse or an ellipsoid is an option also (which I had never come across). If you wanted to simplify it as a cylinder it would be something like 2 -0.005 0 0.01 for the units (2 is for meters), width, length, and height. Looking back at file format references, I guess ellipsoid is also an option, something like 2 -0.005 0.005 -0.01 but I haven’t experienced a data file with an ellipsoid shape. I’m not sure if it matters whether you put the negative sign on width or length if the distribution is axially symmetrical.

Thank you Christopher, very clear. I am sure it will help!