Can someone help me understand the Lovibond number that is listed on Rogue's bottles? Each beer lists the color in degrees Lovibond. I thought degrees Lovibond was supposed to be similar to the SRM number. However, when I look at some of the beers listed on Rogue's website, this doesn't make sense.
The SRM and Lovibond systems are entirely different in concept. The SRM system was invented to get around the shortcomings of the Lovibond system but was scaled so that the two scales approximately corresponded up to say SRM 7 or 8.
In the Lovibond system the beer is put into a cuvet of standard width and examined by northern daylight. It's color is visually compared to a series of colored glass slides made by the Tintometer Company (the manufacturers of the comparator devices). As has been known for years the colors of the glasses do not particularly well match the colors of beers which makes the visual comparisons difficult. Nevertheless, if one takes the transmission data for the Lovibond glasses, computed their colors in Lab space and then finds the closest actual beer color in that space (it has to be the closest average beer color) and then backs out the relationship between that average beer color and SRM one can convert between the two scales and the agreement is pretty good for light beers.
The modern Tintometer is a photometer and, as such, is capable of computing beer color in Lab (or any other CIE derived space) and the people who make it are capable of formulating a mapping between LAB and SRM/EBC as I have described above but only they, apparently, know what that mapping is and they aren't talking. At least they won't talk to me (ignored e-mail requests for this data). A search of the internet reveals an extensive table of EBC vs Lovibond on the Weyermann website but it does not correspond to the closest point in Lab space approach.
For example, the Shakespeare Stout is listed at 135 degrees Lovibond. As I understand SRM, once you get up to about 35-40 SRM it is black. How is Rogue measuring something that is comparitively off the chart?
SRM, EBC and Lovibond colors are all measured by transmitted light. Pour a couple of mm of 'black' beer into the bottom of a glass and shine a flashlight through it. What color is it?
The point is that there is no upper end to any of these color scales. If the beer is too dark to measure in a 1 cm cuvet one simply goes to a half cm cuvet and doubles the color reading. If it's still too dark in a half cm cuvet use a 2 mm cuvet and multiply the color read by 5. This is Lambert's law which says that the log absorption of light is linearly dependent on the path.
One can also use Beer's law (and this is, in fact, what both ASBC and EBC recommend) in which the color of the beer, diluted n:1, with DI water is multiplied by (n+1) - e.g. if the beer diluted with 4 parts DI water measures 10 SRM the undiluted beer's SRM is 50. Same for EBC or °L measurements.
Basically, the higher the number, the darker the beer. Its a flawed system since it only tells about light/dark. I'd go on, but thats another thread...
It is flawed in that it is possible to have two beers measure the same in Lovibond, SRM or EBC which are visibly of different color. If one takes the locus of average beer colors in Lab space and plots the colors of the Lovibond glasses on that same plot (same illuminant, path, observer) they do not lie on the beer locus. Any beer color which lies along a perpendicular to that locus and which contains the point corresponding to a particular Lovibond glass will be best matched by that glass's number and will have the same rating. But these systems do not measure only lightness or darkness because beer colors are quite restricted. One can almost say that the absorption spectra of all beers, when normalized by their absorptions at 430 nm, are the same. Were that true then just the SRM or EBC would constitute a complete specification of visible beer color (for a particular illuminant path and observer). The fact that it is not quite a true statement is responsible for the fact that beers of different color can have the same SRM (or EBC or Lovibond) rating. The near truth allows us, however, to postulate the existence of what I call Beer Coloring Substance and differentiate between beers approximate colors by specifying an effective concentration of BCS in each beer through the SRM or EBC numbers (scaled absorption at 430 nm). Small color differences can be encoded by what I call Spectral Deviation Coefficients which measure efficiently the way in which an actual beer's normalized spectrum differs from that of BCS.
As a practical matter, brewers are stuck with EBC and SRM measurements unless or until they accept the recording of the full absorption spectrum of the beer or some encoding thereof (SRM/EBC + SDC's) and I don't see that happening soon. °L is considered an archaic system used today only by US Maltsters to describe the colors of Congress mashes made from their specialty grains. I guess we have to include Rogue in that group. Note that the modern Tintometer, while it does read out °L also reads in EBC and SRM units.
