Originally Posted by jollyjake
My understanding of this is quite limited to what I've read on this topic, and isn't very definitive.
First, I will try answer your overall question. Degrees Lovibond, and more accurately used SRM (Standard Reference Method) of measuring the color of beer is not linear, but rather logrithmic. This is at the most basic level, mathematically. This means that doubling your Lovibond amount will not directly resemble a beer that is twice as dark.
Now, when reading about the color of beer, you won't take long to figure out it is the biggest PITA for brewers. There is no absolute way to measaure any color of beer. Lovibond used to be the standard, where you would take a set of cards, and compare them to the beer color. This leaves a lot of room for error. The thickness of the beer you look through, the distance from the card to the beer, and the light for observation need to be the same. In addition, I believe there is more than one set of Lovibond card systems. SRM is the measurement of the absorbance of a certain wavelength of light into the beer. The SRM value cannot accurately compare two similar colored beers, like amber, and light brown, but can compare golden hues to amber ones. The wavelengths of light from the color of the beer may be too similar for this type of measurement.
It is described well here:http://www.beersmith.com/blog/2008/04/29/beer-color-understanding-srm-lovibond-and-ebc/
This leads me to believe that the degree Lovibond is an aproximate value when maltsters figure it out.
Now, to relate it to the two references you listed. First of all, the one that claims degrees L is linear is a home brew store. They taylor to homebrewers, and probably don't have a great reference for that statement. Aproximate is almost always good enough for homebrewers. It's also true that the degree L changes (for a same product like Crystal 20L) between maltsters, and even bathces from the same maltster. So, if you take 1 lb 20L from one maltster, and 1 lb of 20L from another, there is a good chance you won't be at 40L. However, if, from the same batch, you add 1 lb of 20L, notice the color, and add an additional 1 lb of that same 20L, there is a good chance it will be close to twice that darkness in color. Now, that doesn't mean you're around 40L, because the scale is not linear.
I think from your references alone, I would ignore the home brew store, but in practicality, the aproximation would be good enough for a homebrewer.
Bottom line, the formula isn't linear.