According to http://www.realbakingwithrose.com/2005/12/sugar.html,
“After harvesting, the plants are shredded and pressed to remove the juices. Insoluble matter is removed and water is added. This syrup is boiled in large steam evaporators. The substance that remains is crystallized in heated vacuum pans and the liquid, now called molasses, is separated from the crystals by spinning it in a centrifuge. At this stage the sugar is known as raw sugar and contains 3 percent impurities. The raw sugar crystals are washed with steam and are called turbinado sugar, which is 99 percent pure sucrose. Although turbinado closely resembles refined white sugar in sweetening ability and composition, it cannot always be substituted in recipes. Its moisture content varies considerably which, coupled with its molasses flavor and coarse granulation, can affect a recipe without careful adjustment.”
This very complete information doesn´t state clearly (al least, I didn’t understand it), if raw sugar can be a substite for light brwon sugar.,althoughiIt does explain that refined sugar cannot be substituted with turbinado sugar.