You should really read the manual I have on the dodecagon.nl How-To's&Manuals page: How-To Change Material Names. With that you can understand what happens when you change material names, even with multiple parts and materials.

In short;
in the obj file: change only the line usemtl room_1_1 into usemtl light
in the mtl file: change only the line newmtl room_1_1 into newmtl light

All the rest remains as is.