set the incandescence to full white (just using the color picker) while you're working on things (for viewport previewing) then when you are done with the texture, plug it into the incandescence channel just like you would the diffuse. i usually stick with .png
i'm assuming you're using maya? if so, try just putting it into a Lambert shader with incandescence cranked all the way up while you work on the texture. Then if you need to render anything, just take your finished diffuse texture and plug it into the incandescence channel