I don't have my tapes on hand at the moment, but 
I am 95% positive it's from the last couple 
episodes during his fight with the bird shinma.  
His posture clearly looks like he is doing 
something active and not just standing around 
talking to Miyu (besides I think he only takes 
his mask off to do that in the manga).  There 
are only three times in the series that I recall 
he takes it off to do any kind of fighting:

-the fight with the mask shinma towards the 
beginning.  But he is either wearing the 
nobleman clothes or in purple lighting.

-umi no hikari arc. But that's all in normal 
lighting, and your cel has a brighter than 
normal lighting tone to it.

-last fight with the bird shinma.  If you want 
more specific you'll need to watch the ep.  
Though I THINK this is from the very end of the 
fight, and the light is actually coming from 
his "finishing move" (ambiguity left for spoiler 
reasons. If you haven't seen to the end and want 
more details feel free to email me.)