i also back this question, since i can`t use planar light sources in sketchup.
It's not a bug but an optimization that we consider the light impact of tiny emissive surfaces only when they are directly visible.
Solution 1: Use lights instead of (or in addition to) very small emissive faces
Solution 2: Use larger emissive faces (Not feasible in your case i guess)
Ok, what is tiny? For example is a light tube of 60 cm x 2 cm tiny? Or an emitter in the size of a post card? (I hope there is a limit that can be adjusted so that tiny is anything below 1 cm. 
) And what isn't tiny?
I got an idea for my current air plane interior. The interior based on repeating segments (see at the linked photo from the web). There are light tubes at each segment. But instead hundred tubes I can insert 30m long tubes over all segements, one at each side for each usage - wall, storage, ceiling. But ... the light effect is still turned off if the light isn't direct visible. So, still I hope there is way to handle emitters.
Or is a light tube of 30m tiny, since the polygon mesh is using tiny polygons? New test - I converted my NURBS light tubes to low poly meshes. Now my tubes are a few 30m long polygons only - still no lighting of invisible emitters. No happy end? One more idea - independent polygon meshes can be joined to one mesh object. Bingo - the scene is full of light.
I'm glad to found a solution. So, what is tiny and could we adjust the limit for a more intuitive use? It's great to see that Enscape can handle a situation like this air plane interior. Only it's needed to get a better control of this monster. I wished I could post a rendering of the scene.
Thomas Willberger Thank you for taking time to answer all my questions yet.
(photo from the internet)
Hey Micha, Ive posted about this problem I had back then and I still hadn't find a solution. I am trying to understand what you mean by: independent polygon meshes can be joined to one mesh object, can you tell me more ?
There is no hard coded defines size. It is more like: as many objects are considered as possible, but the larger ones are more likely to be taken into account. By merging many small ones into one large object you are basically tricking this detection - which may result in slight performane drop but may work nicely
We'll definitely evaluate to allow users to force certain objects/material in reflections. That'd would resolve that issue and others (e.g. mirror scenes) as well.
We'll definitely evaluate to allow users to force certain objects/material in reflections. That'd would resolve that issue and others (e.g. mirror scenes) as well.
Sounds good. Can't wait for the enhancement. 
Thank you.
Any updates to this issue? These lighting features are really important for this software to be usable for many architectural interiors projects. Looking forward to any updates regarding lighting for sketchup...
+1 still having this issue with enscape turning off emitters that I need ON
May I ask, is this still an issue? Or was this corrected in a recent version I haven't installed yet? I'm on 3.4 and I seem to be having this issue.
We'll definitely evaluate to allow users to force certain objects/material in reflections. That'd would resolve that issue and others (e.g. mirror scenes) as well.
+1
