visible light - Why doesn't a lighter flame cast a shadow?

Imagine that you have a titanium screen. It's thin, so it's difficult to see. You can use a torch to heat some areas to incandescence. These areas become easily visible.

Now if we take a very bright light source and shine them at these lit areas, we won't see a shadow that gives us any information about the pattern of the light. Both the cool and hot areas interact with the bright light beam in the same way. The grid itself may be visible, but the pattern won't reflect anything about the relationship between the dark and light regions.

The flame is the same thing. It contains matter (such as fuel or soot particles), but similar matter is also present outside the flame. The imaging light may be slightly attenuated by these particles, but there is no sharp division in their location that corresponds to the shape of the flame. This lack of sharp division will prevent imaging.