What this red cross icon(error icon?) mean on a light? - Rendering

It means that you have 2 or more stationary lights in close proximity. Solution to this problem is switching the lights to static mode and leaving only one as stationary. This situation is caused by technical limitation, each stationary light reserve separate light map channel (at least this is the way i think about it), this enables it to change color and brightness without need to rebuild lighting unfortunately when multiple stationary lights overlap unreal is unable to do so. Lights marked with red cross are automatically switched to stationary mode you should do this by hand to remain good level look, mainly because stationary lights can cast dynamic shadows from dynamic objects like characters and physic. It is good idea to put them in key positions to take full benefit.

Goto FAMA to know more.

@Piroman,
Thanks for your detail explaining.

btw, do you know how to setup this type of lighting?:

Seems, UE4 doesn’t use emissive material to lighting the scene. So I have to use spot light instead.

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hmm, maybe this is only the way, I’ll try it later.
Thanks.
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Just use one spotlight and increase its radius and lenght until it encompasses the whole neon bank.
Simple and effective, and has an added bonus of giving you more accurate results with distance field raytraced shadows, since the penumbra is directly affected by the source size and lenght

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