Use graph editor for simulate neons flickering, curves don't match perfectly and negatives values of light absorbe light from the ambient I have an animation where I use the graph editor for animate the flickering effect of neon litghts inside a garage.
I used cylinders with emission shader for the neons tubes and area light for lighting the scene.
I did that because if I use only emission shader with high emission value I get to much noise, so I can use only area lights but if this are inside the glass of the lamps I get less noise but still too much, If I use only area lights positioned outside of the glass I get almost no noise but it looks not realistic because I don't have the detail of neon lights inside the glass. I'm rendering with low samples and branched path tracing, where the all neons are switched on I can get really good results (almost not noise and good details) with really low time rendering.
For the flickering effect I used graph editor with noise modifier on both (emission shader objects and area lights), by adding this I have two main problem:
1- Even if all the modifier settings values are the same, except for the strenght of light areas and the emission shaders value that have different values (for get less noise from the emission shader I setted it to just 10, for lighting well the scene I setted 40W the strenght value of area lights), at some frames I get the area light already switched on and the neon cylinders still switched off.
I played with the offset value of the noise modifier but if I fix one frame I broke another one.

2- Using noise modifier on the curves in some frames it reach negative value and this seems literaly absorbing lights from the ambient (I'm using Cycles), if I compared rendered frames where the light is exactly 0 the scene look dark but still lighted, on frames where it reach negatives values I get a completly dark patch.
I tried to change the blend type from &quot;replace&quot; to &quot;add&quot; and this don't let curves to reach negatives value but I get a completly different effect, they not seems like neons flickering but like soft lamps, in less words the interpoletions seems too softer.
You can add a Limits modifier to clamp the output to any range you want.
The limits modifier can be added from the N panel in the graph editor.