I’ve sometimes noticed that when the lights slowly dim in a theater–not the house lights, but the “cans” that illuminate the stage–there appears to be a slight bump up in the light level just before they go dark completely. Does this have something to with the way these lights are wired, or could it be that my pupils are opening up in response to the lower light level, and so the light briefly seems brighter?
I should add that I haven’t seen a lot of theater in recent years, so my experience of this predates LED and CFL lighting; if what I saw was a product of something happening with the light itself, is the phenomenon specific to incandescent bulbs?
This might relate to an effect I notice.
If a source of light on the edge of my vision. That I have ceased to consciously pay attention to, or see, turns off. I see it turn off. Odd. I did not consciously see it. But my brain seems to notice and give me a little instant replay of the event.
You are probably not paying attention to the light level. So maybe a change in it, focuses attention and gives a seeming increase in the brightness for a moment as your brain sort of highlights the change for you.
Or the projectionist bobbled the light slider control a bit.
Back when I was doing theater lighting, the most common type of dimmer was the autotransformer (electronic controls were just starting to make their way down to the community theatre level). Unlike the older resistor type, which restricts current, an autotransformer controls the voltage level from 0% to 100%. But while in theory it’s a smooth curve, in reality it very frequently isn’t.
Another factor is the construction: basically, an autotransformer works by moving a brush over a coil, and the point at which the brush contacts the coil determines the voltage delivered (that’s overly simplified, but hopefully you get the idea). Brushes wear out and coils develop flat spots; in addition, dust and other contaminants can build up on the coil — light booths are not exactly the JPL clean room — which can cause further inconsistencies. And since variations in light levels are more noticeable when the lights are dim, one would tend to notice them more.
Quickie vocabulary - in stagecraft, a lamp is what most people would call a bulb, and a fixture or instrument is what people call a lamp. Confused yet?
My WAG is the preheat parameters aren’t set quite right for those instruments.
Preheat is simply running just enough of a “keep warm” power level so the lamps don’t go completely cold, which allows them to start making light faster when turned on and more importantly minimizes inrush current into a cold filament, prolonging the lamp’s life.
Your description makes it sound like the preheat is set too high or its timing is off so there’s a momentary “tug of war” between the “fade to black” power curve which may also be incorrect (LEDs, incandescents, etc. all have different voltage/current vs light output responses) and the preheat function activating.