LED lighting can create a different flicker effect than florescent or high intensity discharge lights (i.e. mercury vapor or high pressure sodium) because the driver module cuts the actual LEDs on and off very quickly with a near square wave light output pattern from the emitters which go from no to full output and back in nanoseconds. Most of our other artificial light sources provide some significant non-binary transition between on and off states where there is still significant light output over the entire time between peak and minimum; for example even when all excitation is removed you will see a brief glow from the phosphor of a florescent tube and the "off" period is far more brief during the operating cycle compared to what you see after turning the lamp off.
More complex LED arrays use a sophisticated driver module that pulses the LED junctions at a rate far faster than the human eye can perceive and the resultant duty cycle (percent of time it is on) provides the observed light level. But depending upon when the camera shutter is open the camera may see a different number of pulsed light events for each exposure because the shutter isn't synchronized to the driver module. The effective LED array light output is based upon duty cycle which depends upon the width (period of time) of the on pulse and the width of the off period. There are a nearly infinite number of combinations of on and off width and frequency leading to the same perceived light output but with very different captured image effects.
Think of taking a pair of equally timed long exposure images and firing a flash manually where one exposure has three flash events of equal intensity and the other has five so if one is correctly exposed the other will be either over or under exposed. With LED lighting you stand a good chance of having this happen especially with a more sophisticated lighting source that is controlled by an advanced PWM (pulse width modulation) controller instead of a simple line frequency fired module which tends to have a slower modulation rate with extended on time. You could simulate the time lag effect of other types of bulbs with the LED driver but then you would run into a problem of color shift because instead of being modulated between full brightness and off the LED would be driven in a more linear fashion producing some output most of the time but at a lesser intensity and resulting different color temperature given the way LEDs produce visible light.
Some arrays attempting to achieve very high specific light output will pulse the LEDs at a higher current than recommended by the manufacturer providing extreme brightness during the brief on event but protecting the LED from excessive heat by running it at a duty cycle that still provides reasonable, albeit shortened, LED life. These types are likely to be the most problematic for photographic purposes but none of them are going to advertise their method of achieving very high light output at a competitive price. A completely flicker free LED array would be DC driven with a 100% duty cycle but this is going to result in a bigger, heavier, and far more expensive array for the same light output so such a device would tend to be sold at a very high price to specialized users.
For a simple test of the camera simulate the scene using incandescent lighting and see if the over-exposure issue disappears. Incandescent lights powered by AC also have a slight flicker but because of the tremendous thermal lag of the superheated filament it is not visible to humans or a practical camera.
LED lighting has some huge advantages over other light sources but like most new technologies it brings additional problems and unwanted side-effects. One area receiving significant study now is the impact of this differnet flicker environent on human health and behavior including increased potential for epileptic events in children. As we learn more, LED driver protocol will change and some of those changes are going to have various impacts on photography that we will have to learn about and adapt.
Rodger
EOS 1DX M3, 1DX M2, 1DX, 5DS R, M6 Mark II, 1D M2, EOS 650 (film), many lenses, XF400 video
Lights don't "flicker" brighter. If they flicker, they flicker off! Call Canon 1 (800) 652-2666
You are getting an overexposure because you are getting more light. Not less light. That is where you need to concentrate your efforts.
