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Green Screen Studio Modifications

We've found that we're needing to use a lot of green screen lately. The effort involved in taking all the kit to site and getting good results is considerable. Therefore we taken the route of a studio set up.

Having done some research, it would seem that best results are achieved when the green screen is lit -2 stops from the subject. However, even at this level there is a fair bit of bleed through on hair, the finer or lighter the hair, the more the effect. I'd read somewhere that an even backwash of magenta helps because it's the opposite side of the colour wheel to green and therefore helps the keying process.

There's no need for high CRI when background lighting the green screen, furthermore the magenta wash would be simply created using a filter gel. So we had a look around and found some Integral 1200 x 300 LED panels designed for office ceiling lighting. These were not dimmable, so our first test was to see how they responded to a constant current source and to determine the internal configuration. We removed the supplied ballast and connected some 4mm test sockets:

First Modification of Integral LED Panel

The tests showed that the fitting pulled about 450mA at 57V and that it was rather bright for our use. We found some dimmable ballasts by VADSBO that would deliver 350mA on two channels. These ballasts use 1-10V dimming signals and are convienently designed for control with a 10K linear potentiometer. Here's the enclosure we constructed for two ballasts (i.e. two pairs of channels)

LED Control Panel

A quick test showed that the panels worked, were very smooth to dim and could achieve smooth illumination across the green screen. However once we set up a camera, we found that there was profound flicker at all frame rates. We pulled out an Oscilloscope and found that the PWM (Pulse Width Modulation) was running at 250Hz. With a shutter angle of 180 Degrees we could see the possibility of getting 4 cycles on on frame and 6 cycles on another. Our solution was to do some quick ripple calculations. These gave a smoothing capacitor value around 450uF which we rounded up to 2200uF just in case:

Smoothing Capacitor in Integral fittings

A few more tested revealed that the fittings would benefit from being half way up the green screen frame, so it was off to the 3D printer to create various brackets. In the end the simplest approach worked best:

Smoothing Capacitor in Integral fittings
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WbAV productions a Berkshire based video productions house, Newbury, Berkshire, RG18 9JE