Interval Exposure Light Painting

At first I wanted to create a better way to capture fire in long exposures. The problem with fire in long exposures is that it usually gets burned out on the sensor. Fire has a lot of dark and bright details but they move around all the time. So I was looking for a way to prevent the bright details from burning out the dark details.

In the back of my head I had an article about the effect a rolling shutter created on fast movements and I thought this could be a good solution. But you would have to open and close the lens very fast, so putting someone next to the camera with a piece of cardboard was out of the question.

I designed a few discs with holes in them so once you let them rotate in front of the lens this would interrupt the exposure in intervals.

After the first experiments you can see in the example photo I soon realized that the wanted effect on fire could be achieved but the whole concept had the potential to do amazing things with any light source because it gives the light source an effect as if it had a strobe mode turned on.

I took a geared 12V motor (around 720 RPM) with a PWM controller to adjust the speed and put this in a printed box. The box was easily mounted on a tripod using a 1/4" to 3/8" adapter screwed into the bottom of the box and the axis of the motor left blank to be able to put different discs on it.

The great question is “how can someone design such a tool”. I have the advantage of using a 3D printer for my tool designs, so I was able to design and print this pretty easy.

If you don’t have a 3D printer at your disposal you can use black acrylic to cut out the discs (or any other material that can be cut in stable discs of this size) just make sure the discs are balanced or they create a lot of wobble if spun fast. As a connector to the motor axis you can use spacers for linear shafts used in mechanics. They are rings with small screws that can be attached to a shaft like the one on the motor. Simply glue one of these rings to the disc in the exact center.

After a few experiments I realized that a few different disc designs were very useful for different effects.
If you create a disc that has evenly distributed open and closed segments you half the exposure which gives any light source a strobe look. The speed and amount of open and closed segments controls how strong this effect is. You can see this effect most commonly in strobed flashlights or those dimmed with PWM. But you can apply this effect to any light source! No matter if it is fire, sparklers, a lamp or car trails.Keep in mind that the amount of openings only influences the speed of the interval-effect. If you have a large variance in motor speed you just need one disc of this type.

If you use a disc that is mostly closed and only has small slits in it you get an effect that can be compared to shooting movement with a strobe flash. Because of the fast and short exposures any light source or lit object will appear relatively static on the photo. But keep in mind that even with small slits you will get a certain amount of movement blur depending on the speed of movement. Any light source (torches, sparklers, etc.) appears as if they are switched on and of real fast.
The amount of open segments in this type of disc influences the frequency of exposures. If you use wider segments you get more movement blur, if you use narrower segments you get sharper photo parts.

Keep in mind that you have to create the open segments with an edge along a line from the center of the disc to its circumference. This is because the further you go from the center the faster a point on a disc rotates. So if the open segments are wider at the outside the image gets exposed correctly everywhere. If the segment was the same width along its length the outer side of the photo would get less exposure then the inner side.

The most important thing to keep in mind is that the exposure is interrupted. This means that you increase your exposure time depending on how many open segments your disc has. If your disc has an even distribution of open and closed segments the exposure is halved. So if you would need 5 seconds for a scene to be lit correctly you will need 10 with the disc. With discs that have small slits this effect is even stronger. I use a disc that has two 10° slits, so only 20° of the whole 360° are open. This means that only 1/18 of the disc is open, which means the exposure time is multiplied by 18. One second exposure would then take 18 seconds.

You can combine this with normal techniques. So you could light a scene normally then cover the lens and put the disc in front of the lens and let it rotate, then remove the cover to create intervalled light streaks. Also keep in mind that there will be a small gap between the lens and the disc, so any light that hits the disc from behind can be reflected into the lens and create flares and strange lights in your photo.

In the photo that can be seen in this step you can see stray light in the top right corner where the spinning disc caught some light from some fellow lightpainters playing around behind me while i took this shot. The sparkles around the stone were made by tracing the edges with a sparkler while the disc was spinning in front of the lens. Then i removed the spinning disc and did the figure on the stone using EL wires.

I have added a small image of the disc used in all the shots of this instructions, so you can get an idea which disc gives what kind of effect.

These examples show some experiments with the finished tool. Almost any kind of light can be used and most of the time will produce quite astonishing results.

The examples are:

Car trails shot from a bridge

A pair of dancers I followed with a flashlight

Burning steel wool swung in a circle

EL wire (the usual blink frequency of most controllers is much slower)

Acrylic glas cut in a round form and lit from one edge and EL wire

Black fiber optics brush moved up and down

Here are the stl files to print the box and the discs. The discs are separated to be printable but need at least 300x200 print area.

The slit disc right is actually a little wrong, it should have been mirrored. I realized this after i printed and glued them together, but left them as they are because this creates a slight irregularity that adds character and makes the pattern look less artificial.