Light Writing Science

Last time I was left alone in a dark hotel room with nothing more than a camera and a flashlight, things got a little…shall we say…experimental. Don’t worry. It was perfectly G-rated and a tiny bit wizardly.

It’s called “light writing”.

Drawing with Reflections

Dark Room Experiment

When you take a photo under normal lighting conditions, the camera sensor is simply picking up the light bouncing off your subject.

But what would happen when there’s minimal or no ambient lighting? If you took a 10-second exposure in a completely dark room, dollars to donuts — what a strange saying — you’re gonna get something like this:

Re-enactment of a 10-second exposure in a dark room

Now what if you took another 10-second exposure, but at the 5-second mark, quickly flicker a flashlight pointed toward the center of the camera sensor? The flashlight wasn’t turned on at the start of the exposure and definitely not at the end…so it’ll probably just turn out the same, right? Wrong. It should look more like this:

Re-enactment of the flicker of a flashlight

Light Science

Like traffic streaks and star trails, light writing takes advantage of the simple fact that the camera sensors are picking up the brightest pixels throughout the duration of an exposure.

For the purpose of this brief explanation, imagine your camera sensor as just a grid of HSL values. And while I assume RAW camera data is stored in a much more complex fashion, this is a nice and simple way to think of it. Plus, ya know, the Internet is too far away for me to investigate further. 🙂

Similar to RGB values, HSL is a common way to represent colors, but rather than using intensity values of red, green, and blue, it uses hue, saturation and lightness. It’s a bit confusing because HSL (lightness), HSV/HSB (value/brightness), and HSI (intensity) are often used interchangeably, but traditionally, for HSL, 100% “lightness” would be pure light (ie. white) and 0% “lightness” would, of course, be pure darkness (ie. black). The camera sensor will essentially “save” the HSL value with the greatest “lightness” throughout the duration of a long exposure.

HSL Cylinder Representation

Interestingly, I’m now kinda wondering if you take the example above a step further and shoot another 10-second exposure in a dark room. If I flicker a DARK purple LED (say, 30% lightness) at the 4-second mark, and then a LIGHT urple LED (same hue/saturation, but, say, 60% lightness) in the exact same spot at the 8-second mark, the camera sensor should pickup the brightest pixel which, in this case, would be light urple. Is this true? No idea, didn’t try it. But I’m definitely a strong “maybe”. 🙂

Light Writing

My point is, these are rambling basics of light writing. All you really need is a light source — be it a small LED light or a tennis ball doused in lighter fluid — and motion.

Simple Tools

The very first time I tried light writing was with my nieces and nephew last Christmas. I didn’t have any lighter fluid on me, so we decided to try it with LEDs. Close quarters…flammable children…I think we made the right choice.

It was Christmas time, so for obvious reasons, we wrote the word “poo”.

Very First Attempt at Light Writing

Comparison of Light Writing Techniques

After the “poo” incident and then the “lonely hotel room” geek out, I recently had a chance to try this again. Only this time, I thought I’d describe some of the basics for anyone who’s interested in trying it themselves.

Reference Picture. Below is a 10-second under-exposed photo of my unwashed 10-year-old Canadian Tire patio furniture atop the questionably-load-bearing deck, without any additional lighting:

Chair with No Additional Lighting

Light Painting. Using a technique commonly referred to as “light painting”, I take the same photo but, this time, swirl the flashlight around my chair throughout the 10-second exposure. Obviously, this is a very basic example, but notice how you can easily highlight specific areas of your photo, like a directed flash. There are a lot of very interesting effects you can achieve using this technique, using different types and colours of lights.

Chair with Small Maglight Painting

Light Tracing. Rather than “paint” the object, try using the flashlight to draw an outline around the object. I don’t know what this is actually called, but I’m gonna go with “light tracing”. 🙂

Sorry, this is kind of a sloppy example, but you get the gist. It’s a 20-second exposure with me standing behind the chair with the flashlight. You don’t actually see me because I’m not standing still long enough for the camera to detect light bouncing off of my person.

Chair with Small Maglight Tracing

Light…oh, I dunno…Scribbling? I’m not really sure what I did before eBay, but I now have this bad habit of buying weird stuff for photography experiments when I’m sitting bored on the bus…Like a $3 fiber optic night light!! Why? Try using one of those to scribble an outline around the same chair! FIRE!!!

Chair with Fibre Optic Scribbling

Try It!

This is just the tip of the iceberg of the crazy wizardry you can do! Google “light writing” images and you’ll find ridiculously amazing effects from wicked-talented photographers!! In the meantime, here are a couple more photos from my earlier experiments:

Chair on Fire!

Early Attempt at Tracing a Chair
Turned out to be quite difficult in a pitch black room.. 🙂

Early Attempt at Tracing a Floor Lamp

Optical Fiber in Action!


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