Making Magnetic Fields Visible with Light Painting
To make otherwise invisible magnetic fields show up in photos, Chris Hill built a device that lets them “light paint” magnetism.
Humans struggle to intuitively understand anything that we can’t naturally perceive. Visible light is just a small range of frequencies in the electromagnetic (EM) spectrum that contains everything from radio waves to gamma rays, but we’re unable to perceive most of that spectrum without the aid of instruments. Magnetism, being an attribute of EM, is also imperceptible to us. We can’t see magnetic fields; we can only see their effects. To make otherwise invisible magnetic fields show up in photos, Chris Hill built a device that lets them “light paint” magnetism.
Light painting is a photography technique in which the camera’s shutter stays open for a long period of time to increase exposure. The dark areas of the frame won’t produce much exposure and won’t show up well in the photo, but anything bright will be very visible. If something bright moves while the shutter is still open, it will leave a paint stroke-like streak in the photo. Photographers use light painting to create interesting effects, but the technique also has practical applications. By tying the brightness of a moving light to data from a sensor, one can produce a light-painted visualization of that data. In this case, that data is the strength of magnetic fields.
There are several ways Hill could have implemented this idea. The method they chose was to light paint with a wearable device. That wearable consists of two parts: a finger piece and a forearm-mounted piece. The finger piece contains the sensor that picks up magnetic fields and LEDs for light painting. The forearm-mounted piece contains the power source and a development board to process the sensor data and control the LEDs. Hill 3D-printed the enclosures for both pieces.
An AAH002-02E Ultrahigh Sensitivity Analog Sensor Board detects the strength of magnetic fields at its location. This is a GMR (Giant Magnetoresistance) sensor paired with a differential amplifier. It can detect very weak magnetic fields and amplify the analog signal to something easier to work with. An Arduino Nano 33 BLE board in the forearm piece monitors the signal coming from that sensor. It then illuminates the LEDs on an Adafruit NeoPixel Stick according to the strength of the signal — higher signal strength means more of the LEDs turn on. The NeoPixel Stick resembles a bar graph, so this is very intuitive.
To create a magnetic field visualization, Hill straps the wearable device to their arm and finger. They then set their camera to leave the shutter open indefinitely for a long exposure. As they move their finger around an area, the LEDs light up in response to the magnetic fields and appear in the long exposure photo. After they’ve covered the whole area, Hill can close the camera’s shutter and it will pop out the light-painted photo with the magnetic field visualization.