Photon: The DIY Incident Light Meter
“tl;dr
We made a light meter from scratch, called it Photon and shared all the info on how we made it.
It has a mechanical keyboard switch for added satisfaction. We tested it, and made a video about it. ”
Incident Light Meters
An incident light meter measures the light falling on your subject, not the light bouncing off it. Point it at your subject, press the button, and it tells you the right shutter speed and aperture for a given ISO. Simple in concept, but surprisingly hard to find cheaply.
This matters because the reflective meters built into every modern camera have a blind spot: they assume the scene averages out to 18% grey. A bride in white dress, a skier on snow, a dark-suited speaker on a dim stage - all fool the camera's meter. An incident reading bypasses that problem entirely.
Why Film Photographers Need One
For digital shooters an incident meter is a nice-to-have. For film it's close to essential. Many classic cameras (the kind you can pick up for next to nothing) have no built-in metering at all. Nail the exposure on film and you get that latitude and tonal quality that keeps people coming back to analogue. Miss it and you've spent real money on a roll that goes in the bin.
Beyond the technical argument, film shooting forces a deliberate pace that digital can't quite replicate. You get 24 or 36 frames, not unlimited. That constraint (and the seemingly endless wait for the lab) is half the pleasure.
Why We Built Photon
Decent incident meters from Sekonic or Lumu cost £150 or more. We suspected a Raspberry Pi Pico and an appropriate light sensor could do the same job for a fraction of that, so we found out.
Sensor BH1750 ambient light sensor (I²C)
Microcontroller Raspberry Pi Pico
Display Small OLED, shows EV / shutter / aperture
Input Mechanical keyboard switch (because why not)
Diffuser (optional) 3D - printed hemisphere, Lambertian surface
Total cost approx £20 - £30 in parts
The mechanical switch was a deliberate choice. Tools you enjoy using get used; a tactile click every time you take a reading makes the whole process feel considered rather than perfunctory.
Does it Work?
We tested Photon against a calibrated Sekonic L-308 across a range of lighting conditions: overcast outdoor, direct sun, tungsten studio, and dim interior. Readings matched within a third of a stop in all but the extreme low-light cases. For film work, that's more than good enough. The video below shows the thinking behind the build as well as some prints we got while using it.
Build Your Own
All the MicroPython code, wiring diagram, and STL for the 3D-printed enclosure are in our github repository. The build assumes basic soldering ability and access to a 3D printer (or a friend with one). The README walks through the full assembly.
Footnote: What About Version 2?
We get a lot of questions about a second version of Photon that uses a dedicated flash sensor. The short answer: it's coming, but we're taking our time to get it right. The sensor hardware is already built and tested and we're folding everything we learned from v1 into the design - the goal is a meaningfully better device, not just an incremental update.
We don't have a concrete timeline yet, so our honest advice is to build v1 now. It's a capable meter and we're committed to making sure there's a straightforward upgrade path from v1 to v2 when it arrives - you won't be starting from scratch.
