I wanted to do some light measurements (illuminance, to be exact), but didn’t want to spend hundreds of euro’s on a light meter. I realized that I actually have a quite good light meter in my pocket: my smartphone’s camera. It doesn’t measure illuminance, but luminance, but that can be worked around.
When getting in to light measurement, you usually are overwhelmed by a load of new units of measurement. A very brief reminder:
- Luminous flux, measured in lumen: the amount of light (think Watt)
- Luminous intensity, measured in candela or Lumen per steradian: the amount of light in a particular direction. Lenses and reflectors can change the amount of luminous intensity (candela), but not the amount of luminous flux (lumen)
- Illuminance, measured in lux or lumen per square meter: the amount of light falling on to a surface
- Luminance, measured in nits or lumen per steradian per square meter: how bright a surface looks. Paint can change the luminance (amount of nits), not the illuminance (amount of lux).
A camera sensor registers the amount of light hitting the sensor while the shutter is open, the luminous exposure, measured in lux-seconds. Usually, this corresponds to the luminance (nits) of the thing you are photographing, which in turn depends its the color and the lighting (illuminance).
So in order to use a camera to measure the illuminance, we need to calibrate away the color.
I found the Galactica Luxmeter app for iOS. It turns the camera on, and converts the current shutter time and ISO (the iPhone has a fixed aperture) in to an exposure value. This EV is then converted to an illuminance measurement with a configurable factor, which allows you to “calibrate” the meter.
To calibrate away the color, I aim the camera at a piece of paper. I found that, contrary to the in-app help, I get more precise results if I make the paper full-frame, instead of only inside the circle. I also found out that the “direct light” measurement (using the front camera) doesn’t work at all.
To calibrate the conversion factor, I measured the illuminance with a professional light meter, and simply adjusted the factor until the reading matched (approximately). My iPhone 6 needed a coefficient of 60.
