Why it beats reading by eye
When you compare a strip with the bottle by eye, your brain silently corrects for the light: under a warm bulb, pink looks different. Here the strip and the chart are in the same photo, in the same light, and the phone compares colours with numbers (in the CIELAB colour space, designed to measure differences the way the eye sees them). It finds which two chart squares your pad falls between and where, instead of just picking the closest one.
For a good photo
- Daylight, in the shade. No flash: glare misleads.
- Strip and chart at the same distance from the phone, flat and without shadows on them.
- Take the photo at the time the bottle says: colours keep changing afterwards.
- If the result says it does not match the chart, retake the photo or re-centre the circles on each square.
What it cannot do
It does not replace a good kit or a photometer, and it inherits your strip's accuracy: an expired or damp strip already gives the wrong colour. Each brand prints its own chart: if your bottle shows other values, change them under "The values on your bottle chart".