Contrast ratio

A scanner turns the camera image into brightness values and splits them into dark and light. Hue barely matters; lightness does. A navy code on white separates cleanly, while a mid-gray or a pale gold code on white leaves the decoder guessing, especially in dim light or with glare.

The WCAG contrast ratio, defined for text accessibility, is a handy way to put a number on two colors: (L1 + 0.05) ÷ (L2 + 0.05), where L1 and L2 are the relative luminance of the lighter and darker color. It is not part of the QR code standard. Formal print verification follows ISO/IEC 15415, which grades symbol contrast from measured reflectance, but for choosing colors on screen the WCAG ratio is a useful proxy.

There is no official minimum ratio for phone scanning, so treat any threshold as a rule of thumb and aim high: dark modules on a light background, as close to black on white as the brand allows. Common problems are pastel or metallic brand colors, gradients that fade toward the background, glossy or foil finishes that reflect light, and a transparent PNG dropped onto a dark slide.

Direction matters too. Light-on-dark (inverted) codes are read by many apps but not all, so a dark code on a light background is the safe choice. Red is a classic trap for laser and red-LED barcode scanners, which see red ink as light; phone cameras handle it better, but only when the red is dark enough. Whatever the colors, test the printed code under the lighting where it will hang.