What you are seeing
The code works in bright daylight and fails after dark, or works under strong indoor light and fails in a dim corridor. Night scanning fails for physical reasons: the camera gets less light, the autofocus hunts, reflections from a nearby lamp can be worse than daylight glare, and a code printed in weak colours loses what little contrast it had. The destination is rarely the problem, because the scan itself never completes. The fix is to make the code readable in the worst light it will face, not in the best.
Likely causes
1.
Too little light for the camera — In dim light the camera raises its sensitivity, which adds noise and softens the module edges. Test: shine a phone torch on the code; if it scans with extra light, low light is the cause.
2.
Weak contrast that daylight masked — Bright daylight can carry a low-contrast code that dim light cannot, because the camera has less signal to work with. Test: compare a black-on-white proof with the original in the same dim light.
3.
Glare from a nearby lamp — A single bright lamp at night creates a hard reflection that daylight spreads more evenly. Test: shade the code or change the viewing angle; if it scans, glare is the cause.
4.
Code too small for dim-light focusing — Small modules need good light and a steady focus, both of which are harder at night. Test: step closer in the dark; if it scans at close range, size and light together are the cause.
5.
Reflective or glossy surface — Glossy lamination and glass covers reflect a nearby lamp straight into the camera at night. Test: view the code from a slight angle so the reflection moves off the symbol.
Step-by-step fix
1. Test the code at night in the exact spot where it will be used, on two phones.
2. Enlarge the code so the modules stay resolvable in dim light.
3. Switch to dark modules on a plain light background with the strongest contrast you can use.
4. Move the code away from direct lamps and glossy reflections, or switch to matte stock.
5. Add lighting near the code, or move the code to a spot that already has steady light.
6. Retest at night after every change, and keep the version that passes in the dark.
Putting this into practice
In practice, daylight is the easy case and night is the real test. A code that only passes in daylight will fail exactly when visitors need it most, such as an evening event or a night entrance. Always run the night test in the real location, because a corridor, a car park, and a street frontage each fail in different ways. Keep a short note of what you tested, what failed, and what you changed, so the next person starts from evidence rather than guesswork. A fix that is written down survives staff changes and reprints; a fix that lives only in memory gets repeated.
Common mistakes to avoid
- Testing a night placement under bright office light and calling it fixed.
- Reprinting the full run before a single proof passes, which turns one failure into many.
- Testing on one phone in ideal conditions instead of two phones in the real conditions where a code that scans in daylight but not at night will be used.
- Changing the artwork and the placement at the same time, so you cannot tell which change fixed a code that scans in daylight but not at night.- Skipping the proof test because the change looked small, when small changes cause most repeat failures.
Quick checklist
- The code passes a test in the real night location
- Contrast is strong with dark modules on a light background
- No lamp reflection falls across the symbol at night
- The corrected proof passes on two phones in the real conditions
- The fix is recorded in the job file so the next reprint starts from a proven setup
- Every printed copy is checked against the approved proof
- The proof is retested after any later change to the link, design, or placement
Related tools and guides
- [QR minimum size checker](/qr-minimum-size-checker)
- [QR contrast checker](/qr-contrast-checker)
- [QR print proof checklist](/qr-print-proof-checklist)
- [QR DPI calculator](/qr-dpi-calculator)
- [QR quiet zone calculator](/qr-quiet-zone-calculator)
- [printed QR tester](/printed-qr-tester)
- [QR size calculator](/qr-size-calculator)
