The complaint
The screen was behind a shop window facing a busy street. The client said the picture disappeared between 11 a.m. and 3 p.m. They assumed the screen was too dim and asked for a 5000-nit model.
What we measured
We took a luminance meter to the glass at three times of day. Outside the window, the pavement in sun was reading about 20,000 nits. The screen face, measured through the glass, was 1,900 nits. The ratio was 1:10. No screen wins that, not at 5000 nits either.
The three real problems
- Reflection. The glass was standard float glass with 8 percent reflectance. At the angle of the afternoon sun it acted as a mirror. Anti-reflective coating or film would cut that to 1 to 2 percent.
- Contrast, not brightness. The content was a light-grey background with white text. At this ambient level, contrast matters more than absolute luminance. Dark background content would have read better at the same 1900 nits.
- Angle. The screen was turned 15 degrees into the shop, so the brightest part of the street reflected straight at the pavement viewer.
What we changed, in order of cost
First, content: switched to a dark background with high-contrast text. Free. Second, a matte anti-reflective film on the inside of the glass. About 40 dollars per square metre. Third, rotated the screen 10 degrees so the reflection went to the wall instead of the viewer. Fourth, only then did we raise brightness from 1900 to 3000 nits at midday using an ambient sensor.
The result
The client could read the screen from the pavement at 2 p.m. The final cost was about a fifth of replacing the screen with a 5000-nit model, and the electricity went up by less than 15 percent because the sensor only raises output for four hours.
The general rule for shop windows
If the screen is behind glass, deal with the glass first. If the glass cannot be changed, design dark content and accept a lower ambient. Buying more nits is the third option, not the first.