Digital signage in the shop window: how bright does the display have to be?

A screen in the shop window does not compete with other screens, but with the sun. What looks brilliant in the shop is often a dark pane outside at eleven in the morning. In consultations clients ask us first about size – the more important number is brightness.

How bright does a window display have to be?

Brightness is given in candela per square metre, in retail usually as nits. A living room television delivers 250 to 400 cd/m², a signage display for interiors 500 to 700. Behind glass with daylight from the side you need at least 1,500 cd/m²; with direct sun on the pane 2,500 to 4,000. Anyone who wants to know precisely measures the illuminance on site with a lux meter: an overcast day brings around 10,000 lux onto the pane, full summer sun over 50,000. For the first value a device with 2,500 cd/m² is quite enough; for the second only a high-brightness display or an LED surface comes into question.

Reflections: the underestimated problem

Even a bright display loses against the reflection of the street if the eye focuses on the pane instead of the picture. Three measures help: an anti-reflective front glass on the device, a slight tilt of three to five degrees downwards and a dark, calm background in the window. Light decoration behind the screen makes it look flatter, not brighter.

Heat behind glass

Professional window displays are rated up to 50 °C ambient temperature and throttle themselves in heat. A television from the electronics store is specified up to about 35 °C. Keep at least ten centimetres from the pane, leave air above and below and avoid closed cladding. The black patches we often see after the first summer are heat damage – and no warranty covers them.

Power consumption and operating hours

A 55-inch high-brightness display draws 250 to 400 watts at full power, an indoor device about half. At 14 hours a day that comes to 1,500 to 2,000 kilowatt-hours a year. Two levers lower the figure: an ambient light sensor that adapts brightness to daylight, and a timer that switches the device off at night or dims it to 20 per cent. Both also extend the panel's life. Many municipalities – Solothurn and Bern too – regulate in their sign regulations when illuminated advertising has to be switched off; moving images by the road usually require a permit. Ask before installation.

Window film or display?

We see it every day: anyone who wants to address passers-by in daytime cannot get past the display. Anyone who wants to play the evening hours does well with film.

Conclusion

Brightness from 2,500 cd/m², air around the device, light sensor and timer – with that a window display works for years. We measure the light conditions on site and show how different brightness classes perform in daylight.