How to Choose Brightness for Window-Facing Digital Signage

Short answer: choose brightness for a window-facing screen by working backwards from the light that reaches the glass, not by buying the largest number on the datasheet. Measure the illuminance the window receives across a working day, decide the ambient contrast the content needs, then select from the sunlight-readable band – typically 1,000-2,500 cd/m² in manufacturer-published figures – and spend the rest of the budget on reflection control and dimming. Direct sun can exceed 100,000 lux, which is why a normal indoor screen behind glass looks acceptable at 9 a.m. and unreadable at 3 p.m.

Why a higher nits number does not guarantee a readable window screen

Brightness is measured in candelas per square metre (cd/m²), which the industry calls nits. It describes how much light the panel emits. It says nothing about how much light comes back off the front surface of the screen, and in a shop window that reflected light is often the dominant term. Two displays with the same peak brightness can perform completely differently ten metres apart on the same street.

Contrast is a ratio, and reflection sits inside it

Contrast ratio compares the brightest part of an image with the darkest part. In a dark room the darkest part is close to zero, so the published figure looks impressive. In a real shop window, daylight reflected from the front glass, the cover glass and any air gap is added to every pixel, including the black ones, and the measured contrast collapses far below the laboratory number. Published contrast figures should never be compared across brands without knowing the measurement setup.

Ambient contrast ratio is the number to design for

Industrial display engineers use ambient contrast ratio (ACR): the contrast actually achieved with a defined level of light falling on the screen. It combines panel output with surface reflectance, so it predicts what a viewer sees rather than what a laboratory sees. ACR is why a 1,500 cd/m² screen with 1-2% surface reflectance can beat a 2,500 cd/m² screen with 6% reflectance in the same window, and why “brighter is better” is bad specification practice. AVIXA V201.01:2021 takes the same position from the standards side: it grades contrast by what the viewer needs to see in the content, and requires measurement in the typical viewing environment including ambient light.

Step 1 – Measure the light at the glass before you specify anything

A phone light-meter app is not a laboratory instrument, but it separates a shaded arcade from a west-facing facade, and that separation is what changes the purchase order. Take readings at the height and position where the screen will sit, not in the middle of the shop floor. Record three conditions: midday on a clear day, an overcast afternoon, and after dark. Note whether direct sun ever falls on the glass, and for how many hours.

• Direct sun on glass: readings in the tens of thousands of lux; plan for reflection control, not just output.
• Bright but indirect: the window acts like a large softbox; contrast suffers less, but veiling reflection from shop lighting still matters.
• Deep reveal or canopy: the glass sits in shade most of the day, so required output drops and the money is better spent on uniformity.

Step 2 – Match the window condition to a published brightness band

The table starts from typical brightness bands published by display manufacturers. Treat them as starting points for a discussion, not as guarantees, and adjust with the measurements from Step 1. The third column is where projects usually get into trouble.

Window condition	Published band to start from	What usually moves the decision
Interior wall, no daylight, controlled lighting	250-400 cd/m² (typical commercial indoor)	Colour accuracy and uniformity matter more than output; a bright screen in a dim room causes glare complaints
Behind glass, indirect daylight, canopy or deep reveal	500-800 cd/m² (typical industrial grade)	Reflections from shop lighting and from the street; glass layers multiply the problem
Behind glass with several hours of direct sun	1,000-2,500 cd/m² (sunlight-readable band)	Orientation, number of glass panes, coating, and whether the screen can be dimmed at night
Exposed facade, no glass in front, sun and rain	Upper end of the sunlight-readable band, with bonded front glass	Thermal headroom, IP rating and maintenance access become the limiting factors, not nits

The bands overlap on purpose. High-brightness LCD lines already cover 2,500 to 4,500 cd/m² across 32 to 86 inches, so 2,500 cd/m² is a mainstream production level rather than an exotic maximum. The useful question is therefore rarely “can anyone make it brighter” but “how much brightness can this cavity dissipate”.

Step 3 – Cut reflection before you buy more backlight

Surface reflectance is the cheapest brightness you will ever buy, because it works on the dark pixels too. Typical published figures put ordinary commercial displays at around 5-8% surface reflectance, industrial displays at 2-4%, and outdoor-optimised assemblies at 1-2%. Reaching the 1-2% band normally requires optical bonding plus an anti-reflective coating on the cover glass; neither alone gets you there.

Glass is part of the optical system, not packaging

Every additional pane between the viewer and the panel adds two reflective surfaces plus absorption. A display placed 200 mm behind a double-glazed shopfront is effectively viewed through four extra surfaces. Where that geometry cannot be avoided, money spent on bonding and coating returns more visible improvement than the same money spent on nits, because it lifts contrast across the whole image rather than only in the highlights.

Step 4 – Treat the street side and the shop side as two different screens

Double-sided window displays are not two copies of the same screen. The outdoor-facing side competes with daylight; the inward-facing side is viewed by people standing in a lit shop a metre or two away, where a 3,000 cd/m² image is unpleasant rather than impressive. The established pattern is independent brightness control per side: one manufacturer-published double-sided window model, for example, is listed at 3,000 cd/m² outward and 1,000 cd/m² inward, set separately. If a quotation offers a single brightness figure for a double-sided unit, ask how each face is controlled.

Step 5 – Plan dimming before the first complaint

A screen specified for noon is over-specified at midnight. In a shopping street that means glare into apartments, light spill on the pavement and, in some jurisdictions, a planning condition. Specify an ambient-light sensor with a defined dimming curve and at least three presets: day, evening and closed. Fixed-schedule dimming is acceptable if site staff can edit the schedule; a dimming feature that needs a technician visit will be left at full output.

Step 6 – Check the thermal and lifetime cost of brightness

Output is not free. Raising backlight current raises internal heat, raises power draw and shortens the life of the light source and the electronics around it. In a sealed high-brightness enclosure the cooling path is the real constraint: a window cavity is a semi-enclosed box with limited air exchange, so heat leaves by conduction to the housing and by radiation from its surfaces. That is why a 2,500 cd/m² panel in a well-designed cavity can outlast a 4,000 cd/m² panel crammed against glass. Ask for the power figure at the brightness you will actually run.

A short spec sheet you can send to any supplier

Send this list to every high brightness window display supplier you shortlist. It forces the conversation onto the two variables that decide the outcome – reflectance and dimming – instead of a single nits figure.

• Measured illuminance at the glass, by time of day, and hours of direct sun
• Required brightness band plus the reflectance you expect on the front surface
• Single-sided or double-sided, and how each side is dimmed
• Available cavity depth, ventilation gaps and mounting method
• Ambient temperature range on site and how heat leaves the enclosure
• Content type: full-motion video, static poster, or menu board with small text
• Acceptance test: measured brightness and uniformity in the installed position, not on a bench

Where VISLAT’s window range sits

VISLAT is a high brightness window display supplier building indoor units in single-sided and double-sided formats with narrow-bezel options, which is the shape most retail projects need once the two sides are specified separately. The published high-brightness window display range covers several panel sizes; the 55-inch ultra-narrow window model suits mullion-constrained shopfronts where a wide bezel would break the glass line, and the wall-mounted light box is published at 2,000 cd/m² for static poster work. Ask for measured brightness and reflectance for the exact model and cavity, and compare those with your Step 1 measurements rather than with a competitor’s headline number.

Frequently asked questions

How many nits do I need for a window facing direct sun?

Start in the 1,000-2,500 cd/m² band manufacturers publish for sunlight-readable displays, then move within it using measurement: a window with several hours of direct sun and multiple glass panes belongs at the top, a window with a deep reveal or canopy at the bottom. No single number is correct for every window, and a supplier who gives you one without asking about orientation and glass is guessing.

Is 2,500 cd/m² too bright at night?

Yes, at full output after dark. Specify an ambient-light sensor with at least three presets so the screen drops to a comfortable level in the evening. On a double-sided unit the inward-facing side is normally configured much lower from the start, so the night-time problem is usually the street-facing side and the light spill it creates.

Does higher brightness shorten the life of the screen?

Running a panel harder increases internal heat and power consumption, and heat is the main ageing mechanism for the backlight and the electronics around it. Buy brightness you will actually use, and confirm the cooling path in the installed cavity, rather than buying maximum output and running it at maximum all day.

Can I just put a normal indoor screen behind the shop glass?

You can, and many retailers do, but the result is predictable: washed-out contrast in daylight because surface reflection dominates, plus a bright rectangle competing with the window display at night. A 250-400 cd/m² indoor panel is designed for a room, not for a window that sees tens of thousands of lux on a sunny afternoon.

What exactly is ambient contrast ratio?

The contrast a display achieves under a defined amount of ambient light falling on its surface, rather than in a dark room. Because it folds surface reflectance into the calculation it tracks what a viewer in a shop window perceives, which makes it the more useful figure when comparing two candidate screens for one location.

Should the brightness figure be verified after installation?

Yes. Measure at the installed position, with the final glass in place and local lighting on, and record luminance at several points across the screen to check uniformity. A bench measurement in a showroom excludes your glass, your reflections and your cavity, and those three decide whether the specification was right.

If you are specifying a shopfront and want a second opinion on the brightness band before ordering, request a project consultation with the Step 1 measurements in hand.