Power and light
How bright should a lit sign be? Nits, codes and dimming
Bright enough to read, never glaring: how sign brightness is measured in nits, what city limits look like, and how to dim a lit sign at night.
Updated
A lit sign should be bright enough to read clearly against what surrounds it, and no brighter. The same letters that look right on a busy, well-lit street can glare on a dark side street or next to homes. Cities that set limits usually cap brightness at night, and a dimmer lets you set the level once the sign is up.
How sign brightness is measured
Three units come up, and they measure different things.
| Unit | What it measures | Where you see it |
|---|---|---|
| Nits (candelas per square meter, cd/m²) | Luminance: how bright the face of the sign is | City sign codes, digital display specs |
| Footcandles or lux | Illuminance: how much light reaches a point some distance away | Codes that compare a sign’s light with the light around it |
| Lumens | Total light an LED or module puts out | LED spec sheets |
A nit reading is taken with a luminance meter aimed at the sign face. A footcandle reading is taken with a light meter at a set distance, once with the sign on and once with it off; the difference is the light the sign adds. One footcandle is about 10.8 lux.
Lumens on a module spec sheet do not tell you how the finished sign will look. The same modules look brighter behind a white face than behind a dark blue one, and brighter on a dark street than next to a lit gas station canopy.
What city limits look like
Many sign codes only say that lighting must not glare onto the street or the neighbors. Others set a number. The numbers vary widely, so treat these as examples, not the rule for your town:
- Nit caps. Night-time limits we found in city codes run from about 100 nits to several hundred. Chelmsford, Massachusetts, for example, caps internally lit signs at 200 nits at night and 2,000 nits by day. A model lighting code from DarkSky International uses 100 nits for signs that cannot be measured with a light meter.
- Light above ambient. The International Sign Association recommends that digital message signs add no more than 0.3 footcandles to the surrounding light at night, measured at a distance based on the sign’s size, and many communities have adopted that method. The DarkSky model code applies a similar limit, 3 lux above ambient, to lit signs in general.
- Rules without numbers. Some codes say the light source itself must not be visible from the street or the neighbors, or that signs must switch off after business hours. The DarkSky model code, for example, lets larger on-premise signs stay lit only while the business is open.
The hidden-light-source rule matters if you want open-face letters with LED neon, because the neon is the light source and it is meant to be seen. Many numeric limits were written for digital message signs, and some cities apply them to every lit sign. Your city’s sign code, or the planner who reviews your sign permit, will tell you which rules apply.
What makes letters look brighter or dimmer
- Surroundings. Brightness is relative. A sign competing with shop windows and streetlights needs more light than one on a dark wall facing a quiet parking lot. That is why some codes measure a sign against the ambient light.
- Face color. White and light faces pass the most light. Dark blue, dark green and other deep colors absorb much of it and look darker at night.
- Lighting style. Front-lit letters show a lit face and are the brightest style. Halo-lit letters put indirect light on the wall, which looks softer, and the glow depends on the wall color. Front-lit vs halo-lit vs open-face compares the looks.
- Viewing distance. Up close, a bright face can look harsh; from across the road it reads well. Judge the sign from where customers will see it.
Brightness by setting
| Setting | What usually works |
|---|---|
| Busy commercial street or highway frontage | Full output at dusk; consider turning it down late at night |
| Street that backs onto homes | A lower night setting, or switch off after closing; check the code |
| Shopping center | Match the neighbors and follow the center’s sign criteria |
| Indoor lobby, reception or salon | Well below outdoor levels; a dimmer is worth having |
| House numbers | Low and warm, enough to read from the street |
Dimming at night
A sign that looks right at dusk, while the sky still has some light, looks much brighter once the sky is fully dark. Two tools deal with this:
- A dimmer. Our LED sign dimmer sets the level on the 12 V side of the power supply. See dimming LED signs for how it works.
- Automatic controls. A dusk-to-dawn photocell turns the sign on when it gets dark, and a sign timer turns it off at a set hour. Photocells and sign timers explains how to combine them.
Some energy codes require controls like these. California’s Title 24, for example, requires outdoor sign lighting to run on a photocontrol plus a time switch, or on an astronomical time switch, and outdoor signs lit both day and night must be able to cut their lighting power by at least 65 percent at night.
How we handle brightness on your sign
Every drawing comes with a lit preview, so you can see how the letters will glow before anything is built. If your city sets a brightness limit, or the sign faces homes or hangs indoors, tell us when you request your drawing and we will include a dimmer on the quote. Browse all sign controls.
After install: set it and check it
- Look at the sign at dusk and again when it is fully dark.
- Stand where customers see it: across the street, in the parking lot, at the nearest corner.
- If there are homes nearby, look at the sign from their side.
- Turn the dimmer down until the letters read easily without looking harsh, and leave it there.
- If an inspector or a neighbor raises a concern, lower the level or shorten the hours on the timer.
