Light your home like a pro: How much light do you need?
July 6, 2017
Lighting can make or break a space, but for most of us, home illumination comes down to guesswork. In this series, professional lighting designers share their tips for creating the domicile of your dreams.
To kick things off, Chris Rush, a lighting designer in Arup’s New York office, offers advice on determining how much light you need in a room and what to buy to provide it.
First, a crash course in the fundamentals. I’ll start with brightness. Most of us think in terms of watts: we know that a 60W bulb is brighter than a 40W bulb. (Lighting designers and engineers use the term “lamp” instead of bulb, but for the purposes of this discussion we’ll stick with the familiar term.) But as technology evolves, this is no longer always true. Different kinds of bulbs — e.g., compact fluorescent, LED, halogen — now emit very different quantities of light for each watt of energy use, making the wattage rating confusing at best and misleading at worst.
For this reason, the industry has moved to a different unit of measurement: the lumen. Professional designers compare light bulbs by looking at their lumens rating, which is always marked on packaging (see the Lighting Facts label). The higher the lumens, the more light emitted.
Another important unit of measurement is the footcandle. Roughly speaking, the footcandle describes the number of lumens delivered per square foot of space, allowing us to consider the density of light arriving at your table, floor, or chair.
Home as a range
Now back to the question at hand: How bright should your living room be?
Start by asking yourself a few basic questions. Do you always like bright light throughout your room, even at night? Or do you want a darker environment for watching TV or relaxing? Do certain zones need more lumens than others — a reading chair, for example?
Once you have a general plan, you can start thinking about footcandle requirements. Authorities like the Illuminating Engineering Society publish huge handbooks giving footcandle recommendations for every possible lighting situation; this is where professionals turn when working on everything from museums to schools. For the home, these guidelines can be boiled down to a typical practical range. One footcandle provides visibility in an outdoor setting (picture a well-lit street at night). Five footcandles is enough to see clearly in a relaxed environment, e.g., a foyer, hallway, or pantry. Fifteen footcandles can create a well-lit space for lounging, dining, casual reading, or quickly sorting the mail. Fifty footcandles will permit more intensive activities like drawing, cooking, or concentrating on a book.
Keep in mind that these are all recommendations. If you have good eyesight and like to ignore fine-print legal disclaimers when going through paperwork, 30 footcandles could be sufficient for your desk.
Doing the math
Once you’ve determined how many footcandles you need in different parts of the room, how do you figure out what to buy to hit this number?
Find the size of your room — for example, 10ft by 15ft, or 150ft². Assuming you want to hit an overall target of 15 footcandles for the space, multiply 150 by 15 to get 2,250: this is the target number of lumens arriving at table height.
So how does this translate in terms of lamps and lighting fixtures on the ceiling or beside your chair? Let’s walk through a few different scenarios.
Assume you opt for one large fixture near the ceiling. For a professional project, lighting designers would prepare very precise calculations showing how many of the lumens generated within the fixture travel downward versus spreading to the walls. For your home, however, you can estimate that about half the light is directed downward. (This percentage will be greater if your fixture has a completely clear glass globe, less if it has a very dark fabric shade.) Since only half the light moves downward and you want 2,250 lumens to arrive at table height, select a fixture that produces double the lumens: 4,500.
If you prefer multiple fixtures (or a multibulb fixture) instead, you could find other ways to reach the 2,250-lumen total. Three 800-lumen bulbs at the ceiling and a few 600-lumen bulbs in table or floor lamps near your reading chairs should work, for instance. This configuration also offers a few advantages over the single-source model. Keeping an adjustable fixture close to your chair when reading gives you more bang for your lumen, as the light is directed exactly where it’s needed. It can also give you more creative freedom with the ceiling lighting.
Recessed ceiling lights are another option, but buyer beware: if uniform lighting coverage is important to you, don’t space them too far from each other. To be safe, you might plan for at least one fixture per 30ft² (assuming an average room height).
Ultimately, of course, there’s no right or wrong answer to home lighting. If you like the look of the space and the brightness level suits your practical needs, you’ve succeeded.
Questions or comments for Chris Rush? Contact firstname.lastname@example.org.