Home Lighting Design

by Eric Strandberg

You might believe that if you have plenty of windows and skylights you won't need to worry about lighting fixtures, but what about that natural light at 5 pm in February? Proper fixtures and placement will ensure adequate and beautiful light for all your needs, whatever the time of day or year.

When I do a residential electric lighting plan, I do the lighting as though the windows and skylights were not there. I need to note their location, so I don't locate a light fixture in the same spot, but I don't rely on them to provide any light. Interior spaces need to be functional at night or on short, gloomy winter days. The Northwest client who says, "I don't need much electric lighting in the bath because I'm putting in wonderful skylights" has forgotten that at 6:30 am in November--when many people are getting ready for work--the skylight will be dark. The same thing can be said, for example, about windows in a kitchen at 5 pm in February. Of course, windows are a fine choice from both an efficiency and an aesthetic standpoint, but we can't rely on them to provide light consistently.

The Temperature of Light

Another way daylight and electric lights interact is with respect to their color, called "color temperature". Many people don't realize how blue (cool) daylight is. Often, when people think of sunlight they think of sunsets or sunrises, in which the light is rather yellow or pink (warm). Most of the day, however, light from the sun is much bluer than light from the most common electric source, the incandescent bulb. Skylight-- light from the clear sky alone--is even bluer than sunlight.

When scientists and lighting designers want to describe these colors precisely, they use the Kelvin scale. On the Kelvin scale, the higher the number, the bluer the light. For example, candlelight is about 1,500 Kelvins (K). A standard incandescent light bulb is about 2,700 K, while sunlight at noon is anywhere from 4,500 to 6,000 K (depending on latitude). Daylight can be from 6,500 to well over 15,000 K. I think it is impractical, unnecessary, and perhaps an aesthetic mistake to try to match daylight color temperatures with the electric lights in our homes. It is odd and even unnatural to see daylight colors streaming from a kitchen or living room after dark.

An incandescent bulb is basically a flame (a burning filament), and over the millennia we humans have been conditioned to associate warm light with hearth, home, and security. However, in certain high-activity areas in homes, it may be desirable to use a whiter light source (3,500 K for instance). This is where fluorescent lights excel. Incandescent sources are naturally rich in the red portion of the visible spectrum. They can be made to appear bluer (that is, more like daylight) by filtering out some of the red and allowing a higher proportion of blue light to pass through. This is very inefficient, as much of the light energy is lost as heat in the filter. Fluorescent sources use phosphors to emit light, and these phosphors can give off cool colored light, warm light, or something in between. This is a much more efficient way to select the color.

Timers and Controls

One of the most common applications for integrating daylight with electric light in residences is the use of photocells, or light sensors. A photocell is a switch that is controlled by light. As the ambient light levels diminish, the photocell turns on the electric light to which it is connected. Photocells are used primarily on porch lights or other exterior lighting applications.

There are pros and cons to the use of photocells. On the positive side, the light automatically comes on at dusk, so if you arrive home after dark, there will be a light or two on when you get there. Having the lights on may also give the house a lived-in appearance and so deter prowlers. On the negative side, the lights are on all night long (unless you have a timer or a manual switch, and then you must remember to reactivate the system). (See "On Photocells and Timers.") If you are controlling an incandescent light, these long hours of operation will result not only in high energy costs but also in lots of burned-out bulbs. A great alternative for incandescent bulbs is to use compact fluorescent lamps (CFLs). They last about ten times as long as an incandescent and use about a quarter of the power (see "A Bright Alternative--From Utility to Consumer," HE May/June '00, p. 11).

There are two major types of photocell: screw-in, and hard-wired. Screw-in photocells screw into existing standard sockets, and the light bulb is then screwed into the photocell. They have the advantage of being easy to install; however, since the device is about 2 inches long, there is sometimes not enough room in the fixture for both the photocell and the bulb. Also, screw-ins put the photocell right in the fixture, which is not always the best place for it (if the fixture is in a shady spot, it may give a false reading). Additionally, CFLs generally don't operate on screw-in photocells.

Hard-wired photocells are installed on the house, in a post (as in a post light), or sometimes on the fixture itself, and the wires run through the photocell just like a switch. The disadvantage of this type of photocell is the expense of installation, though that is generally not an issue in new construction.

Light Contrast

A third way in which daylight and electric light interact is with respect to contrast. Daylight levels are much brighter than normal levels of electric light. Light quantity is measured in foot-candles (FC). A brightly lit kitchen may have 75 -100 FC on the counter, while outside the home, the illumination may be well into the thousands of FC. If there are windows in the kitchen, some of that light will get in and add to the overall light levels, but only near the windows. If you aren't near a window but have the window in your field of view, your pupils will constrict as though you had a bright light in your eyes, making the surrounding area seem dimmer than it is. This can also be a problem in a home office where the desk faces a window. The easiest solution is to orient the desk perpendicular to the window.

Skylights and other advanced daylighting products and techniques can go a long way toward controlling this problem. In my office, as I write this, there is about 80 FC on the desk near the window, but only 8 FC on the desk just 12 ft away from the window. The dynamic nature of daylight is such that at other times of the day, I may have more than 300 FC on my desk, while the second desk has only about 20 FC. (Direct sunlight on my desk measures more than 2,000 FC, necessitating the use of blinds.)

The eye can react to a wide range of light levels, but it can react to only one level (the brightest one) at a time. This is why a large living room with a wall of windows can feel dark during the day but not at night. The overall light levels may be significantly lower at night, but there is no contrast from the window. When the electric lighting is planned, it is important to have a layer of lighting in the interior of each room to help mitigate this effect. Ideally, this inner lighting would be on a separate switch from the one that controls the lights by the windows. Think about washing an art wall with light, lighting some bookcases, or adding some decorative wall sconces on these interior walls.

Daylight: Use It!

Daylight can be a wonderful resource. It can add an essential dimension to our living and working spaces, and can be a vast (and underutilized) source of high-quality light. Using it also provides an opportunity for conservation. Use daylight early and often!

Eric Strandberg is a residential lighting specialist at the Lighting Design Lab in Seattle, Washington. He can be reached at Tel: (206)325-9711, Ext. 28.

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