Distinguishing a Lumen from Illuminance

Light output is measured in lumens. According to The American Heritage Dictionary of Science, a lumen is a unit of luminous flux equal to the amount of light from a source of one candela radiating equally in all directions. A candela is a unit of luminous intensity equal to 1/60 of the radiating power of one square centimeter of a black body at 1,772°C. You can draw two conclusions from this information:

• The higher the lumen measurement, the more light you’ll have to work with from a fixture.
• Authors can easily get carried away when they have too many reference books at their disposal.

Illuminance, which is measured in foot-candles, is the amount of light hitting a point on a surface. A foot-candle is (easily enough) defined as the amount of light produced by one candle on a surface one foot away. We can’t see illuminance, but we do see luminance or brightness, although this is somewhat subjective. (What appears to be dim light to me might be plenty bright to you.) Architects and lighting consultants take all these measurements into consideration when they calculate the lighting needs of buildings.
Comfortable lighting selections and light levels are determined by the tasks that require the lighting, the distance between the light and the task, and the degree of glare. One definition of glare is excessive contrast between the intensity of light on a particular object or surface and the surrounding area or background; indirect glare is the glare produced from a reflective surface. Too much contrast between them causes glare. (Computer screens are a common example.) You can reduce this glare by …

• Installing fixtures that keep the light level appropriate for the task at hand.
• Using a louver or a lens to block or redirect the light.
• Carefully considering the placement and spacing of light fixtures.

Another measurement of lighting quality is how well it enables you to see colors accurately. The better the color rendering, the more pleasing the living space. Color-rendering capability is based, naturally enough, on the color-rendering index (CRI), which measures from 1 to 100. (Natural daylight measures at 100.) The higher the rating on the CRI, the more lifelike and accurate the object being viewed.

Measuring Your Lighting Needs

The NEC calls for a minimum calculation for general lighting and receptacle loads of three watts per square foot of living space. This comes out to approximately one circuit every 575 square feet. That doesn’t amount to a lot, but remember, electrical codes only establish minimum standards. In reality, you’ll want lighting everywhere.
Consider the different areas of your house and their individual needs:

• Kitchen. Overhead lighting, natural light from windows and sliding doors, work light over counters, a light over the stove.
• Bathrooms. Primarily lights over the sink(s) and lights over the bathtub and toilet, depending on the size of the room.
• Dining room. A hanging light over the table, recessed ceiling lights, or possibly wall sconces. This room often has a dimmer switch to tone down the light.
• Bedrooms. Children’s rooms often get ceiling lights. Master bedrooms might depend more on reading lamps and switch-controlled receptacles, although large rooms can use recessed ceiling lighting as well.
• Hallway. You might want a long track light to highlight artwork on the walls.
• Garage. At least one light per bay over the hood of the cars. It’s even better to add one or two at the other end so the trunks are illuminated.
• Basement. Depends on whether the space is finished or unfinished. In either case, you want at least enough ceiling light to cover the entire area thoroughly, leaving no dark spots.
• Closets, storage rooms. At least one ceiling light.
• Outdoors. At a minimum, one light over each entry door and over the garage doors. It’s even better to consider lights to line walkways and illuminate gardens or security lighting for back and side yards.

Your use of a room obviously will determine your choice of lighting fixtures, their locations, and their number. A single fluorescent ceiling light will fulfill all the working requirements of most closets because the requirements are pretty basic: to shed enough light for you to identify and choose your clothes. A kitchen, on the other hand, requires all kinds of light for a modern homeowner. You need lights over counters for close work so you can chop, dice, and mince vegetables instead of your fingers. Overhead lights enable you to read the newspaper and get a better look at what’s hiding in the back of your pantry. A dimmer-controlled hanging light over the eating area lets you tone things down for a late-night meal. Anywhere you’ve got a wall, ceiling, or floor, you can install a light. It’s simply a matter of extending a circuit or running a new one and choosing your fixtures. The science of lighting is a little more complicated.

How Illuminating my home is.......

We are way beyond the point when lighting was simply functional, allowing us to work and not stumble around after the sun went down. If function was all it meant to us, every room in our house would have one huge, efficient, fluorescent light fixture on the ceiling and maybe a night-light or two for after dark. Instead, lighting does much more such as …

• Create a mood or atmosphere.
• Define a space.
• Provide security and safety.
• Highlight artwork or a section of your home.

Your lighting needs will be defined by these factors and others. Before you install a particular type of lighting, ask yourself the following questions:

• Who will be using this area and for what purpose?
• Do I want a traditional or modern look?
• How often will anyone be in this room?
• How much am I willing to spend?
• Is energy conservation important to me?

At a minimum, the code calls for one switch-controlled light per habitable room. Hallways, stairways, and garages also must meet this code requirement. This can be accomplished with permanent fixtures, such as ceiling lights, or through a switch-controlled receptacle into which a lamp can be plugged. Bathrooms and kitchens, however, must have an installed fixture. Your first step is to establish your minimum lighting needs and then choose the style of fixture you want to meet them.