February 2017 — Proper lighting is critical to building operations, as well as the effectiveness and comfort of tenants. Lighting methods are classified according to lighting focus, or direction, and fixture arrangement. The light fixture, or luminaire, serves several functions. It must hold the bulb, protect the bulb, protect users from electric shock, and direct and diffuse the light.
Lighting Focus
A fixture can be designed to focus light in one of the following ways:
- Direct
- Semi-direct
- General (formerly called diffused)
- Semi-indirect
- Indirect
Direct Lighting
In a direct lighting system, 90 to 100 percent of the light from the luminaire shines toward the work surface. This is the most common type of lighting and is used for many types of tasks. The area that is lit depends on the characteristics of the luminaire. For example, light shining downward may be either concentrated in one small area or spread evenly over a wide surface. Sometimes, however, direct lighting creates glare and shadows. The placement of the fixture or brightness of the light can be adjusted to eliminate these problems.
Semi-Direct Lighting
In a semi-direct lighting system, 60 to 90 percent of the light from the luminaire shines toward the working surface. The remainder of the light is reflected toward the ceiling and the upper portion of the walls. This lighting system softens shadows and produces even lighting. Semi-direct and direct lighting have many characteristics in common. As is true with a direct lighting system, unless the fixtures are properly arranged, reflected light or shadows can be a problem when using semi-direct lighting.
General Lighting
In a general, or diffused, lighting system, light is distributed equally to both the upper and lower areas of a room—50 percent upward and 50 percent downward. While general lighting can be pleasing to the eye, it can seem harsh to some. Therefore, a more indirect method of lighting is often desirable.
Semi-Indirect Lighting
In a semi-indirect lighting system, 60 to 90 percent of the light from the luminaire reflects toward the ceiling. Highly reflective ceilings and room finishes are important, as is good surface maintenance. This method of lighting produces a very pleasing room ambience while also providing several good reading areas.
Indirect Lighting
In an indirect lighting system, 90 to 100 percent of the light from the luminaire shines upward and reflects off the ceiling. Indirect lighting should result in highly diffused, evenly distributed light. The ceiling is the main source of light in this type of system, so it should be completely and evenly lit. If the system is well designed, only a small amount of light will shine downward.
Indirect lighting minimizes veiling reflections, shadows, and direct glare. Fixtures should be positioned far enough from the ceiling to prevent excessive brightness, and they should be placed low enough to distribute light evenly over the ceiling. Fixtures usually have a stem of at least 12 inches. Because both the ceiling and upper wall areas are illuminated, surface finishes in the room need to be light colored and highly reflective, and wall and ceiling surfaces must be maintained and kept clean. Poorly maintained surfaces can decrease the light level in the room.
Lighting Fixture Arrangement
Lighting fixtures can be arranged in one of two ways:
- Uniform
- Non-uniform
Uniform Lighting
Uniform lighting illuminates an entire area at about the same level, using any type of lighting focus. Fixtures are normally placed for maximum height and uniform spacing without regard for the location of desks or equipment within the room. This arrangement is useful for open furniture plans or in areas where a high level of churn (relocation of people, workstations, and equipment) is expected.
Non-uniform Lighting
In a non-uniform lighting system, the fixtures are placed high or close to the ceiling, but the spacing between them is irregular. The placement of the fixtures is determined by the location of workstations and machinery and the nature of the task that will be performed in that area.
Most offices have uniform lighting, because workers must be able to use the available lighting anywhere workstations can be set up. Yet, under certain conditions, adequate illumination can be provided by non-uniform lighting, saving up to half the operating cost of a uniform lighting system. For example, if workstations are not close together (that is, if they are generally more than 12 feet apart), and if workstations are rarely relocated, non-uniform lighting should be considered. In this case, large-area (usually fluorescent) fixtures should be used over the work surface or at the sides of the work surface. Fixtures located in front of the work surface may cause reflections, and fixtures located behind the worker can cause shadows.
Task lighting, also known as localized lighting, is an extension of the non-uniform lighting concept. Task lighting has become popular because of the use of systems furniture, where lighting is an integral part of the system. A desk lamp is a simple example of task lighting. Task lighting fixtures are normally low wattage and are placed close to the work surface. Alternatively, a focused fixture can be located some distance from the task. When task lighting is used exclusively, no attempt is made to provide general or ambient lighting.
The task light provides a low-cost, effective method of lighting the area for a given task, but it can create discomforting shadows and reflections. Adding indirect ambient lighting from daylight or another energy-efficient source can alleviate these problems and create a pleasing and effective room and workstation ambience. Building a combination of task lighting and indirect ambient lighting into systems furniture is a method that is currently attracting a great deal of interest because it provides adequate lighting and is energy efficient.
Accent lighting, also known as effect lighting and highlighting, can be used to achieve a certain design effect. Adding wall-wash and focal-point lighting to non-uniform and task lighting helps to maintain a comfortable brightness balance and interesting surroundings. Selective illumination of walls—lighting a single wall, for example, to raise its brightness level in relation to other room surfaces—can be used to create a dramatic effect and provide a point of visual focus.
This article is adapted from BOMI International’s course Electrical Systems and Illumination, part of the SMA and SMT designation programs. More information regarding this course or BOMI International’s new High-Performance Sustainable Buildings credential (BOMI-HP™) is available by calling 1-800-235-2664. Visit BOMI International’s website, www.bomi.org.