THE IMPORTANCE OF QUALITY LIGHTING
The quality of spaces we occupy at home and at work matters tremendously to us - whether we consciously realize or not. Comfortable, well lighted, well ventilated indoor environments put us in a better mood and positively affect health, well-being, and productivity. However, too often we find ourselves inhabiting poorly lighted spaces, generally in office environments, undermining these positive outcomes. Poor lighting most often results from bad design, carelessness, or both, and is not due to insurmountable structural barriers to good lighting.
When it comes to lighting, more is not necessarily better, and "one size fits all" is almost certainly the wrong approach. Successful lighting designs result from a carefully considered analysis of functional and programmatic requirements, the physical characteristics of a space, available daylight, reflectance of room surfaces, and strategies to avoid excessive glare and contrast.
Industry terms used to describe lighting technology and ways to measure lighting performance can easily confuse the layman or even the experienced design professional. Below is a short list of key terms important for the understanding of the remainder of this section.
WHAT IS WHITE LIGHT?
- Color rendering:
- Color temperature:
- Daylight harvesting:
- Illuminance, Illumination:
- Luminous balance:
- Solar heat gain:
What we see as white light can be created through a mixture of the three primary colors of light - red, green, and blue, or RGB (shown below left) - or through a continuous spectrum of nearly equal amounts of energy at all color frequencies of the visible spectrum (shown below right.) While both types of light will appear white, a continuous spectrum renders colors falling between red, green, and blue more naturally.
Humans respond positively to the quality of natural daylight. Yet lighting fixtures, also known as luminaires, generally do not replicate the spectrum of daylight. Instead they have a unique color signature, defined by how much energy is emitted at each wavelength. The color signatures of daylight and some common artificial light sources are shown in the Spectral Distribution Curves (SDC) below. A luminaire's color signature determines if we perceive its light as more natural or more artificial.
Sources emitting the most energy in a particular color range will accentuate that color in an environment. For example, in a grocery store, red wavelengths will enhance the appearance of tomatoes, and green wavelengths will enhance lettuce.
Color temperature refers to the perceived "warmth" or "coolness" of the color of a light source and is measured in Kelvin. Sources below 3,000 Kelvin appear warm white or yellowish, while sources above 5,000 K appear cool or bluish-white. However, color temperature does not determine color rendering capabilities since light sources with the same color temperature may render surfaces quite differently, so both metrics must be considered together. The Color Rendering Index (CRI) is a measure of a light source's ability to reveal colors of objects faithfully when compared to natural daylight. The highest possible CRI value of 100 indicates total faithfulness.
Newer energy efficient light, such as LED, sources have made great progress towards the goal of natural color rendering, coming close to traditional incandescent sources, yet with dramatically longer lives. CHOOSING THE RIGHT LIGHT FIXTURE
There is no rationale or benefit to using one "building standard" light fixture, or, in technical terms, luminaire, for all functions within a building or space. To the contrary, selecting the right fixture for each interior space provides better quality, flexibility, visual interest, and increased energy savings. Best design practice limits the number of different lighting types on a project for ease of maintenance.
Each luminaire has a specific distribution pattern tailors for a specific function as illustrated below. Some are meant to provide light to the walls or ceiling, while other are intended to add sparkle or ambient glow, or to function as focused task lighting. Some luminaires even integrate multiple lamps for different functions. Energy efficient lighting design creatively combines luminaires with varying distributions to provide a balanced and functional luminous environment.
Luminaires are durable devices are should be selected as long-term solutions. Light sources (especially those with shorter lives) should be minimized project-wide to simplify stocking and replacement, and lighting control zones should be established according to different luminaire types and/or functions.USING DAYLIGHT TO IMPROVE PERFORMANCE
Daylight harvesting refers to the ability to realize energy savings by reducing the use of electric lighting at times when daylight is available. Effective daylight harvesting requires an automated control system consisting of a photosensor and controller that adjusts the electric light by either switching or dimming in response to available daylight levels.
Successful daylight harvesting depends on:
- Sufficient daylight to achieve adequate energy savings.
- Physical conditions that promote daylight distribution such as high window headers and light-colored interior finishes.
- Effective control mechanisms to avoid unwanted glare and solar heat gain.
- Lighting control performance that does not distract or annoy the occupants (e.g. with sudden change in illuminance levels).
The following images show some examples of daylight harvesting.
Scene A shows a space where the window-wall ratio and window placement provides for effective daylight harvesting opportunities. In Scene B, the tall windows coupled with a relatively narrow room result in excellent daylighting. Adjacent buildings limit daylight availability in Scene C, but views greatly enhance the occupant experience. The tall windows and relatively narrow spaces shown in Scene D offer excellent daylight design opportunities. However the bottom 30 inches of the glazing do not meaningfully contribute to the daylighting, but increase unwanted thermal heat gain.
Sensitive daylight design optimizes window placement by taking both daylight and views into consideration while limiting the overall amount of glass to what is functionally justifiable. In major renovation projects, ductwork obstructing the tops of windows in older buildings can typically be relocated to take maximum advantage of available ceiling heights within 12 to 15 feet of the perimeter. Automated blinds for solar control should be deployed to limit solar heat gain and glare from windows while maximizing daylight harvesting.
Many of New York City's existing buildings are well situated to capitalize on the benefits of daylight, since most of the City's office buildings construct prior to the 1950s were designed to utilize daylight. Block sizes and orientation have generally resulted in pre-war building floor plates in which daylight reaches a sizable portion of the floor area.
For more in-depth discussion of control strategies conducive to daylighting, refer to the Lighting Controls section of this website. DESIGNING INTERIORS TO IMPROVE PERFORMANCE
Interior design decisions have tremendous influence on lighting efficiency. The placement of interior walls, clerestories, and fixed furniture determines to what extent daylight can penetrate deeper into a space to benefit workers not situated along the perimeter. Good interior design tries to maximize opportunities for daylight harvesting by carefully laying out the various functional areas and incorporating low-cost passive design solutions rather than utilizing expensive technologies.
Comparing the two alternative office layouts for a given suite shown above, we notice that daylight harvesting in private perimeter offices fitted with vacancy sensors, as shown in Scene A, is of limited efficacy because the open office area farther in is shielded from daylight by the row of offices. Reversing the scenario, as shown in Scene B, with open-plan offices located near windows enables effective daylight harvesting that benefits a greater number of occupants. However, if the partitions separating the private perimeter offices from the open-plan area were to feature translucent elements, such as clerestory windows, or to be constructed as all-glass partitions, some daylight could still penetrate deeper into the space.
Low partitions obstruct or absorb less light than higher or full-height ones, making them more efficient for distributing both electric and natural light. Therefore, open-plan offices are typically more energy efficient, and a higher number of occupants can benefit from access to windows and views. DESIGN LIGHTING FOR THE WHOLE SPACE
Current lighting design standards and practices are guided by the quantitative logic of providing a minimum amount of illuminance (measured in foot-candles) on defined task surfaces (usually desks). However, these standards don't address the more intangible overall quality of lighting in a space. As a result, we often encounter unsatisfying environments such as shown in the renderings below.
All of these scenes demonstrate poor quality lighting that wastes energy with excessive levels of contrast and improperly lighted room surfaces. In Scene A, the contrast between the illuminated lower portion of the room including the task surface and the upper darker portion is too stark. In Scene B & C, the spaces feel dark because of the high contrast of dark, light-absorbing surfaces.
To avoid such sub-par outcomes, design teams should keep in mind some simple correlations:
- Dark finishes reduce inter-reflections and hence require more energy to achieve the same level of illuminance as a room with lighter-colored surfaces.
- We judge brightness based on the room surfaces (walls, ceilings, and partitions) within our total field of view. Dark or unlighted areas make a space appear gloomy, even when there is plenty of horizontal illuminance.
- High contrast within a space creates visual discomfort and can affect work performance as the eye struggles to simultaneously adapt to separate brightness levels.
- Increased illuminance on task areas cannot compensate for poor overall lighting quality.
The following illustration shows how increases in a room's surface reflectance will brighten up a space - resulting in significant energy savings since less electric light is needed to reach sufficient illuminance levels.
Perhaps counterintuitively, specular (shiny) interior finishes lead to increased energy use and add visual confusion and glare to the environment as illustrated in Scene A below. Matte finishes as illustrated in Scene B.
This is because specular surfaces reflect in one direction, much like a mirror. They create glare by causing clear images of the light source to be visible. As a result, more light is needed to compensate for high contrast and glare. Matte surfaces, in contrast, reflect light in all directions and prevent glare by obscuring the reflected image of the light source. A matte surface will appear adequately lighting with less light.
The general good lighting design should be to create an overall luminous balance within a space by avoiding glare, over- or under-lighting areas, and excessive contrast. Within such well-designed spaces sufficient and comfortable illuminance levels on the task surfaces are then easily realized.