Basic Concepts of Lighting Design

2020-04-01

Lighting system design is a very important aspect of construction projects, and the best performance is achieved when lighting is specified through detailed calculations and not “rules of thumb”. However, lighting has a key difference with other building systems like HVAC and plumbing: there is a subjective and artistic factor involved. Lighting systems must provide adequate visibility, but they also set the ambiance of built environments.

Effective communication is important in any engineering design process, and this is only possible if the parties involved are familiarized with key technical concepts. This article will provide an overview of the main terms used when specifying lighting systems.

The Lumen: Basic Unit of Luminous Flux

Just like electric current is measured in amperes and water flow is measured in gallons per minute, the lighting output of a lamp or fixture is measured in lumens. Watts are often used to describe the brightness of lamps, but this is an incorrect practice that leads to confusion:

• A few decades ago when all light bulbs were incandescent, wattage could be used to describe brightness because there was a direct relationship between lumens provided and watts consumed.
• However, this leads to confusion when comparing different types of lighting. For example, the lumen output is roughly the same for a 60W incandescent bulb, a 15W compact fluorescent light, and a 9W LED bulb.

A common misconception about LED lighting is that you end up with a darker room due to the reduced wattage. However, this comes from the old practice of describing brightness with watts, when the correct unit is the lumen.

The concept of luminous efficacy describes how efficiently a lamp converts watts of electricity into lumens, similar to the gas mileage (MPG) of a car. Assuming the three light bulbs described above produce 900 lumens each, the luminous efficacy values would be the following:

Lumens are useful when describing the output of a lamp or fixture, but a different measurement unit is required to describe the lighting needed by a specific area. For example, 10,000 lumens is more than enough lighting for a small office, but the effect is barely noticeable in a large warehouse. When describing the required lighting in built environments, the concept of illuminance is used.

Illuminance: Lumens per Unit of Area

The concept of illuminance is used to describe the lighting required for a given occupancy, regardless of size. There are two common measurement units:

• Lux, or lumens per square meter.
• Footcandle (fc), or lumens per square foot.
• 1 fc = 10.7639 lux

Since illuminance is specified per unit of area, room size does not matter. For example, an illuminance of 50 fc has the same meaning for a 500 sq. ft. office and for a 2500 sq. ft. office, with the difference that more lighting fixtures are required for the larger office. Illuminance values are specified by the Illuminating Engineering Society of North America (IESNA) in their Lighting Handbook.

In actual lighting design, the illuminance level varies due to the spacing of fixtures and their beam shapes. However, slight variations are acceptable as long as no areas are too dark or too bright. Illuminance is determined not only by the lighting distribution, but also by room features such as ceiling height and surface colors - manual calculations are very complex, but the process is automated with software in modern lighting design.

Photometry: What is the Beam Shape of Lamps and Fixtures?

Light beam shape is another important aspect considered by lighting designers. For example, spotlights concentrate their output into a narrow beam facing downwards, while troffers in offices spread their output over the largest possible area to achieve uniform lighting.

Do not assume that two light bulbs can be used for the same application just because their bases have the same shape. Using a light source with the wrong beam shape yields poor results even if the lumen output is calculated correctly.

The beam shape of a lamp or fixture is three-dimensional, and product models used in lighting design software include this information for simulation purposes. On the other hand, a 3D beam shape cannot be represented in technical specifications; the beam is described with overlapping 2D figures that represent beam shape parallel and perpendicular to the light source.

Correlated Color Temperature and Color Rendering Index

Describing the color performance of lighting requires two separate metrics, one for the light source and another for the objects it illuminates.

• The correlated color temperature (CCT) describes the color of the light source itself. When dealing with the CCT, no value is considered “the best”, since each lighting hue has different applications.
• The color rendering index (CRI) describes how faithfully the light source renders the colors of objects and surfaces in the room. The maximum CRI value is 100, describing a light source that matches the quality of natural light - a higher CRI is always better regardless of the application.

Using Temperature Values To Describe Lighting Color

Objects glow in a characteristic color depending on their temperature, and this is why lava from a volcano looks red. The same principle applies to stars, where a yellow star like the Sun is hotter than a red star, and a blue star is hotter than a yellow one. In physics, this behavior is described by an abstract concept called a “black body”, which is an object that emits no light except when heated, and each temperature corresponds with a specific color glow.

Light sources are not heated to the temperature implied by their CCT value, but it is a convenient way to assign a numerical value to their color. In most residential and commercial applications, the CCT value of lighting ranges from 2700K (yellowish white) to 6500K (bluish white). In other words, if a light product has a CCT of 4000K, it means it glows with the same color as a “black body” at 4000K, but the light source itself does not reach that temperature!

Although personal preference plays a role in CCT selection, the following principles apply to most lighting designs:

• Low color temperatures like 2700K are perceived as “warm” and they tend to have a relaxing effect. They are preferred in areas like residential bedrooms, hotel rooms and high-end restaurants. Warm colors are not well suited for commercial and industrial settings, where the relaxing effect can be counterproductive.
• High color temperatures like 6500K are perceived as “cool” and they tend to have an energizing effect, enhancing. They are preferred in applications where maximum visibility is required, such as high-precision manufacturing. Cool colors may delay sleep when used in residential and hospitality settings, and extended exposure may be described as “stressful” by some individuals.
• CCT values around 4000K are perceived as “neutral” and they offer a balance point between the two extremes described above. Neutral white is the lighting color of choice for offices, classrooms, kitchens and similar locations where concentration is required for extended periods.

Effect of the Light Source on Objects and Surfaces

Even if two light sources have the same CCT value, their lighting quality may differ. A CRI value of 100 means the light source is a good as sunlight.

• Despite their inefficiency, incandescent and halogen bulbs offer a CRI of 100.
• Fluorescent bulbs tend to have the lowest color rendering performance, and low-tier products can go below 70.
• CRI values for LED bulbs can vary significantly depeding on product quality. Low-end products go below 70, while high-performance products reach values close to 100.

The minimum CRI for a light bulb to get the ENERGY STAR label is 80. Therefore, looking for the label is an effective strategy to avoid lamps with poor color performance, and you can also rest assured that the product has passed rigorous laboratory tests. When dealing with LED bulbs, a higher CRI typically comes with a higher price tag, but there are applications where the CRI metric is very important - consider retail and art galleries, for example.

Conclusion

LED upgrades have the potential to reduce lighting power consumption by over 50 percent. If the space is air-conditioned, indirect savings are achieved by reducing the heat footprint of lamps and fixtures. It can be tempting to simply swap the existing lighting with the most efficient product available, but lighting design must not be overlooked - lighting savings should not be achieved at the expense of quality.