LED Lighting Design 2024: Understanding the Basics

Lighting design is very important in construction projects, and the best results are achieved using detailed lighting calculations rather than “rules of thumb.” Compared to other components such as HVAC equipment and plumbing fixtures, the lighting system is special – there is a subjective and artistic factor involved. Lighting designs must provide sufficient visibility, but also define the ambience of the built environments.

LED lighting is often recommended by energy consultants. The payback period is typically less than three years and major manufacturers offer a five-year warranty. As the initial cost is recovered during the warranty period, LED lighting is a very safe investment.

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Effective communication is important in any project and this is only possible if everyone is familiar with key technical concepts. This article will provide an overview of key terms used when specifying a lighting system.

The Lumen: Basic Unit of Luminous Flux

Just as an electrical current is measured in amps and the volume of water in gallons, the light output of a lamp is measured in lumens . Watts are often used to describe lamp brightness, but this is a confusing incorrect practice:

• A few decades ago, when most light bulbs were incandescent, brightness could be described in watts. There was a direct relationship between the lumens produced and the watts consumed.
• However, this is confusing when comparing different types of lamps. The effectiveness of converting watts to lumens changes depending on the lighting technology.
• For example, the lumen output is approximately the same for a 60W incandescent bulb, a 15W compact fluorescent bulb, and a 9W LED bulb.

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

The concept of luminous efficacy describes how well a bulb converts watts into lumens, similar to the fuel consumption (MPG) of a car. If the three lamps described above produce 900 lumens each, their luminous efficacy values ​​are as follows:

Just like a car with high MPG has lower fuel costs per mile, a light source that produces many lumens per watt has lower energy costs. From a financial perspective, LED upgrades are among the best energy efficiency measures for buildings.

Lumens are useful for describing the output of lamps. However, a different unit of measurement is used to describe the lighting required for a specific area. For example, 10,000 lumens is more than enough light for a small office, but its effect is barely noticeable in a large warehouse. The concept of illuminance is used to describe the lighting required in built environments.

Illuminance: Lumens per Unit Area

The lighting needs of a given occupancy are described with an illuminance value, which is not affected by size. There are two common units of measurement for illuminance:

• Lux (lx) – lumens per square meter.
• Footcandle (fc) – lumens per square foot.
• 1 fc = 10.7639 lx

Since illuminance is specified per unit area, the size of the room does not matter – an illuminance of 50 fc has the same meaning for offices of 500 square feet and 2,500 square feet. The difference is that the larger office needs more lights to reach 50 fc. The IESNA Lighting Manual provides design illuminance values ​​for each occupancy classification.

In real lighting projects, illuminance varies due to luminaire spacing and beam shapes. However, slight variations are acceptable unless some areas are too dark or too light. Illuminance is determined not only by lighting distribution, but also by room characteristics such as ceiling height and surface colors. Manual lighting calculations are complex, but the process is automated with software in modern designs.

Photometry: What is the shape of the lamp beam?

The shape of the light beam is another important aspect considered by lighting designers. For example, spotlights focus their output into a narrow, downward-facing beam, while troffers spread their output over the largest area possible for even illumination.

• Do not assume that two lamps can be used for the same application just because their bases are the same shape.
• Bulbs with the wrong beam shape will result in uneven lighting even if the total lumen output is correct.

The shape of a lamp's beam is three-dimensional and can be simulated by lighting design software. Lighting cut sheets describe the beam with 2D shapes, parallel and perpendicular to the lamp, since a 3D beam cannot be represented completely. However, product models used for software have the complete beam simulated in 3D.

Color temperature and color rendering index

Two metrics are used to describe lighting color performance – one for the light source and one for the objects it illuminates.

• The correlated color temperature (CCT) describes the color of the light source itself. Each CCT has different applications and no color can be considered “the best”.
• The color rendering index (CRI) describes how faithfully the light source reproduces the colors of objects and surfaces. The maximum CRI is 100, describing a light source that matches the quality of sunlight – a higher CRI is always better, regardless of the application.

Using temperature values ​​to describe lighting colors

Objects glow a characteristic color depending on the temperature, which is why lava from a volcano appears 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 with an abstract concept called “black body”, which is an object that only emits light when heated, and each temperature corresponds to a specific color.

The bulbs are not heated to the correlated color temperature, but it is a convenient way to assign a number to their color. If a lamp has a CCT of 4000K, it will glow the same color as a “blackbody” at 4000K, but the light source itself does not reach that temperature. In most residential and commercial buildings, the CCT ranges from 2700K (yellowish white) to 6500K (bluish white).

The ideal lighting color depends on personal preferences, but the following principles apply to most lighting designs:

• Low color temperatures like 2700K are considered “warm” and tend to have a relaxing effect. They are preferred in areas such as home bedrooms, hotel rooms and high-end restaurants. Warm colors are not suitable for commercial and industrial environments, where the relaxing effect can be counterproductive.
• High color temperatures like 6500K are considered “cool” and tend to have an energizing effect, raising awareness. They are preferred in applications where maximum visibility is required, such as high-precision manufacturing. Cool colors can delay sleep when used in homes and apartments, and some people may become stressed by these colors after long exposure.
• CCT values ​​around 4000K are perceived as “neutral” and considered balanced. Neutral white is the ideal lighting color for offices, classrooms, kitchens and other areas where people congregate for long periods.

Effect of light source on objects and surfaces

Even if two lamps have the same CCT, the lighting quality may differ depending on the color rendering index. A CRI of 100 means the light source is as good as sunlight.

• Despite their low energy efficiency, incandescent and halogen bulbs have a CRI of 100.
• Fluorescent bulbs tend to have the lowest CRI values, and low-quality bulbs fall below 70.
• CRI values ​​for LED bulbs can vary greatly depending on the quality of the product. Low-cost products fall below 70, while high-performance LEDs reach values ​​close to 100.

The minimum CRI for an ENERGY STAR bulb is 80, which means you should look for labeled bulbs to get high-quality lighting. The label also means that a lighting product is trustworthy as it has undergone rigorous laboratory testing.

When dealing with LED bulbs, a higher CRI typically comes with a higher price. However, there are places where the CRI metric is very important, such as art galleries and shopping mall stores.

Conclusion

LED upgrades can reduce lighting costs by 30-90% in most cases, depending on the types of bulbs replaced. In air-conditioned spaces, the reduction in LED heat production also results in small cooling savings.

It may be tempting to simply replace your existing bulbs with the most efficient LEDs available, but lighting design should not be overlooked. Electricity savings are welcome, but should not be achieved at the expense of quality.

If you have a New York property covered by Local Law 88, you must update lighting systems to the Energy Conservation Code by 2025. However, you can achieve a higher ROI by exceeding the efficiency level required by the code , by using the opportunity to improve the quality of lighting.