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A well-designed lighting system provides just the right amount of light for its intended application: less lumens yield poor performance, but more lumens represent wasted energy. Like in any building system design, the optimal capacity can only be specified with an adequate calculation procedure. Lighting designers use the photometric data of proposed fixtures as a starting point. For the calculation procedure they consider the project's location, indoor or outdoor, as well as environmental conditions like temperature and dirt.

The purpose of lighting calculation is to reach a suitable illuminance level for the intended application. The concept of illuminance describes lighting delivered per unit of area, and it is typically measured in foot-candles (lumens per square foot) or lux (lumens per square meter). In the case of New York City, meeting the lighting efficacy requirements of the NYC Energy Conservation Code is also important.

Illuminance levels for each location are not arbitrary; they are established by industry organizations such as IESNA (Illuminating Engineering Society of North America). When specifying a lighting system, design engineers aim for the illuminance values established in the IESNA Lighting Handbook. Achieving the exact specified values is not feasible, but a lighting design is satisfactory if the variation is slight.

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Lighting calculations can be carried out manually, but this approach demands considerable man-hours and is impractical in modern building design. A more effective approach is using automated software calculations, allowing the lighting designer to focus on the best decisions, while a computer handles repetitive tasks.

The lighting design specialization is available for professionals from different backgrounds. For example, architects, electrical engineers and interior designers all have a knowledge base that is suitable to complement lighting design.

The Lumen Method: Basic Lighting Calculations
The lumen method provides a simple hand calculation approach to estimate the illumination achieved with a proposed lighting distribution. The method becomes impractical for complex room geometries or very large projects that are split into multiple areas. It can be summarized in the following steps:

Calculate the Room Cavity Ratio (RCR): The RCR describes the ratio of vertical area to horizontal area in a room. The RCR formula varies depending on room geometry.

Obtain surface reflectance: The reflectance is the fraction of light reflected from a surface, and it is strongly influenced by texture and colour. Reflectance values are required for ceilings, walls and floors.

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Obtain photometric data: When you purchase a lighting product, its specifications include total lighting output and its spatial distribution. This is fundamental information for lighting calculation procedures.

Determine the Coefficient of Utilization (CU): Based on the RCR and surface reflectance, it is possible to determine the CU, which indicates how efficiently the lighting output is delivered to the working plane.
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