Lighting is an essential element in architectural design, significantly impacting aesthetics, functionality, and energy efficiency. The right illumination can transform a room, creating warm, exciting, and inviting atmospheres that enhance the overall experience of the occupants. Beyond its visual appeal, lighting also plays a crucial role in enabling occupants to perform tasks safely and comfortably, minimizing eye strain, and boosting productivity. In addition, properly designed lighting contributes to energy efficiency, making it critical to strike a balance between aesthetics and practicality in the design process.
Achieving this balance requires a deep understanding of key lighting metrics that inform effective design. While these metrics—luminous flux, illuminance, and luminance—are commonly used in the lighting industry, their theoretical foundations are not as widely comprehended. This blog aims to provide architects and lighting designers with a clear understanding of these three fundamental metrics, enabling more informed decisions during the design phase to optimize both aesthetic and functional outcomes.
What is Luminous Flux?
Luminous flux is the measure of the total amount of visible light emitted by a source per unit of time [1] is expressed in lumens (lm), which quantifies how much light is perceived by the human eye. Luminous flux describes the light output of a source itself. When discussing luminous flux in relation to fixtures, two terms are commonly used.
The first is “generated lumen”. Generated lumens represent the total amount of visible light produced by a light source as it is created (see picture below). This measurement indicates the light output without accounting for any losses that occur due to factors such as heat or absorption by the fixture. Essentially, it reflects the raw light-emitting capability of the source itself, often determined under ideal conditions in a controlled environment.
Another term is called “output lumen”. Output lumen, on the other hand, refers to the actual amount of light that is delivered to the environment after considering various losses. This measurement takes into account the efficiency of the light source and any light that may be obstructed or absorbed by the fixture or the surrounding environment.
There are cases where light sources with high generated lumens yield low output lumens due to losses during the emitting process. Consequently, output lumens are more practical and widely used in the lighting industry. Generally, a light source with higher output lumens is perceived as brighter than one with lower output lumens. Whether considering downlights or custom-length fixtures, understanding output lumens is essential for making informed decisions about the intensity and quality of light required for specific environments, such as task lighting in an office or ambient lighting in a hotel lobby.
Illuminance vs. Luminance: What’s the Difference?
Illuminance
Illuminance is a measure of the amount of luminous flux (light) that falls on a surface per unit area [1]. It is expressed in lux (lx) or foot-candles (fc), where one lux is equal to one lumen per square meter, and one foot-candle is equal to one lumen per square foot. This metric is critical in assessing lighting conditions, as it considers both the intensity of light and the area over which it is distributed. Illuminance is influenced by various factors, including the light source’s output lumens, the angle of incidence, and the surface’s reflectivity.
Illuminance is the most widely utilized metrics in lighting design, as it provides a standardized way to evaluate and ensure that spaces meet specific visual requirements. Different activities necessitate varying illuminance levels; for example, the Illuminating Engineering Society (IES) provides guidelines suggesting that office spaces typically require illuminance levels at desktop ranging from 300 to 500 lux (around 30 fc to 50 fc), while specialized tasks such as surgical procedures may demand levels exceeding 1,000 lux (100 fc) [2#1]. Moreover, the design process involves considering not only the quantity of light but also its distribution and uniformity across surfaces. Inappropriate illuminance can lead to visual discomfort, increased error rates in tasks, and even contribute to long-term health issues, such as eye strain and circadian rhythm disturbances.
Lighting design has traditionally posed significant challenges, as it relied heavily on experiential knowledge. Prior to the advent of advanced technologies, it was not feasible to obtain illuminance levels until the installation of lighting systems. However, significant advancements in software, such as AGI32, ElumTools (integrated with Revit), and Visual, enable designers to conduct photometric simulations with very high accuracy. The tools allow for the assessment of light distribution and illuminance levels within design spaces prior to construction. This capability has become a critical component of the design process, facilitating the optimization of lighting conditions and ensuring compliance with applicable standards and guidelines. Furthermore, many jurisdictions now mandate the submission of photometric reports as part of the permitting process, underscoring the growing importance of data-driven approaches in contemporary lighting design.
To verify if a space has appropriate illuminance levels after construction, an illuminance meter (also known as a light meter or photometer) is needed. The device utilizes a photosensitive sensor (shown
Photo by: Tech Equipment
as the white hemisphere in the photo below), often a photodiode or phototransistor, that responds to light across a defined spectrum. The collected data is processed and displayed digitally, providing users with precise illuminance readings. Advanced illuminance meters may incorporate features such as cosine correction, which accounts for the angle of incoming light, ensuring accurate measurements regardless of the light source’s position. Some models include spectral sensitivity adjustments, allowing users to measure specific wavelengths for specialized applications like plant growth or lab lighting. Data logging capabilities and connectivity options enable integration with software for comprehensive analysis and reporting.
Illuminance has a profound impact on the lighting industry by shaping how spaces are designed and utilized. Being aware of illuminance during the design process is essential for creating environments that meet specific visual and functional requirements. As the industry increasingly adopts data-driven approaches, understanding and applying illuminance principles will be key to developing smarter, more adaptable lighting solutions that cater to diverse needs across various sectors.
Luminance
Unlike illuminance, which measures the amount of light that falls on a whole surface, luminance quantifies the amount of luminous flux emitted, transmitted, or reflected from a surface in a given direction per unit area, expressed in units of candelas per square meter (cd/m²). It represents the brightness of a surface as perceived by the human eye and is influenced by both the intensity of the
light source and the surface properties, including color and texture. Lighting designers often rely on horizontal illuminance to establish appropriate light levels in various projects. However, there are situations in which horizontal illuminance does not accurately represent the light levels perceived by occupants within a space. In such cases, luminance provides a more effective measure for estimating brightness perception, as it accounts for the intensity of light emitted or reflected from surfaces as viewed from specific angles. When specific lighting requirements are necessary, luminance measurements can offer a more reliable assessment compared to horizontal illuminance alone. Therefore, incorporating luminance levels alongside horizontal illuminance can enhance the overall evaluation of lighting conditions, leading to more effective design outcomes that better align with user experience and the design objectives for the space.
To measure luminance, a luminance meter is needed. The instrument employs photometric sensors, often based on photodiode technology, calibrated to the spectral sensitivity of the human eye, typically following the CIE standard luminosity function. The device typically features a calibrated optical aperture, allowing for precise measurement of light from specific areas or angles, and incorporates features such as data logging, statistical analysis, and connectivity for integration with other measurement systems.
Photo by Konika Minolta
Conclusion
Understanding the interplay of lighting metrics is vital for architects and lighting designers seeking to create well-lit, inviting environments. Each metric introduced in this blog offers unique insights into the lighting dynamics of a space, emphasizing the need for a balanced approach that considers visual comfort, functionality, and energy efficiency. As the lighting industry increasingly acknowledges the importance of data-driven methodologies, considering these metrics during the design phase will significantly enhance the effectiveness of lighting designs. This comprehensive understanding ultimately leads to more adaptive and intelligent lighting solutions that cater to diverse user needs, ensuring that both aesthetic and practical requirements are met in contemporary architectural projects.
For expert assistance in achieving optimal lighting solutions, contact Light Plan Design today.
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References:
[1] Pentland, A. P. (1982). Finding the illuminant direction. Josa, 72(4), 448-455.
[2] DiLaura DL, Houser KW, Mistrick RG, Steffy GR. 2011. The lighting handbook reference and application, Tenth edition. New York (NY): The Illuminating Engineering Society of North America. 1328 p.