calculations: the word alone has a reputation of being both uninteresting and intimidating. However, lighting and its related calculations are a necessary component in any successful project, though more commonly a necessity in non-residential applications.
Generally calculations can be considered in two tiers, primary calculations (what is needed for the specific project) and secondary (what is needed to meet industry standards such as ASHRAE 90.1 and LEED). Additionally, calculations quantify light in two different ways: illuminance, ambient light measured in footcandles and watts; and luminance, the visual character of a space.
1. the complexity of the space determines the type of calculation. There are several project variables that dictate the quantity and detail of the calculations necessary. The calculations required to light a fairly simple room differ greatly from those required in more complicated spaces. Cambridge-based Lam Partners principal Keith Yancey explains, 'If it is a very basic geometry and simple task, we are usually just doing illumination calculations.' Illuminance is the result of light reaching a surface directly and by interreflections from surrounding surfaces. There are two types of illumination calculations: The first type is used to determine how much light reaches a surface point directly from a luminous surface, and the second type is used to determine how much light reaches the same surface or point from a secondary source of light, which is, luminous by reflection.
However, the complexity of the space is directly proportional to the type and quantity of calculations needed. 'When you are talking about public spaces or the exteriors of buildings like façade situations,' Jeff Gerwing of Detroit-based SmithGroup notes, 'then sometimes the numbers become a little less important in terms of illuminance levels. At some point, it becomes more about the perception of a space than what the numbers say.' However, it is always imperative that there is enough ambient and task light in the space.
Finding the appropriate balance between the qualitative and quantitative analysis is often a challenge. Vikram Sami of Lord Aeck Sargent Architecture in Atlanta offers an example of how these two methods work together. 'If you have 50 footcandles on a surface, usually you would be okay,' he explains. 'But if you have a surface next to it that is 1,000 footcandles, 50 footcandles appears really dark because the eye adjusts physiologically to the brightest spot in the room.' It cannot be assumed that the complete solution is solved once the quantitative analysis is calculated. Sami continues, 'Numbers are not the whole answer. You need to go further and analyze them experientially.'
2. computer technology aids the calculation process. For most designers, including Jim Nicolow, also with Lord Aeck Sargent, developing a project design happens intuitively initially, and 'then we iteratively ask the questions: Is it working? How is it working?'
While hand calculations are fairly straightforward for spaces with simple geometry and tasks, they are rarely used today. Given the prevalence of computers, most designers work with spreadsheet programs and lighting calculation software, including: AGI-Light, Lumen Micro, Lightscape, SketchUp, and Radiance, to name but a few.
Some computer calculation programs are formulated to perform illuminance calculations in which the main outcome is the required footcandle numbers and/or the watts-per-square-foot information. This is important, especially from an energy code perspective, but there is often a need to know more about the overall visual character of the space. This is where luminance calculations come into play. Luminance describes how light makes objects and surfaces appear, and includes the determination of contrast and luminance ratios.
For luminance calculations, Gerwing finds it imperative to use 3D lighting analysis tools. 'Not only can you get the technical information, but you can also get the actual image of what the space is going to look like. It becomes more about where the light is being placed and less about just a hard number.'
Of course, no program is without limitations. If designers are considering spaces with backlit elements, even 3D programs aren't always sufficient. For specific issues like this, both Yancey and Gerwing stress the importance of building physical, mockups, which have particular importance when working with daylighting.
3. do mockups, especially when calculating for daylight. 'Calculating artificial light is about as easy as it gets,' Gerwing explains, 'because you are talking about a static system. Obviously, with daylighting, you are talking about a dynamic system that is constantly changing throughout the day. It is an unending variable.' Because of this, a lighting designer cannot simply rely on one calculation for the space, as its results would only reference a specific time and day. The complexity of calculations increases tremendously with daylighting projects, although most lighting programs are sophisticated enough to derive some basic daylighting computations. Physical models, however, allow a designer to see how the daylight will interact with the room volume.
Generally, though, daylighting is not a stand-alone component of a lighting plan. Even in the most advanced daylit spaces there is a need for a supplemental electric lighting system. Yancey explains, 'We are still going to design our electric lighting to a certain level when it is dark outside. Most spaces will not be used only during the day. Even in late afternoon, when you have very little daylighting, the need for electric lighting is present.'
4. investigate special requirements for code and standard compliance. The tension between the functional and aesthetic aspects of calculating light becomes further obvious when a project must comply with codes. Requirements for state code compliance have become more stringent in recent years. Gerwing explains how 'Regulations have basically required states to lower the limits for allowable lighting power, and it is becoming a big issue in the lighting industry. It gets to the point where codes start to limit the wattage so much that you begin sacrificing aesthetics.' For Gerwing, the bigger issue is always visual, solving the problem functionally in a way that retains the aesthetic intention. Sami warns of relying too heavily on the calculations that are part of the LEED daylighting credit. He explains that the LEED credit requires a daylighting factor calculation, 'which honestly is not always a great solution. The daylight factor was developed for conditions in England, which are pretty cloudy. It's great if you are working in seattle, but for most places, it's not a great thing to go by. You can do the calculation for the LEED credit, but it is always worthwhile to do the simulations in one of the [more sophisticated] programs.'
Depending on the project, calculations can act as best practice guidelines or to reinforce code compliance. In either case, the process of lighting calculations provides a balance between aesthetic and functional considerations.
Jessica N. Johnson is an architect in Raleigh, North Carolina. She received her Master of Architecture from North Carolina State University, and is currently participating in a teaching fellowship at the school.