Caveat Metrics

May 11, 2011 / no comments


Daylighting metrics are methods for measuring the quantities of daylight in a space during a period of time. More and more, metrics are becoming the dominant means by which daylighting in a space is evaluated. With the imminent adoption of the International Green Construction Code and other codes mandating daylighting, the use of metrics will become even more integrated into the daylighting evaluation of buildings. While evolving analysis tools provide new and exciting capabilities, they also present new challenges to the designer or consultant.


Metrics have the inherent benefit of providing better information on the performance of a space than traditional rule-of-thumb methods. They are fast, adaptable, and instill confidence in the client, and the flexibility of digital modeling allows many variations of a design to be tested quickly at early stages of the design process. Unlike rules-of-thumb, metrics are more easily capable of evaluating non-orthogonal spaces, and they are becoming more accessible as more and more software provides daylighting analysis tools. And if that were not enough, increasingly, clients demand to see statistics and false-color grids in order to be convinced that their building will perform well, achieve credits, or meet codes.

But, like all things, metrics have downsides: metrics can be deceptively convenient. It seems as if it should be relatively easy to just build or import an architectural computer model into a simulation program and run the metric, but this is not so. Each software has its own rules for producing correct output. These include ways in which geometry should be modeled, whether or not to include a ground plane and how to define materials. Different lighting simulation engines have different ray-tracing methods (e.g. backwards versus forwards), and different simulation settings. The same basic variable likely has a completely different name from one program to another, and of course, the software interfaces are different – certain programs allow control over lighting variables, while other programs keep the user from accessing or modifying those variables.

Conversely, one benefit to the increased focus on daylighting metrics is their increasing accessibility. Plug-ins like DIVA-for-Rhino and the su2rad script allow widely used softwares like Rhino and Sketchup to interface with Radiance, the premiere calculation engine. While this overall accessibility is positive because it allows daylighting analysis to be employed more freely, making it more of a player in design decisions, it also makes education about the proper use of those metrics much more important.

The first step in understanding metrics is to know what metrics currently exist and what information they can provide. A useful guide to daylighting terminology was provided by Kevin Van Den Wymelenberg in an Architectural Lighting article in 2008, where he defines several of the daylighting metrics currently most in use today: Illuminance, Daylight Factor (DF), Daylight Autonomy (DA), Continuous Daylight Autonomy (CDA), and Useful Daylight Illuminance (UDI).

As a quick overview, the main distinction between various metrics is between the so-called “point-in-time” (Illuminance, Luminance) and annual, climate-based calculations (DA, CDA, UDI). Point-in-time calculations measure light levels at a specific date and time, under a specific sky condition. These calculations are more intuitive because they mimic how we experience the world: we see the light levels change from one moment to another. Annual or climate-based calculations, on the other hand, use weather data to simulate lighting levels over the length of an entire year. As such, they are more comprehensive than point-in-time metrics, but are also a more abstract, less intuitive way of measuring lighting. While they provide a more comprehensive performance evaluation, they may not show as clearly why one scheme performs better than another. Daylight factor, which originated in the cloudy climate of Britain, is neither point-in-time nor annual, as it uses an evenly illuminated (overcast) sky condition to measure interior-to-exterior light ratios.

Once designers have some idea of which type of calculation to use, they are faced with the issue of whether or not they can use it. Currently, the majority of lighting calculation software provides only illuminance and luminance calculations on a point-in-time level (for example, a clear day on September 21st at 9:00 AM). In general, there is a movement towards using annual, climate-based calculations rather than point-in-time, but the critical issue is that most commonly used daylighting programs do not support climate-based metrics. At present, 3dsMax and AGi32 only calculate illuminance and luminance (point-in-time). Daysim is the only widely used lighting engine which can perform the annual calculations.

The given metric may not really deliver answers to the questions at hand. From an architect and owner’s perspective, there are usually several critical questions posed to the consultant about daylighting. The first two are: how often will we be able to dim or turn off the electric light, and how will daylighting affect thermal performance? Currently, there is no good metric to directly answer those questions. Christoph Reinhart and Jan Wienold have developed one metric, called Daylight Availability, which perhaps comes the closest. In their paper “The Daylighting Dashboard – A Simulation-Based Design Analysis for Daylit Spaces,” they document the metric. It combines DA (Daylight Autonomy) and UDI (Useful Daylight Illuminance), and shows, in one false-color grid, the assessment of areas that are likely to be overlit (requiring shading), well lit by daylight alone, or partially daylit (requiring supplemental electric light). It is possible that this metric, or one like it, could fill the void.

The final part in the daylighting metrics process is the output. Once a metric has been chosen and run, the programs produce either a rendered image, a false-color image, or a grid of numbers as a result. The job of the daylight analyst is done, right? Of course not. This step can be the most challenging of all. Expressing daylighting analysis results in an intelligible way, and presenting them to a client can be difficult. There is no formula for the best way to do it, and it often comes down to what the particular situation requires. The fact is that it is difficult to synthesize in a single image the variability of lighting conditions over the day and year, and when multiple design options like shading devices, materials, or orientations are added, the complexity expands proportionally. Given this, there is a tendency to become metric-happy and produce copious studies for different times and under different conditions; this often overwhelms the client who, unfamiliar with the format, may barely understand a single false-color grid, let alone a set. Even for sophisticated daylighting designers, the useful conclusions may be hidden in the sheer mass of output.


Outputs produced with DIVA-for-Rhino

There is no single metric which can answer all questions; each provides only part of the story. Annual calculations provide information about lighting levels, but not about glare, thermal costs, or aesthetics. One idea beginning to gain acceptance as a solution is the concept of a “dashboard”. Dashboards, as laid out by Reinhart and Wienold, are meant to show summary results of many metrics in a single side-by-side view, although, it should be noted, that synthesis is still left to the consultant.


Reinhart and Wienold, “Daylighting Dashboard” concept image

Lastly, the architect’s and owner’s question, “What will it look like?” still prevails. False-color grids and numbers don’t read as quickly as does an image, and after all, a large part of the value of daylighting design is improving the visual quality of the space. Images may contain the least amount of hard data, but they tend to go the furthest in illustrating daylighting concepts to clients.

As we enter this new phase of daylighting analysis, it is important to know the strengths and shortcomings of each metric and to be informed as to how to properly use them. The increased predominance of the computer does not change the fact that it is the designer who must know how to use the tools, how to understand the results, and how to effectively communicate the results to team members and clients.

5_DIVAforRhino Visualization_KLagios.jpg

Radiance Visualization using DIVA-for-Rhino

Images credit: Kera Lagios(1-3,5), Christoph Reinhart and Jan Wienold (4)


Basic Sustainable Lighting Concepts: On Building Design

April 25, 2011 / no comments

Although they say there are no bad ideas, here are a few good ones regarding lighting to help you navigage the greenwash out there and get to the real facts. This is the first part of an ongoing series outlining design principles for sustainable lighting design.

Thin buildings need less help

The thinner your building is, the less it will need to rely on artificial life support systems like HVAC and electric lighting to operate. Standard windows can light twelve to fifteen feet into a space. Windows with a daylight control and delivery system, like a light-shelf, can push it even further, up to thirty feet in some instances. More daylight = less electric light.

Orient your building east-west

The path of the sun has changed little over the past few millennia. By now we have a pretty good idea of where the sun goes and of the most effective methods for using that sunlight. Of course, east- and west-facing windows get sunlight, but only for half the day. If the major axis of a building is oriented east to west, the southern exposure will be able to harness that energy almost all day long – if designed correctly (not too much, not too little).

Easy does it on the glass

High-performance glass is a wonderful thing, but it’s still no replacement for a solid wall, in terms of insulation and reasonable cost. In these energy-crunching design times, we need to optimize our building designs so they accept just enough daylight and reject the rest. Too much glass and you could end up with heat-gain and heat-loss problems and glare issues. Too little and you could have a cave.

Bring up that window sill, too! The glass that extends to the floor has little practical value except aesthetics, which is a debatable, fickle thing. There’s nothing like seeing that trash can pushed up against the glass…

Lower partition heights and fewer offices, please

Private offices are sought-after the world over by the power climbers, but they stink for utilizing the space as well as possible. Consider opening up your office design to more community spaces, putting the bosses right out there with everyone else. Private spaces will still be necessary, but limit them.

Workstation heights have to come down too. It’s kind of a corporate slap in the face to be given an office without a door or window – you sit at the bottom of a cubicle well all day. By lowering the heights of the partitions, you open up people’s views to perimeter glass, let the daylight penetrate deeper into the space, and encourage more interaction and camaraderie.

Lighten up!

How would you like being told what your favorite color is? People take it very personally – designers especially. Whatever your preference is, we, as lighting designers, respectfully ask that you pick light colors with higher reflectances. How building surfaces reflect light has a lot to do with how the space feels, either with daylight or electric light. If the finishes are too dark you create a cave, and then need to pump in way more energy to light the space adequately.


A Daylighting Pattern Language: Bilateral Lighting

February 9, 2011 / no comments

In Christopher Alexander’s book A Pattern Language he points out: “When they have a choice, people will always gravitate to those rooms which have light on two sides, and leave the rooms which are lit only from one side unused and empty.” He touts that “this pattern, perhaps more than any other single pattern, determines the success or failure of a room.”

Why is this? Alexander goes on to point out that his experiments had been rather informal and drawn out over many years. But the trend is very real. He also recalls that light on two sides was a tenet of the Beaux-Arts design tradition. What is it about certain patterns of light that attract people or enhance space and volumes effectively? Looking to the past may be the best way to design daylighting for the future.

In last September’s entry, ‘A Daylighting Pattern Language’, Robert Osten used Le Thoronet abbey as an example of how well small apertures were designed for introducing daylight into the interiors of large masonry constructions. Before the use of sophisticated computer models and analytical studies, architects based their designs on common sense, common practice, and a basic understanding of the relationship between architecture and the sun. Why are some spaces much more successful than others? When it comes to light, and especially daylight, it’s a human response: what feels good.

Let’s examine some reasons why a room or space with bilateral light is more successful than one with light only from one side.


The first, seemingly obvious, reason is brightness balance and contrast ratios. For example, sitting in a space and talking to a person in front of a window without the benefit of light on their face (from another window or skylight on the opposite side of the room) is not only uncomfortable, but since their face is in silhouette, it is difficult to read their expression, giving us little or no information about their mood or response.

In work environments, the contrast ratio could cause discomfort, and result in eye fatigue. R. G. Hopkinson in his book Architectural Physics: Lighting, published back in 1963, discusses the issues of glare and contrast in great detail, showing how it affects much more than simply the quantity of light. Today, from simple evaluations to exhaustive studies, we continuously find proof that high contrast and glare will affect productivity in the workplace.

Not only is bilateral lighting preferred for being more comfortable, but it instinctively feels more natural. Balanced light – light coming from more than one direction – is more akin to ‘natural’ light. In the great outdoors we have both direct sunlight and light from the sky itself – light coming from all different directions. This helps provide depth, giving us clear information about shapes and forms. A strong light source from one direction tends to flatten our views, providing less visual information. Dramatic, but a bit unnatural, and uncomfortable over long periods of time.


Notice how the unlit wall, relative to the illuminated walls on either side,
looks dim and cavernous in contrast with the daylit atrium above.

Since many large or multi-story buildings tend to introduce daylight through sidelighting, it is critical to balance the overall lighting in the space. The more daylight introduced from one side, the more light it will take from the opposite side to offset cavernous effects. Of course, introducing more daylight through an additional window or clerestory is the most effective approach since it will maintain the same exterior intensities. If daylight cannot be provided, supplemental electric lighting should be designed to fill in the gradients where daylight is lowest. Washing walls and surfaces with light and using light colors will increase the perception of balanced light in the space.

The relationship between daylighting apertures and electric lighting is key. There is a lot to be learned from historic, daylit buildings. We should not ignore those patterns throughout the history of great architecture that have always met with positive responses from the humans who use them.


Photos: © Lam Partners

A Daylighting Pattern Language: Deep Apertures

September 27, 2010 / no comments

Le Thoronet is one of three wonderful Cistercian abbeys in Provence, built around 1170. In the mid-twelfth century this part of southern France was not a major tourist destination. The monks who built Le Thoronet were avoiding the political intrigues and feudal power struggles of the cities by locating in a remote area, and they weren’t necessarily welcoming company. And they were building for eternity, too, so the walls are thick, sometimes over three feet thick. As with a lot of ancient masonry construction, this has a salutary effect on the way daylight works in the interior. Why?

In contrast to today’s vogue for all-glass buildings, how is it that massive masonry construction can result in wonderful daylighting? This has a lot to do with contrast control, which is related to the deep apertures created through the thick walls.

Because sunlight is such a powerful light source, a major challenge with daylighting is to moderate the contrast between very bright exterior views and the relatively much darker interior surfaces. In particular, the interior face of the wall containing the window often tends to be the darkest surface in the entire space, since it may receive no direct daylight at all. This can result in very harsh contrast at the apertures. But, when the aperture has depth, the sides of the opening provide extensive surfaces with a brightness that is intermediate between exterior and interior, graduating the contrast.

2_Le Thoronet_Nicola Comodo.jpg

Without this kind of buffer, the contrast is often more than the eye can comfortably accommodate. If splay or architectural ornamentation is present in that zone, the contrast gradient is even more improved; the ornamentation itself is beautifully rendered by the raking light and brightness gradient from exterior to interior.

In addition, those surfaces, especially the sill, diffusely reflect daylight into the interior – for example, illuminating the ceiling even though most of the original daylight source is heading for the floor.

3_Le Thoronet_Glen Craney.jpg

These are daylighting principles we would do well to emulate in our designs today. We’re rarely going to have walls three feet thick to work with, but we can accomplish similar effects by, for example, positioning our apertures against flanking walls or piers. In this house by Tadao Ando, the room surfaces perpendicular to the apertures have a brightness intermediate between the view outside and the darker interior surfaces. In addition, they diffuse daylight back onto the inside surface containing the aperture, which further softens contrasts.

4_Ando house.jpg

The deep aperture approach lies in stark contrast to just treating daylight apertures like simple holes in the wall. Besides improving contrasts, the deep aperture uses daylight as a powerful expression of the extension of space.

5_Ando house.jpg

Photo Credits: Betina (1), Nicola Comodo (2), Glen Craney (3), Lam Partners (4-5)

Dawn of the Daylighting Codes

December 21, 2009 / no comments

It’s pretty safe to say that people like daylight and sunlight. Daylight is good for people, since it sets our biological rhythms, gives us a connection to the weather and time, keeps us physically and mentally healthy, and obviously allows us to perform visual tasks. It’s no wonder then, that architects through the ages have designed architecture to effectively introduce sunshine and daylight into building interiors – not only to sustain human life, but to allow it to flourish.

Daylighting has been an integral part of the built environment throughout architectural history, and structures that are thousands of years old are still revered for their daylighting qualities. “The history of Architecture is the history of man’s struggle for light – the history of the window,” wrote Mies van der Rohe.

It’s only within the last 75 years or so that daylighting has been supplanted by electric lighting as the primary source of interior daytime illumination. Ever since the introduction of air-conditioning, and especially of modular gas-discharge lighting (i.e. modern fluorescent lamps), windows and skylights have been getting smaller and floor plates have been getting larger. Our luminous environments have been deemed adequate and appropriate based on a simple numerical criterion, horizontal footcandles. However, in recent years, especially with the ‘green’ movement, there has been much more pressure to re-introduce daylight back into our interiors and create daylit architecture once again.


But what exactly is ‘Daylit Architecture’? It’s difficult to define. For architects it may be about beauty and ergonomics; for engineers it tends to be focused on energy and economics. Fortunately, with recent studies, we finally have hard evidence showing that daylight in schools improves test scores, and daylight in the workplace improves productivity. In retail, it boosts sales; in hospitals, it reduces recovery time. These studies embolden the stance of the ‘quality’ seekers.

But, on the other side are the energy tyrants. They want to see fewer windows in architecture since windows are terrible insulators. The criticism is real. News stories are unfolding about LEED buildings and how they are not living up to their touted energy claims. But the LEED points for daylighting and views have nothing to do with saving energy. It’s all about interior environmental quality.

So now, there is a bigger push to improve energy usage and enforce ‘green’ building codes. LEED, CHPS, and other programs give you the option of getting daylighting points. A ‘green’ code will require it. There has been overwhelming support for some type of daylighting requirement or code, but the problem seems to be in writing one. Most would agree that, if introduced properly, daylighting can save energy associated with interior illumination. The more difficult aspect is quantifying quality. How do you require architecture to beautifully introduce daylight and sunlight into itself?

Codes requiring access to daylighting are relatively new to the United States. Title 24 in California already requires daylighting in certain buildings. There’s a rich history of codes requiring access to daylight. An English law dating back to 1663, Ancient Lights, is a form of easement that gives owners of a building with windows a right to maintain access to daylight. Justinian Code in the sixth century AD included sun rights, laws to ensure that every homeowner had reasonable access to the sun. And, many modern European codes require daylight and views for workspaces and classrooms.

Get ready for daylighting codes across the United States. Come late spring 2010, ASHRAE will have introduced its new Standard 189.1, which is basically a ‘green’ standard that goes beyond the energy-saving measures published in ASHRAE Standard 90.1. It also contains a lot of language about minimum amounts of windows and required illuminance from daylight. The other big player is the International Code Council, with their new proclamation, the IgCC, or ‘International Green Construction Code’. In that particular code, the daylighting portion will most likely be broken into two sections: energy and indoor environmental quality. This approach makes the most sense for both camps. We want enough daylight and views to elevate the human spirit, but not so much as to cause glare or unnecessary energy usage associated with excessive cooling loads.

It won’t just be footcandles and daylight factors anymore. Relatively new metrics such as Daylight Autonomy, Daylight Saturation Percentage, Useful Daylight Illuminance, and Daylight Glare Probability may become common language within these new daylighting codes.


It’s probably time that we have some sort of code that protects and even encourages our access to our greatest energy source, the sun. How it is written makes all the difference. It cannot reward poor design, or suffocate good design.

Great daylit architecture comes from the brilliant architects and designers who create it, not from a formula or code. But gone are the days of overly-glazed façades used in the name of ‘daylight’. Responsible practice must produce sustainable architecture, even if it has to be mandated.

Photo Credits: Elinnea (1), Roryrory (2), Stephen Lee (3), Lam Partners Inc (4)

Photo of the Month: June 2009

June 14, 2009 / no comments


Pantheon, Rome, Italy

The Emperor Hadrian, who commissioned Rome’s Pantheon (completed around 126 AD), was an architectural enthusiast. The building incorporated amazingly innovative technologies, so it’s not surprising that it also used the best available lighting technology of the day: daylight.

With the exception of a little light coming through the doorway, all of the daylight in the space enters through a single oculus at the peak of the dome. It’s about 30 feet in diameter, which sounds big, but if we take that as a fraction of the floor area below (142 feet in diameter), the oculus works out to be about 4.5% of the floor area. And that provides very comfortable levels of interior daylighting, in a space that’s also 142 feet high.

There’s no glass in the aperture – it’s open to the sky – so if we corrected for modern low-E insulating glass at 70% light transmission we’d get the same amount of daylight from an aperture of about 6.5% of the floor area. To also account for absorption by lightshelves, baffles, lightwells, etc., we might double it, to around 15%. That’s not a bad rough starting point for thinking about a modern daylighted building. And by the way, Rome’s not in “the south”: it’s at virtually the same latitude as Boston, so the sun-angles there would be the same also.

If Hadrian could have built an all-glass building, would he have chosen that instead? Would that have been better? We’ll never know, but I know my vote…

Photo Credit: stanrandom