Many people would argue that the healthiest lighting at night is no lighting at all. Studies are revealing that biological rhythms are offset, sleep patterns are disrupted, even breast and prostate cancers are more likely with disrupted circadian rhythms, due in part to improper lighting at improper times of the day.
The human race evolved under both light and dark. The light-and-dark cycle sets our circadian rhythms and is responsible for our good physical and mental health. Relying on the sun, moon, and stars has for centuries provided us with high-quality, healthy lighting. That’s why there is such an interest in daylighting our buildings, not only to save energy, but to put us in touch with a natural light spectrum that changes throughout the day and provides us with healthy lighting. However, we sometime need to augment this cycle at times when tasks must be performed and there is no available “natural” light.
Exterior lighting provides useful illumination at night mostly through electric means, and there are a plethora of electric light sources available to light our cities, towns, and campuses at night. What’s the best choice? Unfortunately, that question is far too many times answered by “what’s the cheapest?” – cheapest to purchase, operate, and maintain. “What’s the most energy-efficient?” is another, more admirable, question.
Both metrics are easily quantifiable and, as a result, are used almost exclusively in decisions about what light source to use. But since we are primarily lighting for humans, we should be asking “what’s the healthiest lighting to provide at night?” This is a more difficult question to answer. We may find that the healthiest lighting at night is no lighting. But, if we determine that some sort of illumination should be provided for some given task, what kind of light is best?
Most of America’s highways, streets, and pathways are illuminated with high-pressure sodium lamps, a yellow, monochromatic source. Many lighting designers, though, prefer metal halide over sodium vapor due to its “whiter” color and superior color-rendering properties. However, designers have had a tough time justifying this qualitative aspect when compared to the efficiency and long life of sodium.
Then, a number of years ago, studies started to show that cool or bluish light (white light with relatively high color temperatures, 5000K to 6000K) improved visual acuity in off-axis seeing tasks. There was even discussion about measuring this benefit for exterior lighting applications, since off-axis tasks were very important for good nighttime vision, (scotopic) driving, and walking. Finally, designers had a scientific reason for recommending metal halide over sodium for ‘people-centric’ tasks.
On the other side of the coin is the research saying that blue light at night is bad. Studies are starting to reveal that exposure to light with higher concentrations in the blue spectrum at night may actually be bad for our health. Apparently, it is especially harmful to people prone to macular degeneration. Cool blue light in the morning and during the day, at high levels of illumination, is crucial in setting our circadian rhythms by producing serotonin in the body.
Conversely, warm-colored light at night does not suppress melatonin, which is needed for proper sleep cycles. This is why some claim that night lights should be amber or red so as not to disturb sleep patterns through the night, or that people should stop working on their computers or watching TV at least an hour before going to bed, because of blue-rich light emanating from the visual displays.
And then there is the psychological side. Most people prefer a warm tone for low levels of illumination. It feels more natural. The sun gets warmer closer to sunset. Firelight has a nice warm glow. Dimming an incandescent lamp warms its color temperature. Kruithof’s amenity curve reinforces the notion of relating color temperature to the illuminance level.
Just to complicate matters, many exterior applications are beginning to embrace the burgeoning technology of the LED. It seems to make sense from a maintenance standpoint, since LEDs have a very long lamp life. However in order for these diodes to be very efficacious, they must be in the very cool or blue end of the spectrum, about 6000K. When this color temperature is used for outdoor applications of 10lux or less, the resultant lighting system looks very unnatural, not to mention what it does to skin tones.
On the Boston Common, like in many communities across the nation, there is a mock-up of several different styles and manufacturers of LED pedestrian lanterns. One evening, as we were observing the differences between the luminaires, we received an unsolicited opinion from a passer-by who commented on how the warm-white LED lantern looked the best. I tend to agree. The blue light at night simply looks unnatural. I often wonder why induction lighting is not more common for exterior lighting applications – the color temperature and rendering properties are superb, and it is rated at twice the life of most LED systems.
Ultimately it comes down to good lighting design:
1. Identify what needs to be illuminated and what can remain dark, in order to create useful contrast and manage energy usage wisely.
2. Highlight features to reinforce a hierarchy of events and provide orientation for the user.
3. Arrange light sources in clear, understandable patterns to create optical guidance for wayfinding.
A comprehensive nighttime visual environment must provide safety, foster a sense of security, be convenient for performing tasks, and appear aesthetically pleasing. When it comes to choosing the source, it should have a high color-rendering index and a nice warm color somewhere in the 2700K range. And, remember the rule of “everything in moderation”! Providing low levels of well-considered lighting will reveal the environment to the user much more effectively than flooding an area with high levels of potentially glary light, and chances are we’ll all be healthier for it. We won’t be breathing air that is polluted by power plants used to produce electricity to power exterior lighting, and we won’t be subjected to luminous energy that disrupts our biological rhythms. A win-win situation for everyone!