Light quality and the role of color temperature

There has been a lot of confusion as to what constitutes “light” and how various man made “artificial lighting sources” affect human’s ability to see in the dark.

Researchers at the University of California, Berkeley have been working on a “cross-over chart” to compare various lighting of different light spectrums and how it relates that to what “the eye” can perceive, given one source as opposed to another. This is to enable lighting design engineers to call out “adequate lighting” that takes into consideration color temperature instead of simply, the physical output measured in lumens, lux, or candlepower. Currently only half of the light equation is being considered.

Full-spectrum white light, when reflected off objects makes the objects appear brighter and more visible to the human eye at greater distances than monochromatic HPS (high pressure sodium) or LPS (low pressure sodium) light sources measured at identical lumens (or candlepower) outputs.

Measuring any light by its lumen output simply depicts what the lamp can produce at its source. Doing so, would be the same as measuring the brightness of any light source by using the lamp’s electrical current consumption in watts to describe the degree of brightness of any lamp. This of course, is not a good indication of its lighting performance.

The most important property in lighting is how much light reaches an object (Reflected light bouncing off an object.) Not how much light is being generated at the source. Hence what affects the perception of “reflected light” makes it more important in the majority of lighting applications than the lamp’s physical light output if viewed directly.

The degree of visual acuity and how it affects perception is influenced by the color spectrum the object is viewed at. This phenomenon is referred to as scoptopic vision.

Most people are not aware that the color of light is a factor of “color temperature” measured as “degrees of Kelvin” as in nature’s full color spectrum light – the sun.

Natural sunlight at noon has a color temperature of 5,250 degrees Kelvin.

In the morning or at sunset when the sun rays have to penetrate more air in the atmosphere as it is angled low on the horizon, it appears red or orange. This is measured at the low color spectrum of around 1,000 to 2,000 degrees Kelvin. Hence this is the reason why humans have more difficulty seeing details whilst driving at dusk when light is reduced & reddish in color (monochromatic) as compared to noon-time sun when the light is rendered in “pure” white. When viewing printed colors, it is also more accurate viewing it at noon, than say early morning where the light is hinging towards the blue spectrum or evening in towards the red.

Hence if an “artificial light” can be engineered to emulate that of natural sunlight, then what will be perceived is naturally more visible – thus making the light “appear brighter” when in actual fact if measured physically using a photometer, produces less lumens than a monochromatic light at higher lumens. This is a result of the cones & rods in the human eye that aligns itself better to absorb more details at the mean temperature produced by the sun – natural light. The converse of such would be nocturnal animals whose eyes are designed to view details in low light and in the blue spectrum.

Last but not least, with higher lumen outputs, glare is also generated and such is counter-productive to “visual acuity.” It even becomes a hazard when driving at night as “glare” reduces the amount of light that enters the eyes of the driver as the iris of the eye automatically closes to a smaller aperture to protect itself hence farther reducing visual acuity. This is counterproductive – despite the fact that it appears to be “brighter” on paper because of the higher lumen output; you are not seeing more but less.

This is a problem reflected in the current crop of conventional street lighting using HPS (high pressure sodium bulb) that produces light at 2,200 degrees K. or SOX (low pressure sodium) lamps at only 1,700 degrees.

This phenomenon is still being studied by lighting researchers in order that visual acuity measurements can be developed and used instead of simply relying on Lumens to determine what is “adequate lighting.” Few lighting engineers around the world are even aware of such a phenomenon. They are still measuring light by using the old method; solely applying physical light output (i.e. lumens, lux or candlepower) without taking light frequencies into consideration for any specific application. Until a “cross-over chart” can be developed to factor in color temperature against physical light output, most lighting engineers would still be essentially designing “lighting applications” somewhat in the dark.

Recommended Lighting Levels for Exterior Lighting
From the International Dark-Sky Association — Information Sheet 77

The Illuminating Engineering Society of North America, (IESNA, or IES) in its current IES publications provides recommended illumination levels for outdoor lighting. We summarize some of these recommendations below.

It is important to note that these values are recommendations, not standards. Standards are set at the federal, state, county, or community level.

We give below a summary of the lighting illuminance levels in foot-candles, as that is the most common unit used in the United States. However, lux is the preferred international unit, and the IES does officially recommend its use in the United States. A level of ten lux is about one foot-candle (10.76 lx = 1 fc, exactly).

The Illuminating Engineering Society of North America, (IESNA, or IES) in its current IES publications provides recommended illumination levels for outdoor lighting. We summarize some of these recommendations below. It is important to note that these values are recommendations, not standards. Standards are set at the federal, state, county, or community level. We give below a summary of the lighting illuminance levels in foot-candles, as that is the most common unit used in the United States. However, lux is the preferred international unit, and the IES does officially recommend its use in the United States. A level of ten lux is about one foot-candle (10.76 lx = 1 fc, exactly).

IES also states that the “Low” values are appropriate wherever there is a requirement to maintain security at any time in areas where there is a low level of nighttime activity.

Insolation is a measure of solar radiation energy received on a given surface area in a given time.
The insolation into a surface is largest when the surface directly faces the Sun. As the angle increases between the direction normal to the surface and the direction of the rays of sunlight, the insolation is reduced in proportion to the cosine of the angle. This is known in optics as Lambert’s cosine law. This ‘projection effect’ is the main reason why the polar regions are much colder than equatorial regions on Earth. On an annual average the poles receive less insolation than does the equator, because at the poles the Earth’s surface is angled away from the Sun.

Use the map below to identify the yearly average solar Insolation levels in your area.

International Dark-Sky Association (IDA)

(www.darksky.org)

Mission

The mission of the International Dark-Sky Association (IDA) is to preserve and protect the nighttime environment and our heritage of dark skies through environmentally responsible outdoor lighting.

Goals

1. Stop the adverse effects of light pollution, including:
   • Energy waste, and the air and water pollution caused by energy waste
   • Harm to human health
   • Harm to nocturnal wildlife and ecosystems
   • Reduced safety and security
   • Reduced visibility at night
   • Poor nighttime ambience
2. Raise awareness about light pollution, its adverse effects, and its solutions; and
3. Educate about the values of environmentally responsible outdoor lighting
4. To promote responsible legislation, public policy, and standards

Background

The International Dark-Sky Association (IDA) seeks to preserve dark skies worldwide for the benefit of society by promoting good outdoor lighting practices and educating the public on the rewards of preserving the stars. Light pollution is an increasing problem, threatening astronomical facilities, ecologically sensitive habitats, our energy consumption, and our human heritage. IDA was founded in 1988, by Dr. David L. Crawford and colleague Dr. Tim Hunter, to address the growing problem of light pollution.

The IDA has published a number of articles and helpful guides and a quarterly newsletter highlighting the leading topics in dark sky preservation. This is in addition to award winning and highly popular meetings and events plus a number of programs and features, such as the Fixture Seal of Approval Program (FSA) or the International Dark Sky Communities (IDSC), Parks (IDSP), and Reserves (IDSR) Programs.

IDA incorporated in 1988 as a tax-exempt non-profit organization operating exclusively for educational and scientific purposes.

SolarPath and IDA

SolarPath HighLight Series ML fixtures have been officially approved by the IDA as DarkSky compliant meaning they do not cause light pollution. click here to visit the SolarPath products page on the IDA website.
Planet earth at night – bright areas are a result of light pollution

USGBC – LEED
The U.S. Green Building Council is a 501(c)(3) non-profit community of leaders working to make green buildings available to everyone within a generation.

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