About 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

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).