Solar Lighting: Making Steps Off-Grid towards a Sustainable Future
Vision and color. Many studies have been done on how the human eye performs to different light levels and colors. Photopic vision uses cones of the eye and is generally referred to as "day" vision or vision under high illuminance levels with good color perception. Photopic vision is the basis for the development of light level criteria published by the IESNA. Even illuminance meters measure light based on a photopic response. Scotopic vision utilizes rods of the eye and is generally referred to as "night" vision. Scotopic vision like that of someone under a moonlit sky will have poor visual acuity and color perception. To evaluate human vision in conditions such as exterior night time lighting where light levels are higher than that of scotopic but less than photopic levels a third category is used. Mesopic or "dim light" vision uses both rods and cones. Studies have found that the spectral distribution of a light source can affect the perceived brightness of a space. Correlated color temperatures of 5000K to 6500K or higher (more bluish-white light) will be perceived as brighter because of the additional engagement of rods in the eye. In effect, identical streetscapes could be lit to the same illuminance levels with one site having a more scotopically rich color spectrum; in this scenario the scotopically rich site will always appear brighter. This concept is especially relevant to LED sources which are more efficient in higher CCT ranges.
Luminaire performance.Regardless of the light source technology used in a luminaire, all luminaires can be evaluated by the horizontal pattern of light (lateral distribution) and by the amount of light in vertical angles. A plan view of the lateral light distribution or pattern of light on a task plane can be organized into five main groups. Established by the IESNA and represented by roman numerals Type I through Type V, these distribution classifications give designers a quick idea of what the pattern of light from a luminaire will look like. For example a Type I distribution may provide the best distribution for a narrow bike path where a Type V distribution would be best suited for luminaires mounted in the middle of a large open parking area.
In addition to lateral distribution, in 2007 the IESNA revised the previous cut off classification groups that defined the amount of light in vertical angles. Luminaires once classified as full cut-off, cut-off, semi cut-off and non cut-off now use the Luminaire Classification System (LCS) published in the IESNA TM-15-07 document.
The Luminaire Classification System (LCS) defines distribution of light from a luminaire within three primary solid angles: back-light, up-light and front-light. These three primary angles are further divided into 10 secondary solid angles. The secondary angles are described as back-low (BL), back-mid (BM), back-high (BH), back-very high (BVH), up-low (UL), up-high (UH), front-low (FL), front-mid (FM), front-high (FH) and front very-high (FVH). The new LCS system allows for a much more detailed evaluation compared to the old system. This is beneficial to designers evaluating the environmental impact of a luminaire and will provide key data about light spill, light pollution and glare.
Example of lateral distribution types for exterior luminaires Source: IESNA Light Distribution Patterns RP-33-99 |
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The LCS added a back-light, up-light and glare rating system, otherwise known as BUG. The BUG rating system provides a rating of 0-5 which describes how much of the luminaire lumens are present in each solid angle. For example, B1-U3-G1 describes a luminaire with very minimal back-light, moderate up-light and very low glare. This BUG rating system provides designers with a quick indicator of a luminaires vertical distribution performance. A quality solar luminaire should have good control of light to make the most of the lumens produced without creating glare.