NIGHT SKY AND DARK ENVIRONMENTS: BEST MANAGEMENT PRACTICES FOR ARTIFICIAL LIGHT AT NIGHT ON BLM-MANAGED LANDS TECHNICAL NOTE 457 63 5.5.3 Consider Melanopic/Photopic Ratio When Selecting Light Sources The melanopic/photopic (M/P) ratio is a metric for evaluating the biological effects of light on humans and most vertebrate wildlife, by comparing the proportion of melanopic light (light that affects biological function such as circadian rhythm) to photopic light (light that supports vision) for a given light source. The M/P ratio represents the ratio of light producing biological signals to the brain, compared to the light producing visual responses. The M/P ratio is expressed in a decimal form, for instance, 0.3 or 0.9. The lower the number, the smaller the light’s biological effects. The higher the number, the greater the light’s biological effects. However, biological effects of light also depend on the light’s intensity, duration, and timing. In general, lower M/P ratios and lower light levels are better for areas with sensitive species. If available, consider the M/P ratio when selecting light sources. Notes: Consideration of the M/P ratio provides a more accurate assessment of the biological effects of lighting, rather than consideration of only CCT. For example, because white light contains colors of many wavelengths, two light sources may both appear white to the eye but have substantially different biological effects. Increasingly, lighting manufacturers use the M/P ratio to better communicate the likely biological effects of lighting. However, at this time, M/P ratio is not widely used to describe commercially available lighting products. 5.5.4 If Necessary, Remove UV Light Content with Filters or Housings Reduce or remove the UV content of the light with UV filters or glass housings when the use of high-UV content lighting such as metal halide or mercury light sources cannot be avoided. Notes: Filters/housings may be unavailable or inappropriate for some lights and lighting applications. Luminaire modifications not approved by the manufacturer may void the product warranty, cause damage to the luminaire, or present a hazard. 5.5.5 Use Consistently Colored Light Sources Where lighting requirements allow, use consistently colored light sources to minimize aesthetic effects. Notes: Lighting requirements in some situations may require lights of different colors (e.g., lights with a higher CRI for task lighting where color rendition is important). Note also that different colors affect perceived brightness differently (e.g., increasing the short-wavelength content of light sources increases perceived brightness more than an equivalent increase in long-wavelength content). 5.6 BMPs for Minimizing the Level of Illumination The following BMPs suggest ways to use the lowest possible level of illumination to meet lighting needs for a particular area. These practices prevent over-illumination. 5.6.1 Use the Lowest Possible Level of Illumination that Meets the Lighting Needs Use the lowest possible level of illumination that safely meets the lighting needs for a particular area. Refer to the IDA-IES Model Lighting Ordinance (IDAIES 2011) for maximum lighting densities (lumens/ ft2) of hardscape areas for the applicable lighting zone performance standard (see Sections 5.1.2 and 5.4.5). This practice reduces ecological and human health effects, while saving money and preventing wasted energy. Notes: Determining minimum safe lighting levels likely requires a qualified illumination designer or engineer, especially for public use areas. The use of two very differently colored light sources creates an aesthetic problem.
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