Alliance for Lighting Information


by David M. Keith

Uplight is visible radiation that either directly escapes from luminaires in an upward direction or is reflected by surfaces in an upward direction. By this definition, uplight is what is produced by the combination of the luminaires and illuminated surfaces from the ground up to the height of the luminaires. Furthermore, by this definition, uplight is not the same as skyglow, because any effects on the visible radiation once it rises above the luminaires is not included - particularly the atmospheric scattering that redirects radiation to make it skyglow.

In addition this uplight definition does not include surfaces between the tops of the luminaires and the "bottom of the atmosphere" - surfaces that may exist in some locations and not in others. For example, a small road near the stream in the bottom of a mountain canyon lined with evergreens will have an entirely different radiative transfer for uplight into the atmosphere compared to a similar roadway in the middle of open plains - or along the coast - or in a forrested area in summer - or in winter.

Instead the definition of uplight addresses the lighting system, the intentionally illuminated surfaces and the immeduate surrounding. These are elements that can be reasonably predicted by the lighting designer without excessive assumptions or variability. The model is not intended to be comprehensive but relative. Although the uplight model is not intended to predict the precise contribution to light pollution from the particular lighting system, comparisons between the uplight produced by two lighting systems will accurately predict the relative contributions. If the model predicts that one system will produce half the uplight of another, then it should be reasonable to expect that the light pollution produced by radiation from that system will be half the light pollution of the other.

The Unit Uplight Density (UUD) model is useful for developing comparisons, based on the concept of "all other things being equal". Therefore, if the spectral power distributions (SPD) produced by two systems are significantly different, then the spectral aspects are not eqaul and they need to be considered as well. The Rayleigh Scatter Index (RSI) can be used to estimate the spectral effects from different sources - which again are not absolute values but suitable for comparisons. The results from RSI evaluations of SPDs is available here. Additional discussions about these and related topics can be found here.

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