How Wall Finishes Can Save Lighting Energy
My most recent lighting column in Electrical Contractor Magazine describes the role of surface reflectances in the overall efficiency of lighting applications.
Because surfaces and objects in typical spaces reflect light, they can play a part in lighting efficiency as extensions of the lighting system. By controlling room surface reflectances, light levels can be improved, creating opportunities to save energy.
Click here to read this article at the Electrical Contractor website.
Craig that was very informative – as usual.
As a counter point, I offer the following regarding the use of RCR and CU, and the zonal cavity calculations these are designed to support:
While its obvious that with indirect lighting, use of high reflectance ceiling surfaces is required, or efficiency suffers – the effect of wall and floor finishes is far less simple. Contrast is an effective design tool in producing greater perception and visibility on a task or object. Use of bright white walls to increase efficiency will actually create issues of low task surface illuminance, which will demand more light than is necessary. The same applies to displays – where an object lighted to X Fc on a white wall, will less bright than the same object on a darker surface. The 50% wall and 20% floor reflectance values have become a defacto standard of sorts, as it represents a good neutral wall surface reflectance, while the floor being darker produces strong contrast in the horizontal visual plane. Interestingly, this has come from the practice of design finding these values generally desirable, not from scientific study, so is unlikely to be changed much, other than slight variations that occur from style trends – like the short lived white wall stripped bare minimalism that assumed we all wanted to live in Arthur C Clark futuristic shiny white plastic modernism – which is more hospital than hospitable.
The reflectance values shown in the RCR charts and the RCR values themselves are only used for zonal cavity calculations – a crude and inaccurate calculation of average footcandles within a room. Zonal cavity calcs offer no fidelity, so is too overly generalized to be of real value in precision design practice. For example, it assumes a red wall of 50% reflectivity has the same visual impact as a gray wall of the same reflectance – which fails to recognize human visual response to both color and illuminance in processing information. Zonal cavity is also numb to intended space use, treating retail spaces that need vertical illuminance the same as offices which need horizontal, as one and the same. The underlying assumption of the whole Zonal Cavity calculation itself is skewed completely to horizontal illuminance, as this is all it can calculate, so fails to consider any other needs. Yes, there are those who only do Zonal calcs and think they are doing good – that does not mean this is good or effective design practice.
While of some value in assessing general apporaches to the most generic of spaces, to use RCR and CU values as a basis for energy saving recommendation leads to one conclusion – all walls, floors and ceilings should be high reflectance white, as this produces the greatest average illuminance per watt consumed – all other approaches or surface finishes are then a compromise top energy efficiency. This could not be further from the truth, thus the fundamental error of overly generalized average illuminance calculations.
To produce a desired contrast between vertical and horizontal surfaces, it may be necessary to use a luminaire that places a great deal of light on walls painted a low reflectance color. This is the a principal often used in display illumination, yet the CU/RCR values would indicate this as a poor performing solution.
Real energy efficiency from color and reflectance manipulation comes from an understanding of contrast, visual fields, task level illuminance, brightness and glare influances. When well controlled, these can all add up to significant opportunities to cut energy use. Without consideration, these factors will make whatever general illumination calculations completed irrelevant.
With the range of available software today to evaluate design quickly, with real fidelity, including energy used to attain a desired end result, the use of crude estimates for average illuminance is out dated.