Energy conservation, especially in regards to lighting, can be analogically explained using the example of a highway bridge. Basically, it is more practical and economical to lower the river or sea below a giant bridge as opposed to raising the bridge itself.
Similarly, when you need to reduce a lighting load, you ideally target, and adjust both the visual task size and contrast. To effectively measure these parameters, we use the Relative Visual Performance (RVP) index. According to the index, the RVP reading shoots up with an increase in contrast, no matter how slight.
Further, when dealing with 10-point types, such as large posters, you will notice that the RVP may continually rise up to 1.0, even when the retinal illuminance is nominal. However, once a certain point is reached – that is, the given plateau range, the RVP will remain static, even when the illuminance is increased. This knowledge is especially essential for street lighting experts who need to ensure the contrast levels are adjusted accordingly to aid elderly people or people with sight problems to see the roads properly as they wall and drive.
As we increase the contrast, we must find a way to considerably minimize the glare, whether direct or reflected, as a form of sight aid. Remember, glare can greatly reduce people’s abilities to read written signs and directions and interfere with vision when driving at night.
Unlike during the day, when the high ambient light levels do not trouble our vision on the roads, even if the oncoming cars have their headlights on, driving at night is an entirely different ballgame. For one, there’s minimal ambient lighting, and most cars have their headlights on at this time, which creates glare. The (direct) glare from both headlights and streetlights allows for very little contrast, and this can be blinding when at high levels.
Reflected glare, on the other hand, is produced when high luminaries shine on polished or glossy surfaces in the field of view, thereby reducing the contrast. For instance, you may experience problems reading fine print on a shiny screen at night. Even worse, reflected glare increases with an increase in luminance, which means a better solution is needed.
One efficient way of minimizing reflected glare while indoors is to install dynamic light finishes around the walls, in addition to all major vertical surfaces. That way, you will significantly increase the volumetric brightness within that particular room and hence make it easier for an occupant’s eye to get used to it, just like it happens during the day.
In summary, the RVP index puts the entire surrounding into perspective when analyzing the lighting quality and requirements. In fact, illuminance levels are only a minor aspect of lighting design. To get a good job done and attain acceptable levels of energy efficiency, you have to define the visual task sizes, implement glare reduction, and get a visual map of the surrounding.
Reference: LD+A Magazine | March 2017 – ‘Energy Advisor – Beyond the Numbers’