The quality of light is at the same time the most important aspect to consider and the most often overlooked. It can’t be measured with a light meter and requires expertise to execute. It has many elements and all of them contribute in some way to visibility. Normally the better the quality of light, the better we can see objects. Improving visibility should be the primary reason to install lighting.
Many confuse quality and quantity of light. The public, police and elected officials often assume that more light will improve visibility, and therefore safety. In reality, there is very little linkage between the two. Unlike quality, quantity is relatively easy to measure. It can be modeled by lighting design programs and measured in-situ, using light meters that measure either illuminance (lux or foot-candles) or luminance (cd/m2 or nits).
Describing the quality of light often involves a lot of technical acronyms like CCT, CRI or SPD, but these terms all relate to the color of the light. White light is used primarily indoors. However, it can range in color from warm to cool. Warm-white light from a traditional incandescent lamp looks more yellow than most florescent lamps that appear cooler or “bluer”. While we may accept cooler white light in offices or other commercial uses, most still prefer to have warmer or neutral-white in the home.
On the other hand, exterior lighting has historically been predominately much warmer, appearing more yellow than white. This is because High Pressure Sodium (HPS) has been the most energy efficient light source for the past several decades, and therefore used extensively for street, road and area lighting. Other sources like Metal Halide have been employed in areas in which color rendition has been more important than energy costs, like retail display lots and illuminating building facades.
In addition to improved color rendition, white light improves visibility. In recent studies it has been shown that you can reduce the quantity of illumination by 50% using neutral white light, compared to HPS with the same visibility.
The most recent technology to be introduced for outdoor lighting is Solid State Lighting (SSL). Of all the types of SSL, Light Emitting Diode (LED) is the light source that has garnered a significant market share. This is primarily because it is very energy efficient and its efficacy is growing rapidly. Efficacy is measured in lumens per watt. However, LED has the added advantage of being able to direct its light precisely to the target area without the use of reflectors. This adds to the LED fixture’s overall efficiency because little or no light is lost bouncing around in a reflector, which is commonly used in other technologies.