Because quality of light is such a broad term, and because it includes so many different aspects, precisely defining it is difficult. It’s like asking what “quality of food” might mean. Quality of food could refer to the quality of the ingredients used in a dish, and to the expertise with which they’re combined—the proportions, the seasoning, the cooking method and time, and so on. It could also refer to your experience of serving that dish in a restaurant—an experience that includes presentation, taste, price, and how it makes you feel.

Quality of light is similar. It refers on the one hand to the ingredients of light—the different measurable aspects of luminaires and the light they produce. And it refers on the other hand to the perception of light—how light influences the look of spaces and objects in different applications, and how it affects the people who use and manage those spaces. 

Several well-known measures quantify the physical characteristics of light, such as Color Rendering Index (CRI), Unified Glare Rating (UGR), and Correlated Color Temperature (CCT). These measures are calculated from the photometric measurements and are typically published as part of a luminaire’s technical specifications. But being able to fully describe the link between the perceived light attributes, physical light characteristics, and technology variables is still insufficient to describe the overall light quality because critical application knowledge is missing. For example, how we perceive colors and whites depends on the application and the observer. The goals and requirements for illuminating an office space, for example, differ significantly from the goals and requirements of illuminating a fashion store. Also, the observers’ preference and adaptation state will play an important role. A lighting designer for example will always choose high-quality lighting components, but the designer’s artistry and expertise to create a lighting solution that is appropriate for the space being illuminated is equally important for a successful design. This gives the users of the space the high-quality experience that they expect and desire.

Furthermore, like most of the new technologies, solid-state lighting also provides new possibilities to differentiate in quality of light like fast temporal response, multi-primary control, and small form factor, but also introduces some unwanted visual effects that were absent or less dominant for the conventional, well-established, lighting technologies.