Quality of light is both objective and subjective; a science and an art. On the one hand it can be measured; on the other hand it depends on personal judgments. It certainly depends heavily on the application. For example, for road lighting, quality of light involves providing good visibility while limiting glare. In a fashion store, quality of light means matching the image of the brand and attracting the target audience by, for example, creating high contrast areas within the store. These different definitions make it difficult to pin down what high quality of light means to a customer, particularly when people’s ages and cultural differences also need to be taken into account. A general rule of thumb is that the quality of light is high if the lighting serves its purpose.
Professionals and their clients generally look for high quality of light, but the big question is how they can decide which of the different options is best. There are different methods of demonstrating and explaining lighting, and it is not always clear which method will most faithfully showcase what the light and quality of lighting will be like after installation. We'll now look at the most relevant ones.
One way is to present and share data on certain quality of light parameters, which can help some professionals if they have sufficient knowledge to interpret the data. Quality of light can be defined in terms of illuminance and luminance, color rendering index (CRI), color temperature, glare factors etc. What is essential is to present all the data in a consistent way so that clear comparisons can be made easily.
A second common approach is to show what the end result will look like. A range of methods are available – both computer-based and manual – and each with their pros and cons. Hand rendering can show how light works in a space. Even the highest end renderings still only give an impression (compared to seeing something in reality), and one advantage of using a pencil and paper is that it is immediately apparent that reality is not being presented, but an impression. This leaves room for the imagination, which for some situations and applications such as architectural lighting, can be beneficial. Some skill is needed though to perform hand renderings effectively.
Computer rendering methods can be fairly quick, taking the elements necessary to make lighting calculations and then projecting them onto the surfaces that the computer is told are present. These systems assume a limited number of inter-reflections in the space. This method can have value in seeing very approximately where light falls and giving some idea of intensity.
Higher up the tree is ray tracing software, which can provide good quality images by tracing the paths that rays of light take and taking into account the materials that have been used. The quality of the output then relies entirely on how faithfully the material attributes are linked to reality. For example, not just specifying ‘light wood’ but exactly what type, texture and other features of the wood.