Lighting designers consider four fundamental factors that determine the ability of a person to properly see an object or task. While some may seem fairly intuitive, they are important to remember when making decisions about lighting installations.
The first factor is the Size of the object relative to how close or far it is from the observer. If an object or task is further away from someone, higher lighting levels might be needed to help see the object. As a small object is brought closer to the eye, the visual angle, which we interpret as size, is increased, making it easier to see. This is the principle that the familiar eye-test chart utilizes for testing vision.
The second factor is the Brightness or luminance of an object. This depends both upon the Quality of light hitting the object and the Quantity of light hitting the object. The more white light that hits an object, the more light can reflect back into the eye and be seen by the observer. In the world of photography, ‘quality’ describes the hardness/softness of light, as in soft diffuse shadows or hard crisp ones.
The third factor is Contrast or the difference between the brightness of the visual object (task) and its immediate background. For example, it is much easier to read black lettering on a white background, than on a dark-coloured background, or a light grey text on a white background. If there is little contrast between an object and its background, we can sometimes have a difficult time locating the object. This is the same principle as camouflage that animals use to hide in plain sight from predators.
The fourth factor to consider is the amount of Time that the object can be viewed. The longer the eye can focus on the object or task, the more details can be taken in. This factor is extremely important when trying to view objects that are moving, which is why sports arenas and race tracks require such high levels of illumination.