Visual comfort criteria measure the ability of an individual to carry out tasks comfortably in terms of their photo-sensory perception of their environment. They are dependent on many factors including: light intensity, direction of light source, reflection of surfaces, contrast of surfaces, the nature of the task being undertaken and the photo-sensory response of the eye. International recommendations exist that specify the minimum level of illuminance required to provide visual comfort for various tasks and locations. As with thermal comfort temperatures, these light levels were derived experimentally and reflect an average person engaged in a specific activity within a defined environment.
A specified light level could be provided by either natural light or artificial light, but in order to satisfy energy concerns it is often necessary to maximise the degree of daylight contribution and minimise the artificial lighting contribution. However the introduction of natural light into a space can have certain drawbacks, namely glare and unwanted solar heat gain, thus blind control can have an important impact of the visual environment within a room. Indeed, a misconception that has originated from the international recommendations is that we need to provide tightly controlled uniform lighting levels to achieve occupant visual comfort. In fact it is the contrast in light levels between the front and back of the room that we should try to limit, not the variation in the overall illuminance. Preferences to different combinations of illuminance and colour temperature vary from person to person depending on gender, age and past experience. An individual can have a tolerance to a wide range of light levels, so much so that reading performance does not show any significant variation between 100 and 5000 lux.
Different people react to windows, views, natural light and direct sunlight in different ways. Research has shown that such factors can have real psychological benefits to occupants, but whether they have a positive affect on productivity depends on the task being undertaken. The variable nature of natural light is one of the fundamental parameters of human life; it provides us with a perception of the running time and the conditions for psychological well-being in closed environments. Cool daylight which is filtered through trees or shading devices can provide visual relief to mentally counteract the thermal rigors of hot days and a carefully placed sparkle of sunlight can help relieve the impression of cold in the winter. Empirical visual comfort assessments do not take many of these factors and other psychological factors into account. Therefore when designing an automatic control system for a blind it is often wise to provide an occupant override.