Many of the factors of importance for visual comfort in daylit environments are the same as those for artificially lit environments. However, the direction, quantity, quality, variability and information content of daylight is significantly different to that of electric light, which will cause differences in the levels of visual comfort in the two lighting conditions.

Glare

Much is written about the need to place and specify luminaries to minimise glare. The glare generated from daylight is quite different from that of luminaires. It is not a small source – generally it comes through windows, a far larger source than a luminaire. However, glare from daylight still has the potential to significantly reduce the visual comfort of a space.

Significant amounts of research suggest that glare is a very important factor in the levels of visual comfort in daylight. Glare from daylight has been the subject of numerous studies in relation to visual comfort. Ralph Hopkinson began studying glare from daylight in the late 1950s and several more researchers have since joined the field including Chauvel, Osterhaus and Velds to name a few.

The glare caused from daylight may be either disability glare, disabling the worker in his/her task, or discomfort glare, producing uncomfortable conditions making the task more difficult. Several methods have been proposed for evaluating the impact glare, particularly from windows, has on the visual comfort of people. Each of these methods and the research into daylight glare in general will be discussed in greater detail in Section 3 – Glare.

Luminance Ratios

In a similar fashion to glare and contributing to glare, high levels of contrast between luminances of surfaces can create visual discomfort in an environment. If the contrast between the task and the background or general surroundings is too high the eye may have difficulty adapting causing discomfort. Windows are generally a very bright surface, often creating a high contrast with the window surround and the office task – particularly VDT screens that are also placed in a vertical plane.

As with glare, there has been significant research into the recommended luminance ratios under artificial lighting conditions, however, it is more limited for daylighting. The research into preferred luminance ratios in daylit environments is largely based on testing the validity of using the recommendations generated for artificial light. Section 4 – Luminance Ratios will investigate what the current opinions on acceptable levels are and what impact these may have on the preparation of assessment methods.

Illuminance and Uniformity

The recommended illuminance for particular tasks is very well documented from sources such as the Illuminating Engineering Society of North America (IESNA) and the Chartered Institute of Building Services Engineers (CIBSE) etc. Again, these are primarily prepared for artificially lit environments, although daylight recommendations generally follow similar patterns.

This report gives more consideration to the level of uniformity needed to create a visually comfortable environment. As with the level of illuminance, there are several recommendations as to the required uniformity of that illuminance. However, it is far easier to achieve a uniform illuminance with overhead luminaires than it is with windows providing side-lighting. The inherent directionality of daylight causes an uneven distribution of light across the space (high illuminance near the window, which decreases rapidly with distance into the space). Existing research has been analysed in Section 5 – Illuminance and Uniformity to determine what levels of uniformity are acceptable and how they relate to a potential assessment method.

View

Possibly the most important difference between solely artificially lit (ie: windowless) environments and daylit environments is the view. Artificial light cannot provide the same level of information that daylight can. A view from a window provides information about the weather, the time of day and the activities outside. It is argued that the tolerance of occupants to glare etc is proportional to the quality of view. Even though a window may produce a large amount of discomfort glare, the occupants may still be happy due to the high quality of the view.

It is commonly accepted that a view is desirable, although it may not necessarily add to the visual comfort. All buildings with external windows will have a view of some sort; but how can the quality of the view be assessed?

There is very limited research on the view component of daylight. What there is will be reviewed in section 6 – View. Its effect on visual comfort and potential measures to assess the quality of the view will be discussed.