1B Clearly category 1 objects are not going to be able to be displayed daylit areas, this notwithstanding an annual exposure value for objects in this category is a useful measure for occasions where these objects for one reason or another are required to be illuminated at higher light levels, these reasons are more likely to be for conservation condition reports and photography rather than display however if for technical reasons, for example control system failure, lighting levels exceed those that are desirable this measure can be used to determine and compensate for these eventualities. Thompson proposes 200 KLux Hours Category 2 objects may be exhibited satisfactorily under controlled daylight conditions. Category 3 objects do not require exposure levels to be predetermined. In reality the light levels for these objects will be constrained by the display conditions of other objects around them. Now we have some parameters to use as goals for day lighting let us consider what we are controlling.
Daylight varies enormously throughout the world.
In Northern Europe and Scandinavia we see extreme variations in daylight availability throughout the year, at the equator there is little variation throughout the year in hours of daylight or solar angles, what variation there is tends to be climate dependent. It is extremely important to use daylight availability data for the location of the project. Obtaining this data can sometimes pose problems, we usually manage to obtain this form local meteorological offices or architecture and Building engineering schools. The first table shows actual daylight recorded in Geneva averaged over a ten year period. As you can see the daily variation is significant and the month to month variation is also dramatic.