Measuring the luminance and chromaticity distribution of the sky

One of the prerequisites for the design of buildings that make good use of daylight for internal illumination is a knowledge of the daylight climate in the place where the building is to be situated. The amount of light likely to enter a building is often quantified using Daylight Factor, the ratio of internal to external illuminance, which can be measured by taking some internal and external readings with a luxmeter. A luxmeter is a photometric device that uses a photocell with V-? and cosine correction. In other words its response is corrected so that it responds in a similar way to the human eye and accepts light from all directions from the zenith to the horizon. A typical daylight factor that might be used as a design parameter for an office space is 2-5%. Prior to the 1980s little detailed data had been gathered on the worlds daylight climate and designs used an overcast sky and the daylight factor calculation to predict the amount of light reaching the inside of buildings. During the 1980s, the concept of daylight coefficients was developed by Tregenza and Waters (1983) in order to refine the process of daylight design. Use of daylight coefficients allows spatial variations in sky luminance to be taken into account in daylighting design but requires knowledge of the luminance distribution of the sky. The problem here is that the sky is constantly changing and its luminance is difficult to predict. Under the auspices of the International Commission on Illumination (CIE) a fairly long term, world-wide, programme of measurement and study was started in the early 1990s to quantify sky luminance and other parameters to produce a set of 15 Standard Skies for use by architects. The programme was known as the International Daylight Measurement Programme (IDMP). The aim was to give a basis for predicting the frequency distribution of these skies and thereby facilitate more evidence based daylight design. Part of the measurement programme involved the design, construction and use of specialised sky scanners. Two companies, PRC Krochmann of Germany and EKO based in Japan, produced the scanners used in the programme. These scanners were highly sophisticated and expensive items and were provided only at a small number of the IDMP stations those designated as of Research Class. A sky scanner typically is required to scan the entire sky vault or hemisphere, measuring the luminance in candelas per sq. m. of a series of 145 sky patches, each defined by an acceptance angle of 10 or 11 degrees. In order to avoid errors due to changes in luminance during a scan it was felt to be desirable that a scan be completed in about 1 minute.. Scans were taken at approximately 15 minute intervals over a long period (typically a year) in order to obtain data of statistical significance. It would seem that very few of these scanners are in use today and that interest in the IDMP seems to have declined since the programme came to an end in about 1997.