Daylight availability: global and diffuse Evgh, Evd--Algorithm 1.31

These empirical equations give the frequency distributions of global illuminance (the illuminance on unobstructed ground from both skylight and sunlight), and of diffuse illuminance (skylight alone). By computing first the solar altitude at regular intervals of time through the day, for every day of the year, and then the distribution of illuminance at these times, the total annual hours that a given illuminance is exceeded can be found.

The equations are based on measurements made at Nottingham UK and may be applied to other sites in north- west Europe with similar climate.

a. Global illuminance

Input
Solar altitude, gs radians

Equation
The mean global illuminance, in kilolux, under actual skies is

The fraction of the mean illuminance at solar altitude gs exceeded on 95%, 90%, ... 10% and 5% of occasions is as follows:

percentile
95% 0.15
90 0.26
80 0.32
50 0.90
20 1.56
10 1.83
5 1.98

Source
Tregenza(1)

b. Diffuse illuminance

Input
Solar altitude, gs radians

Equation
The mean diffuse illuminance, in kilolux, under actual skies is

The fraction of the mean illuminance at solar altitude gs exceeded on 95%, 90%, ... 10% and 5% of occasions is as follows:

percentile
95% 0.23
90 0.35
80 0.52
50 0.96
20 1.40
10 1.60
5 1.80

Source
Tregenza(1)
References
  1. Tregenza P R Measured and calculated frequency distributions of daylight illuminance Lighting Research and Technology 18 (2) 71-74 (1986)

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