Question-and-Answer Resource for the Building Energy Modeling Community
 | 1 | initial version |
Just wrote my comments two times as comments, which got deleted by the site :/
Please be careful about the idea of using the daylight factor, whenever you measure something in a real building. The daylight factor assumes (and is calculated as such by software) a theoretical "overcast sky", which you will probably never find in nature. So by definition, you compare the results of different sky distributions - and one should start worrying if that leads to agreement. The one exception is cases where the distribution has no impact. Say, you building consists of a glass bubble, where it does not make any difference how the luminance is distributed over the sky hemisphere as long as the integral is correct. But for such cases, there would not be much to validate.
Often, as long as you know direct and diffuse illuminance from measurements, sunny sky conditions give better agreement. Here, you know that most of the illuminance is attributed to the sun direction, and not knowing the exact distribution of the "blue sky" has little impact. But beware - this changes immediately if a bright white cloud comes into sight, which adds some illuminance from its direction, but gets attributed to the entire hemisphere in your diffuse illuminance measurement.
Best, Lars.
| | 2 | No.2 Revision |
Just wrote my comments two times as comments, which got deleted by the site :/
Please be careful about the idea of using the daylight factor, whenever you measure something in a real building. The daylight factor assumes (and is calculated as such by software) a theoretical "overcast sky", which you will probably never find in nature. So by definition, you compare the results of different sky distributions - and one should start worrying if that leads to agreement. The one exception is cases where the distribution has no impact. Say, you building consists of a glass bubble, where it does not make any difference how the luminance is distributed over the sky hemisphere as long as the integral is correct. But for such cases, there would not be much to validate.
Often, as long as you know direct and diffuse illuminance from measurements, sunny sky conditions give better agreement. Here, you know that most of the illuminance is attributed to the sun direction, and not knowing the exact distribution of the "blue sky" has little impact. But beware - this changes immediately if a bright white cloud comes into sight, which adds some illuminance from its direction, but gets attributed to the entire hemisphere in your diffuse illuminance measurement.
Best, Lars.
