1. Daylight measurements and luminance ratio determination
Jan KAŇKA, Jitka MOHELNÍKOVÁ, Jaroslav VYCHYTIL *, Josef Filla
* CTU in Prague, Faculty of Civil Engineering, Department of Building Structures

2. INTRODUCTION
Shading obstructions and influence of their surfaces on daylight simulations in simulation tool Světlo+ was studied.
The luminance ratio values were determined on basis of daylight measurements on models.
The measurement data were compared with the simulation outputs.
The comparison of the results is useful for correction of the standard luminance ratio for dark shading obstructions.
It was proven that the standard value of the luminance ratio is overestimated for some designed cases.

3. About Luminance Ratio
Standard recommendation for km = 0.1 is used as a default value for daylight factor calculations.
It means it is considered that luminance of shading obstructions is adequate to one tenth of the mean sky luminance Lm [cd.m-2].
Modern software tools with material databases and luminance models give more accurate values for simulations of external surface influence on daylighting .
Nevertheless the luminance ratio has importance of for practical tasks in building design practice.

4. Measurement - Models
Measurement in Prague and in Brno

5. Measurement - Models
Models in Brno: The model of the court-yard was completed of a wood board box of dimensions 1.5 m × 0.6 m × 0.6 m. The wood floor surface – light brown colour, walls with white painting. Some measurements were completed in the model alternative – one internal surface in the model was covered by a black cloth.

6. Results
The luminance ratio km [-] was determined from the measured values of luminance and simulated in software Světlo+. Finally the measured and simulated values were compared in accuracy factor af [-]: LR determined from measured data,
LR determined from daylight simulations
Position of measurement
1 - Left - top
4 - Centre - top
7 - Right - top
2 - Left - middle
5 - Centre - middle
8 - Right - middle
3 - Left - below
6 - Centre - below
9 - Right - below
Luminance of surface L [cd.m-2]
678.1
729.8
607.6
534.2
526.2
552.1
326.3
359.5
316.8
Horizontal illuminance EH [lx]
5910
6000
5720
5440
5630
5500
5420
5310
Luminance ratio km1 [-]
determined from measurements
0.36
0.38
0.33
0.31
0.29
0.22
0.19
0.21
Luminance ratio km2 [-]
determined from simulations
0.39
0.30
0.32
0.24
Accuracy factor
af [-]
0.95
0.98
0.88
1.03
0.92
0.99
0.86
0.89
0.85

7. Results
The luminance ratio km [-] was determined from the measured values of luminance and simulated in software Světlo+. Finally the measured and simulated values were compared in accuracy factor af [-]: LR determined from measured data,
LR determined from daylight simulations

8. CONCLUSIONS
Daylight measurements and calculations focused on the luminance ratio determination were completed for models of court-yard shaded due to surrounding building facades.
The measurements compared with simulation outputs are in acceptable compliance.
Results of the presented evaluations confirmed that the value of standard luminance ratio km = 0.1 is convenient for limited design tasks.
It is acceptable for obstructions with very light colour surfaces.
In case of dark surfaces this value is not recommended to be used as an input value for daylight simulations.
The value of luminance ratio should be minimised for value km = 0.05 or less in case of dark surfaces of shading obstructions.
The luminance ratio should be reduced up to 1/10 of the present standard value in case of large black surfaces of constructions.

9. Thanks for Your Attention
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