Calculation Methods of Utilisation Factors for Interior Lighting

Prior to the use of computers for the production of photometric data, a series of approximations were used in these calculations for the determination of utilisation factors and uniformity over a specified surface, for interior lighting installations.
This investigation has considered and shown the effects of these approximations at each step of the current method detailed in the CIBSE publication Technical Memoranda 5 (TM5) (1). It has developed mathematical models for a single luminaire based on flux distribution grids calculated by both
ɸ = E x A and ɸ = Iave x ꙍ:
Where:
ɸ = total flux required in lumens
E = reference surface illuminance
A = area of the reference surface reference
Iave = average intensity in angular zone
ꙍ = very small solid angle
Thus enabling the evaluation of the direct flux for any configuration of luminaires.
The presentation of this data has been produced in three acceptable formats, to enable utilisation factors to be determined as easily as the present method, but with far greater accuracy. One of these allowing values to be obtained direct from tables the other two being slightly more accurate and giving additional information, but requiring the assistance of short computer programmes. Examples of calculations using all three methods are included and tables of comparisons constructed.
Spacing to height charts and tables have been developed for a number of luminaires and values of Mid-Area Ratios (MARs) compared to those defined by using a standard square array of sixteen luminaires and values calculated from devised installations where actual luminaire numbers and positions were taken into account.
The accuracy of the application of aspect factors, for the calculation of illuminance of modern luminaires from measured intensity values has also been investigated and compared with the results using aspect factors derived from polar curves in inclined axial planes.
ꙍꙍꙍAn investigation into the accuracy of the present trend of subdivision of a luminaire into small elements, for the purpose of illuminance calculations has also been carried out. The results being compared to those obtained by measuring each element separately and summing the totals.