Lighting quality recommendations for VDT offices: a new method of derivation

An experiment in a mock-up office space gave occupants control over dimmable lighting circuits after a day working under pseudo-random lighting conditions. Data analysis indicated that the lighting experienced during the day influenced the changes in lighting made at the end of the day. Occupants chose to reduce screen glare if any existed. Even after allowing for the effect of glare, desktop illuminance at day's end varied with the illuminance experienced during the day. Regression of these end-of-day choices relative to the illuminance experienced during the day can yield a preferred illuminance, equivalent to the daytime illuminance at which no change was preferred at day's end. Using this method, preferred illuminances in the range 200 to 500 lx were derived. Preferences for luminance ratio were also derived. Interestingly, the deviation between participants' lighting preferences and the lighting they experienced during the day was a significant predictor of participant mood and satisfaction. List of symbols LC The participant having control over lighting choices at the start of the day NC The participant who did not have control over lighting at the start of the day, but who got to express their preference at the end of the day VDT G % Fraction of VDT screen area – when dark – occupied by visible glare image, > 40 cd/m 2 (%) VDT G L Mean luminance of visible glare image (cd/m 2) E D Illuminance measured on the desktop, close to the VDT, but on the opposite side from the task light (lx) LMM Natural logarithm ratio of maximum to minimum luminance in the field of view; luminance values averaged over a target approx. 1° square VDT-LC G L VDT G L of luminous conditions chosen by the LC participant at the start of the day, and prevailing during the day (cd/m 2) LC E D E D of luminous conditions chosen by the LC participant at the start of the day, and prevailing during the day (lx) NC E D E D of luminous conditions chosen by the NC participant at the end of the day (lx) LC LMM LMM of luminous conditions chosen by the LC participant at the start of the day, and prevailing during the day NC LMM LMM of luminous conditions chosen by the NC participant at the end of the day