While BS EN 12464-1 provides the hard legal numbers, true Office Compliance and visual excellence are guided by the Chartered Institution of Building Services Engineers (CIBSE) Lighting Guide 7 (LG7). LG7 dictates how light should make a space *feel*, not just how bright the desks are.
Eradicating the "Cave Effect"
LG7 specifically warns against the "cave effect," which occurs when lighting is only directed downwards onto the desks, leaving the walls and ceilings in gloomy shadow.
If you put a 500 Lux spotlight directly over a desk but paint the walls dark grey, the employee feels isolated and claustrophobic, staring into a bright monitor while surrounded by darkness. The contrast ratio causes severe eye strain. LG7 mandates that the architectural surfaces (walls and ceilings) must be illuminated to balance the brightness of the task area, creating a visually open and airy environment.
Wall and Ceiling Illuminance Ratios
To achieve LG7 compliance, the lighting design must ensure that the walls receive at least 50% (and the ceiling at least 30%) of the illuminance provided to the working plane (desks).
If the desks are at 500 Lux, the walls must be illuminated to at least 250 Lux, and the ceiling to 150 Lux. This is mathematically proven by lighting software (like Relux or Dialux). To hit these numbers, lighting designers cannot rely solely on flat LED panels. They must integrate wall-washers to push light outward, or use suspended luminaires with an "uplight" component to blast light onto the ceiling slab.
Cylindrical Illuminance for Facial Recognition
LG7 requires an average cylindrical illuminance of 150 Lux at head height to ensure good facial recognition and comfortable interpersonal communication across the office.
Cylindrical illuminance measures light hitting a vertical target from all sides. If an office only uses narrow-beam downlights, the light hits the top of a person's head, casting deep shadows over their eyes and making them look intimidating or ill. By ensuring a high cylindrical illuminance (using wide-beam optics or bounced indirect light), colleagues' faces are softly and evenly lit, fostering a more collaborative and friendly workplace atmosphere.