What are the required illuminance values and measurement planes for Visual Acuity and Melanopic Light Intensity, and how long must Melanopic light be present daily?

• A. Visual Acuity: 215 lux on a vertical plane; Melanopic Light Intensity: 250 equivalent EML at 75% of workstations, measured on a horizontal plane, 4' away; Melanopic light must be present for at least 6 hours per day
• B. Visual Acuity: 320 lux on horizontal plane; Melanopic Light Intensity: 200 equivalent EML at 50% of workstations, measured on a vertical plane; Melanopic light must be present for 8 hours per day
• C. Visual Acuity: 215 lux on a horizontal plane; Melanopic Light Intensity: 250 equivalent EML at 75% of workstations, measured on a vertical plane facing forward 4' aff; 4 hours per day
• D. Visual Acuity: 150 lux on horizontal plane; Melanopic Light Intensity: 300 EML at 100% of workstations, measured on a vertical plane; Melanopic light required for 2 hours per day

Answer: (C)

Two distinct goals drive these requirements: providing adequate light for clear visual tasks and delivering enough melanopic light to support circadian health. Visual Acuity focuses on task visibility, so the illuminance is specified on the task plane where people actually work. The 215 lux on a horizontal plane reflects typical desk-task lighting and uses a horizontal measurement because it represents the light reaching the eyes during common activities like reading or computer work. For circadian-relevant lighting, the metric is Equivalent Melanopic Lux (EML). The standard calls for 250 EML at 75% of workstations, measured on a vertical plane facing forward at about 4 feet from the user. A vertical measurement captures the light entering the eyes during forward viewing toward the workstation, and the 4-foot distance standardizes how far the eye is from the light source. Melanopic light must be present for at least 4 hours per day to provide a meaningful daily circadian stimulus. This option aligns with those specifics: 215 lux on a horizontal plane for Visual Acuity, 250 EML at 75% of workstations on a vertical plane facing forward at 4 feet, and 4 hours per day.