Approximate horizontal diffuse illuminance from the sky on a
fully unobstructed surface (lux), as a function of solar
altitude in radians, for the given sky model.
Screening-grade — these aren't the per-sample-aware values a
Perez All-Weather model would return, but they capture the
dominant effect (sky is brighter when the sun is higher) and
give the year-integrated metric a physically-meaningful shape.
Uniform: 10000 lux during daylight, 0 at night. A
useful "compliance-style design sky" — corresponds to the
classic 10,000 lux outside reference often used for
daylight-factor work.
CIE Overcast: scales with sin(altitude). Roughly
8000 + 12000·sin(α) lux — peaks near 20 klux at zenith
altitude, drops to 8 klux at the horizon. Above-horizon only.
Approximate horizontal diffuse illuminance from the sky on a fully unobstructed surface (lux), as a function of solar altitude in radians, for the given sky model.
Screening-grade — these aren't the per-sample-aware values a Perez All-Weather model would return, but they capture the dominant effect (sky is brighter when the sun is higher) and give the year-integrated metric a physically-meaningful shape.
sin(altitude). Roughly 8000 + 12000·sin(α) lux — peaks near 20 klux at zenith altitude, drops to 8 klux at the horizon. Above-horizon only.