晝光率 Daylight factor

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晝光率 Daylight factor

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晝光率 Daylight factor

或稱晝光係數。

晝光率的物理量是以室內任一點照度與當時該室外為晴天狀態下之水平面照度的比值。

DF = (Ei / Eo) x 100%

DF:晝光率
Ei :室內照度(lux, lx)
Eo:全天空照度(lux, lx)



參考資料

Daylighting

General Principles and Commitments

Achieve a minimum of daylight factor of 2 percent (excluding all direct sunlight penetration) in 75 percent of all space occupied for critical visual tasks. Provide automatic dimming controls or accessible manual lighting controls, and appropriate glare control.

Technical Guidance

Introduction

Requirement: "Achieve a minimum of daylight factor of 2 percent"

Light levels in a building space are typically measured in footcandles or lux. Light from electric lights is fairly constant so exact levels of lighting can be obtained through the selection and layout of light fixtures. When it comes to daylighting the light source is the sky vault which is outside the building so it becomes necessary to install glazed openings (windows and skylights) to admit adequate daylight to meet space needs. Designing for exact levels of light from daylight is difficult since the light source is constantly changing depending on time of day and weather conditions. Instead of calculating exact levels of light (footcandles or lux) in a given space from daylight a daylight factor was created to measure relative light levels. The daylight factor is best defined as:

DF = Iin/Iout x 100%
Where:
DF = daylight factor
Iin = illumiance due to daylight on the indoor working plane
Iout = illumiance outdoors on a unobstructed horizontal plane

There are three possible ways that daylight can reach the indoor working plane. They are:

Visable light directly from the sky vault (define)
Light reflected from exterior surfaces.
Light entering the space and reflecting from interior surfaces.

All three of these components need to be accounted for to determine the daylight factor.

A daylight factor of 2 is a typical level one would want to achieve for an office space. It assumes that 2% of the total light that is outside the building ends up on the working plane or desktop. Assuming an average outdoor illumiance of 2500 footcandles and a daylight factor of 2% the indoor illumiance on your desktop would be 50 footcandles (2% x 2500 footcandles = 50 footcandles). Other typical daylight factors for various spaces are:

Discussion Groups – 14
Residential Living Room – 1
Residential Kitchen – 2
Office - detail work – 4
Office - drafting – 6
Office - corridors – 0.5
Schools - classrooms – 2
Schools - art rooms – 4
Hospitals - wards – 1
Hospitals - waiting rooms – 2
Sports facilities – 2
Warehouse - bulk storage – 0.5
Warehouse - medium size storage – 1
Warehouse - small item storage – 2

Calculating a daylight factor based on your building design includes window/skylight sizes, overhangs/lightshelves, glass types, and exterior/interior reflectances can become very complex. A simple rule of thumb to approximate the daylight factor for daylit spaces using vertical windows is:

DF = 0.1 x PG
Where:
DF = daylight factor
PG = Percentage of glass to floor area

Assuming a 1000 sf office space has 200 sf of windows. The daylight factor would be:

DF = 0.1 x (200/1000)
DF = .02 or 2%

For more detailed calculation methods which take into account the three major components (light from sky vault, light reflected from exterior surfaces and light reflected from interior surfaces) that make up the daylight factor refer to the references in this guidance.

Requirement: "(excluding all direct sunlight penetration)"

Traditional daylighting excludes direct beam sunlight which typically introduces unwanted heat, glare and extreme contrast resulting in increased energy use and uncomfortable conditions. The sun is constantly changing positions (altitude and azimuth) during the day and throughout the seasons. The MOU guidance recognizes that excluding "all" direct sunlight penetration may not be possible or desirable. If the building design incorporates both passive solar heating and daylighting excluding the direct beam would not be desirable since this is the main source of heat. The intention of "all" would be in an office/classroom/business type environment where passive solar heating (direct beam sunlight) would not be part of the design and would lead to poor daylighting. Direct sunlight should not be used when calculating the daylight factor.
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