Daylighting - Best (English)

Japanese: 採光 - さいこう（英語表記）daylighting

Also known as daylighting or natural lighting, it is an architectural technique that allows daylight to enter a building through its windows to brighten the interior, make things easier to see, and create a pleasant atmosphere.

[Kunio Matsuura]

Daylighting requirements

The first priority is to maintain a level of brightness appropriate to the room's use. Carefully consider the size and placement of windows, and try to distribute brightness as evenly as possible throughout the room. Direct sunlight entering through a window can cause unpleasant glare, so make sure to provide shades or other suitable blocking measures. Also, pay attention to window placement so that there is no shadowing or light curtain reflection (a phenomenon in which an oil painting on a wall or a piece of paper on a desk shines and becomes difficult to see).

[Kunio Matsuura]

Light source for daylighting

It is usually called the light source of daylight. Its source is the sun, but on the earth's surface there is direct sunlight that passes through the atmosphere from the sun and directly enters the earth, blue sky light that is scattered by the atmospheric layer, and overcast light that diffusely passes through clouds or is reflected from clouds. Blue sky light and overcast light are collectively called skylight. Since direct sunlight may not be available depending on the weather, only skylight is considered as a stable light source for daylight. The quantity that indicates the brightness of light incident on a surface is illuminance, and its unit is lux. The normal surface illuminance of direct sunlight outside the atmospheric layer is approximately 126,800 lux.

[Kunio Matsuura]

Brightness due to daylighting and its standards

Skylight is considered a stable light source for daylighting, but even though it is not as strong as direct sunlight, it constantly fluctuates, and the illuminance inside a room fluctuates accordingly. Therefore, it is meaningless to determine the illuminance value in terms of lux as a standard for the brightness required from daylighting. This is where it differs from nighttime lighting such as electric lights. There is a very common idea that inside a building there are bright rooms and dark rooms. This concept is unrelated to the weather, and it is the concept that rooms with large windows are bright and small rooms are dark. This concept is thought to correspond to the ratio of the brightness inside a room to the brightness outside, so the ratio of the indoor illuminance E to the outdoor illuminance E _s (total sky illuminance) from the sky without obstacles at that time is called the daylight factor D, and this is used as an index of brightness from daylighting. In this case, both illuminances are considered excluding direct sunlight ( Figure A ). The values shown in the table are recommended as the standard daylight factor for the required brightness inside a room. For example, a school classroom has a daylight factor of 2%, which means that it has an illuminance of 600 lux on bright days, 300 lux on normal days, and 100 lux on dark days.

[Kunio Matsuura]

Daylighting calculation

When designing a building, daylighting calculations (daylight lighting calculations) are sometimes performed to predict the daylight factor of a certain room and to see whether it meets the standard daylight factor shown in a table . The principle of daylighting calculations is the same as that of lighting calculations for artificial lighting. A window surface is considered to be a surface light source with a large light-emitting surface, and the illuminance due to this is calculated and divided by the outdoor illuminance (total sky illuminance) to obtain the daylight factor.

[Kunio Matsuura]

Daylight quality

If the daylight factor in a room is high, the daylight illuminance is also high and the room is always very bright, but no matter how bright it is, if the distribution of brightness in the room and the direction of the incoming light are not appropriate, the room is said to have poor quality daylighting. Quality daylighting differs depending on the use of the room, but in an office or classroom, for example, it meets the following criteria: (1) the distribution of daylight factor is uniform, (2) there is no unpleasant glare from the windows themselves, (3) the windows have shades that can control (block or appropriately introduce) direct sunlight, (4) the windows are positioned so that there is no shadowing or light curtain reflection, and (5) the reflectance of the interior surface finish is appropriately high and the field of view is bright.

[Kunio Matsuura]

Lighting method

The most common type is side window lighting, which can be one-sided or two-sided. One-sided lighting is the most common, and most offices and classrooms use this type. Its advantages are that the main light comes from one direction, so there is no light curtain reflection ( Figure B ), and if the window is transparent, you can have a view to the outside. Its disadvantages are that the distribution of illuminance is uneven, so the back of the room is often not illuminated properly, and it is prone to obstruction by neighboring buildings. Two-sided lighting does not cause illuminance shortages, but since the main light comes from two directions, it is unsettling, and it is prone to silhouettes (backlighting makes it difficult to distinguish faces, etc.). High window lighting can be thought of as a side window lighting with a high window position, and although the illuminance distribution is good, there is no visibility and poor ventilation. It is used in factories, gymnasiums, and museums. Skylight lighting (top lighting) is lighting from windows on the roof or ceiling. Its advantages are that the illuminance distribution is uniform and that it is not affected by neighboring buildings, but its disadvantages are that it does not have a view to the outside world and that it is prone to glare from direct sunlight. Of course, it can only be used on one-story buildings or the top floor, and is used in large factories and gymnasiums. Top window lighting (top lighting) is a method of lighting from vertical or nearly vertical windows near the ceiling, and there are sawtooth roof lighting and koshiro roof lighting ( Figure C ), which are often used in factories, and one used in museums ( Figure D ). Sawtooth roof lighting has windows facing north, so it creates a stable lighting environment with little direct sunlight and little glare. Monitor roof lighting was originally conceived as a way to exhaust heat, steam, and waste gases from factories, but now only the form remains and the windows are used only for lighting. In the case of top lighting used in museums, as shown in Figure D , a window must be provided to avoid light curtain reflection on the painting surface and to keep the viewer's area dark so that their image is not reflected on the painting surface.

[Kunio Matsuura]

Daylighting and artificial lighting

When the required illuminance is around 500 or 750 lux, as in modern offices, natural lighting alone is not enough to provide this level of illuminance during the day, and artificial lighting must be used to supplement it. In particular, the back part of the room, far from the windows, requires artificial lighting at all times, even during the day. On the other hand, from the perspective of energy conservation, it is desirable to make use of daylight as much as possible during the day. The key to modern daylighting design is finding a balance between these two. It has also become practical to install devices that measure the amount of daylight and automatically turn artificial lighting near windows on and off or dim it.

[Kunio Matsuura]

Lighting and regulations

The Building Standards Act and its Enforcement Order stipulate that occupant rooms (rooms used continuously for living, working, etc.) must have windows with an area that is effective for letting in natural light. The window area is set as a ratio to the floor area of the room, and must be at least one-fifth for kindergartens, elementary, middle, and high school classrooms, and at least one-seventh for residential rooms, hospital rooms, etc. However, not all windows can be said to be effective for letting in natural light, and care must be taken as windows that are too close to the boundary line of neighboring properties may not be effective.

[Kunio Matsuura]

[Reference] | Lighting | Windows

Daylight factor (Figure A)
©Shogakukan ">

Daylight factor (Figure A)

Side windows are useful for removing obstacles caused by light curtain reflection (Fig. B)
©Shogakukan ">

Side windows that are useful for removing obstacles caused by light curtain reflection (Fig.

Factory top light (Fig. C)
(a) and (b) are sawtooth roofs, (c) and (d) are ridge roofs. ©Shogakukan ">

Factory top light (Fig. C)

The light from the top of the museum (Fig. D)
©Shogakukan ">

The light from the top of the museum (Fig. D)

Standards for required brightness based on daylighting [Table]
©Shogakukan ">

Standards for required brightness based on daylighting [Table]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

Japanese:

昼光照明または自然照明ともいう。建物の窓から昼光を取り入れて室内を明るくし、物を見やすく、またよい雰囲気をつくるための建築的技術である。

［松浦邦男］

採光の要件

第一にその室の用途に応ずる明るさを保つことである。窓の大きさと配置とをよく考え、できるだけ室内が一様な明るさの分布となるようにする。窓から直射日光が入ると不快なまぶしさ（グレアglare）を感ずることがあるので、日よけなどでこれらを適当に遮蔽(しゃへい)できるようにする。また手暗がりや光幕(こうまく)反射（壁面に展示された油絵や机上の紙面などが光って見えにくくなる現象）が生じないよう窓配置に注意する。

［松浦邦男］

採光のための光源

通常、昼光光源とよぶ。その源は太陽であるが、地表では太陽から大気を透過して直接的に入射する直射日光と、大気層で散乱される青空光および雲を拡散透過するかあるいは雲から反射される曇天光とがある。青空光と曇天光をあわせて天空光とよぶ。直射日光は天候により期待できないことがあるので、安定した昼光光源としては天空光のみを考える。ある面への入射光の明るさを示す量は照度であり、その単位はルクスである。大気層の外側での直射日光の法線面照度は約12万6800ルクスである。

［松浦邦男］

採光による明るさとその基準

採光のための安定した光源として天空光を考えるが、これも直射日光ほどでなくとも絶えず変動するので、室内の照度もこれに従って変動する。そこで採光による必要な明るさの基準として何ルクスというように照度の値を決めてもあまり意味がない。この点が電灯などによる夜間の照明とは異なる点である。建物の内部で明るい部屋、暗い部屋というごく一般の通念がある。これは天候のよしあしに無関係で、窓の大きい部屋は明るく、小さい部屋は暗いという概念である。このような概念は室内の明るさと外の明るさとの比に対応していると考えられるので、室内照度Eとそのときの野外での障害物のない天空からの野外照度E_s（全天空照度）との比をとって昼光率Dと名づけ、これを採光による明るさの指標とする。このとき両方の照度とも直射日光を取り除いて考えている（図A）。室内の必要な明るさの基準としては、表のような値が基準昼光率として推奨されている。たとえば学校の教室は昼光率2％であり、これは明るい日600ルクス、平生300ルクス、暗い日100ルクスの照度があることを意味している。

［松浦邦男］

採光計算

建物を設計する際、ある部屋の昼光率を予測し、表に示した基準昼光率を満足しているか否かを検討するために採光計算（昼光照明計算）を行うことがある。採光計算の原理は人工照明における照明計算と同じである。窓面を発光面の大きい面光源と考え、それによる照度を求め、これを野外照度（全天空照度）で割って昼光率を得る。

［松浦邦男］

採光の質

室内の昼光率が大きいと昼光照度も大きくつねにきわめて明るいが、いくら明るくとも室内の明るさの分布や入射光の方向が適当でないと質の悪い採光といわれる。質のよい採光とはその部屋の用途によっても異なるが、たとえば事務室や教室では、(1)昼光率の分布が均一であること、(2)窓そのものによる不快なまぶしさがないこと、(3)窓に直射日光を制御（遮蔽または適当な導入）できる日よけなどがあること、(4)手暗がりや光幕反射が生じないような窓配置であること、(5)室内の表面仕上げ反射率が適当に大きく視野が明るいこと、などを満たしているものである。

［松浦邦男］

採光の方式

もっとも多いのが側窓(そくそう)採光（側光）であり、片側(かたがわ)採光と両側(りょうがわ)採光とがある。なかでも片側採光が多く、事務室や教室はほとんどこの方式である。その長所は、おもな光線が1方向からくるので光幕反射を生じないこと（図B）、透明窓にすれば外への眺望が得られることなどであり、短所は、照度の分布が不均一で部屋の奥が照度不足となりやすく、また近隣の建物などによる障害を受けやすいことである。両側採光は照度不足となることはないが、おもな光線が2方向からくるので落ち着きがなく、シルエット現象（逆光となって顔など見分けがつかなくなる現象）が生じやすい。高窓採光は、側窓採光で窓の位置が高い場合と考えてよく、照度分布はよいが見通しがきかず、通風もよくない。工場、体育館、美術館で用いられる。天窓採光（頂光）は屋根または天井にある窓による採光で、長所は、照度分布が均一であること、隣接建物の障害を受けないことなどであるが、短所は、外界への見通しがないこと、直射日光によるまぶしさを生じやすいことなどである。もちろん平屋または最上階にしか使えず、大面積の工場や体育館に用いられる。頂側窓採光（頂側光）は天井付近の鉛直または鉛直に近い窓面から採光する方式で、工場によく使用される鋸(のこぎり)屋根採光、越(こし)屋根採光（図C）と、美術館に用いられるもの（図D）とがある。鋸屋根採光は窓面を北向きにするので、直射日光がほとんど入らないまぶしさの少ない安定した光環境をつくれる。越屋根（モニタールーフmonitor roof）採光は、工場の熱、水蒸気、廃ガスの排気のために考えられたものが原形であるが、現在は形だけが残り窓面は採光のためだけに使われている。美術館に用いられる頂側光では図Dに示すように、絵画面での光幕反射を避け、また鑑賞者の場所を暗くして鑑賞者の姿が絵画面に映らないように窓面を設けねばならない。

［松浦邦男］

採光と人工照明

最近の事務室のように必要な照度が500あるいは750ルクス程度になると、昼間は採光だけではこの照度は得られず、人工照明で補わなければならない。とくに窓から離れた部屋の奥の部分は昼間でも常時、人工照明を必要とする。一方、省エネルギーの観点から昼間はできるだけ昼光を利用したい。現在の採光設計の要点は、この両者間のつり合いをいかにとるかというところにある。昼光の量を計測して窓際の人工照明を自動的に点滅または調光する装置を設けることも実用化されている。

［松浦邦男］

採光と法規

建築基準法と同施行令では、居室（居住、執務などのために継続的に使用する室）に採光に有効な面積をもつ窓を設けることを定めている。その窓面積はその部屋の床面積に対する比率として定められ、幼稚園、小・中・高等学校の教室では5分の1以上、住宅の居室、病院病室などでは7分の1以上である。ただし、どんな窓でも採光に有効であるとはいえず、隣地境界線に近すぎる窓は有効でなくなるおそれがあるので注意を要する。

［松浦邦男］

[参照項目] | 照明 | 窓

昼光率〔図A〕

光幕反射による障害除去に有利な側窓〔図…

工場の頂側光〔図C〕

美術館の頂側光〔図D〕

採光による必要な明るさの基準〔表〕

出典　小学館　日本大百科全書(ニッポニカ)日本大百科全書(ニッポニカ)について　情報 | 凡例

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