Day Light Factor in Office Spaces

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DAY LIGHT FACTOR IN OFFICE SPACES

BACKGROUND:


Day lighting admitted into building through “holes” in external fabric (windows, roof lights, etc.) which in adverse climates generally incorporate glass or an alternative transparent material to heat loss.

The energy efficiency and sustainability became increasingly important issues in the field of architecture. Often day-lighting is recognised as a key strategy in reducing the energy consumption by reducing amount of electrical lighting in a building. And even, day lighting is also known for having its positive effects on human health and productivity. So by all these, the energy standards and green building rating systems have strongly recommended that designers incorporate day lighting strategies into building design. However, this recommendation is frequently ignored due to the complexity in adopting day lighting design and due to the visual discomfort caused by the excessive sunlight penetrations and sun glare.

Many days lighting analysis have been developed and used by students, designers and consultants. From physical modelling to computer based simulations, analysis methods are used to predict the performance of day lighting systems before a building is completed.by using all these methods, we can measure or calculate various values like illuminance levels. However it is not easy to get accurate predictable or often even matching values between techniques due to variance in variables and also due to the sky selections.

INTRODUCTION:

Day lighting is a highly cost-effective means of reducing the energy for electrical lighting and cooling. But architecture education often reduces the aspect of day lighting to eye-catching effects on facades and scarcely discusses its potential effects – not just on cost, but health, well-being and energy.

The light matters will explore the often unexplored aspects of day lighting and introduce key strategies for you to better incorporate day lighting into design: from optimizing building orientations to choosing interior surface qualities that achieve the right reflectance. These steps can significantly reduce your investment as well as operating costs. And while these strategies will certainly catch the interest of economically oriented clients, you will soon discover that day lighting can do so much more.

The main study of this paper is to understand the dynamics of day light factor in relation to building spaces and to understand day light as a design element. The development and effects of day light and its factor in office buildings.

ABSTRACT:

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

Day light factor:

The concept of daylight factor (DF) was developed in the United Kingdom in the early 20th century. Day light factors are used in architecture and building design in order to assess the internal natural lighting levels.

Daylight factor is defined as the ratio of the internal illuminance at a point in a building to the unshaded, external horizontal illuminance under a CIE overcast sky (Moon and Spencer 1942). The main intension of using this type of ratios rather than absolute values, was to avoid the difficulty of having frequent and often severe fluctuations in the intensity of day lighting (Waldram 1909). In the beginning, sky factors were used to calculate the day light illuminance from the sky dome to a particular point in the building. Later on, the sky factor was evolved into the daylight factor, as the light reflected from external obstructions, light from the glazing and internal refelectances were added as well (Waldram 1950).

Day light factor is the ratio between indoor illuminance and outdoor illuminance. It can be measured for a specific point or for an average of a space. The following formulae shows how to calculate day light factor from illuminance levels.

""

DF = 100 * Ein / Eext

Where

DF = day light factor

Ein : illuminance due to day lighting on the indoor working place

Eext : illuminance outdoors on a unobstructed horizontal plane.

Daylight reading at a reference point in a room can be made up of three components:

Sky component – the light received directly from the sky

Externally reflected component – the light received after reflection from the ground, building or other external surface and

Internally reflected component – the light received after being reflected from the surfaces inside a building.

All these three components need to be accounted for to determine the day light factor. Day lighting factor was used to obtain the minimum lighting requirements, but it does not gives the idea of good day lighting. Even though the day lighting factor was used most widely to measure day lighting and for majority of practitioners, the consideration of day light begins and ends with day light factor (Nabil and Mardaljevic 2005). For the last fifty years, this design practice has largely remained unchallenged with a few notable exceptions (Kendrick and Skinner 1980; Tregenza 1980). So it is widely accepted , quantitative performance measure for day lighting.

Day light factor has also got different supporting qualities for the design usage in the buildings. Depending on the building designs, the day light factors have the capacity to influence the design choices. Coming to the different aspects where the day light factor can affect the building design were like, the geometry of the building, surrounding landscapes and surrounding buildings. Not only the external properties, due to the day light factor, there a considerable affects in the interior of the buildings also. They are like the colour, reflectance, transmittance, diffuseness, secularity, etc., have an impact on the day light factor.

In general, the daylight factor measures are intuitive and easy to communicate. To calculate the day light factor, there are a number of methods existing. They range from simple calculations to the BRE split flux method to advance calculation techniques based on radiosity (Lighting Analysts Inc. 2006), Lumen Designer (Light Technologies Inc. 2006) and/or Raytracing (Ward and Shakesphere 1998). Using radiance, for any geometry of building and any type of material, we can calculate the day light factor.

The day light factor can also have the influence on day light designing in the particular buildings. Some form giving features, which are generally accompanied with the good day light, were influenced. Like the high window- head heights, high ceiling reflectance, large facades, wall finishes, narrow floor plans, high transmittance glazing, sky light openings, etc.

If we go for the “the more the better” approach, the maximum day light penetrating into the building in a fully glazed building will be more. We can have the enough day lighting, but often those building exhibit comfort and energy related problems. Generally these are conditions which we see in commercial buildings.

There are few limitations of the day light measures. They are; Design recommendation based on the day light factor will be same for all types of façade orientations. Day light factor for all types of building locations will be same. Daylight factor does not consider the season, time of the day, variable sky conditions. Actually this bears very important consequences. Day light factor cannot help to prevent the glare strategies for different façade orientations, even though the problem of glare is most prominent.

Day light factor in office spaces:

A day light 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 illuminance of 2500 foot candles and a day light factor of 2% the indoor illuminance on the desktop would be 50 foot candles.

2% x 2500 foot candles = 50 foot candles

Other typical day light factors for various spaces are:

Various spaces

Day light factor

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 day light factor based on the building design includes window/skylight sizes, overhangs/light shelves, glass types, and exterior/ interior refelectances can become very complex. A simple rule of thumb to approximate the day light factor for day lit spaces using vertical windows is

""D = 0.1 x PG

Where

D = daylight factor

PG = percentage of glass to floor area.

For example, a 1000sft office space has 200sft of windows, and then the day light factor will be

DF = 0.1 x (200/1000)

= .02 or 2%

The day light factor levels can be broadly classified into three categories:

Under 2

Between 2 and 5

Over 5

Not adequately lit

Adequately lit

Well lit

Room looks gloomy under day light alone

Artificial lighting may be used partly

Artificial lighting generally not required except at dawn and dusk

Artificial lighting will be required

But glare and solar gain may cause problems

Daylight factor contours:

The day light factor inside a room will vary according to position. Day light factors are often given as working plane contours. Tall windows provide deeper day light penetration, while multiple windows provide more even day light distribution.

Assessment of day light factors:

The day light factor inside an existing building can be measured directly using a photometer which gives a direct reading of the day light factor.

In the design stage, day light factors can be predicted using:

  • Design tables;
  • Waldram diagram;
  • Computer programs;
  • Scale models;
  • Day light factor protractors.

Day light factor analysis:

In spite of the day light factor popularity as an indicator of day light performances, daylight factor has some serious limitations.

  • Day light factor doesn’t take consideration, the location of the building.
  • DF cannot represent the change in illuminance levels indoors, due to the temporal variations of the sky luminance.
  • The orientation of the facade of the building has no effect in day light calculation.
  • Da y light factor doesn’t assess glare caused by the day lighting.
  • Glare is a major issue requiring careful considerations during the design stage.
  • Day light factor is not specific about the climate.

LITERATURE REVIEW:

Light is as much a “material” for building as the stones, bricks, and other components used in construction; for, although there would be no wall without its structural components, the wall has no real existence for us unless it corresponds to a sensual impression, gained with our eyes and substantiated with our minds.

  • Derek Phillips

Natural light is recognized and varied for the qualities that it offers. Man’s religion, health and culture has huge positive impact due to day lighting, since ages. The need for natural illumination in interiors has been investigated by lighting engineers, physicians and psychologists for several centuries, and thus had a profound effect on day lighting, the science of natural illumination. Research on the effects of the deprivation of light, the need for view and effects on biological processes have confirmed the factual bases for this need.

Characteristics of Natural Light:

Natural light that enters any room consists of two basic parts, namely direct sunlight (insolation) and diffused skylight. Roger Neal Goldstein in his Research on Natural Light in Architectural Design states that by carefully orienting a window or a roof-light one can allow for the penetration of both the elements of natural light, if desired.

According to Millet (1996), the luminous effect of all light sources depends on four factors:

  1. The Source ( its intensity, its directional characteristics, its colour)
  2. The Geometry (its relationship between the source and the receiver or receiving surface)
  3. Different Surfaces inside the Space (The surfaces that receive and modify light, becoming secondary light sources in themselves by reflecting, redirecting and colouring light ; and the person who views the source and illuminated surfaces as he or she moves around.)
  4. Movement and Visual Perception of the Observer.

He also declares that by observing how light behaves, we can work with it to reveal architecture.

THE SOURCE:

According to Millet(1996), “Each particular place has its light. Light expressing place encompasses two distinct aspects:

  • The place itself, its physical feature and characteristics that determine how it differs at any given moment from any other place and
  • The particular set of changes that take place within it over time, creating distinctive patterns of diurnal and seasonal changes.

Sukhtej Singh Gill in his paper on the Study of the Characteristics of Natural Light in Selected Buildings by Le Corbusier, Louis. I.Kahn and Tadao Ando mentions that “these meanings change the way light interacts with the built environment.”

"The window is a major component of the spatial record between inside and outside. With its size relative to the solid wall, it determines the direction in which attention is focused. With its details, it defines the transition between the room and landscape." ( Millet 1996, p. 96)

It is in this transition of natural light from exterior to the interior that a space is transformed and its character is defined.

INTENSITY:

The intensity of light is judged based on the perception of the objects it tends to highlight. Lam (1977) states that "Perceptions of the luminous environment always include an affective component an evaluative or emotional response to the perceived state of affairs."

Judgements in a space, such as light or dark, bright or gloomy, interesting or dull, sparkle or glare, depend on whether or not the luminous environment meets our expectations and satisfies our needs for visual information by emphasizing what we want or need to see rather than the actual luminance levels in a space.

DIRECTIONAL CHARACTERISTICS:

The directional characteristics of natural light are defined out of the movement of the sun across the sky. The depth shadows and the contrast between two surfaces are born out of the directional characteristics of the source of natural light once interrupted by a physical plane or boundary. These are associated with the character of a space and help to draw a meaningful relation with the source at all times of a day, month or season of a year.

Tannizaki (1997) describes that the beauty of a Japanese room depends on a variation of shadows, heavy shadows against light shadows.

Light is an indelible part of our experience of life. It helps us understand the overall depth of the scene by showcasing the difference between the bright and the dark.

COLOR:

The use of color can alter the perception of a space. Lighter colors reflect more light and the room appears to be of a greater size. When painted with dark colors that tend to absorb most of the light falling on the surface, the same room appears much smaller in size. The light that filters through a colored glass can change the hue of the base color of the surface on which it falls.

Apart from this, natural light changes in color as the case is when comparing the morning, afternoon or evening light from the sun, but this change is compensated by our expectation in the way we perceive the same space at different times of a day.

According to Philips (2004), " the experience of natural color, for whilst the physical color of our world as experienced in daylight alterations from dusk to dawn, the changes are part of our experience; we compensate automatically, a white wall appears a white wall even if in the evening it may be warmer, or is coloured by sunlight, or altered by cloud formations.... it is the color we regard as natural".

THE GEOMETRY:

Light is not perceptible without form and vice versa says Sukhtej Singh Gill in his paper " The Study of Characteristics of Natural Light in Selected Buildings Designed by Le Corbusier, Louis. I. Kahn and Tadao Ando".

He quotes Millet's statements that " Natural light that renders form visible is always changing, but we perceive the form as stable due to out perceptual process.... Shadows help in the perception of the form and the spatial depth...Extreme brightness or darkness can dissolve a form as they tend to blur the details and obscure the firmness of a material... Silhouetting is one of the ways in which light from the roof glazing can reveal the structure."

IMPORTANCE:

Research identified benefits of daylight, sunlight as well as view for people’s health and

Well-being.

  • People believe that working under natural daylight is better for health and well-being than electric Light (Veitch et al., 1993/1996)
  • Less eyestrain (Cowling et al., 1990), higher productivity (Visher 1989), more effective learning (Heshong et al 1999)
  • Daylight is better for psychological comfort, for office appearance and pleasantness, for general health, for visual health, and for colour appearance of people and furnishings (Heerwagen et al.,1986)
  • Having a window, or having access to daylight, improves satisfaction with lighting (Veitch et al.,2003)
  • In deep working spaces people have a strong preference for being near the windows (Christoffersenet al., 1999)

REFERENCES:

  1. A Study of the Natural Light In Selected Buildings Designed by Le Corbusier, Louis. I. Kahn and Tadao Ando by Sukhtej Singh Gill;2006
  2. Natural Light in Architectural Design: Element and Determinant by Roger Neal Goldstein;MIT;1976
  3. Day light benefits in health care buildings, by prof. DTG Strong.
  4. http://www.arefianatelier.com/interior-light-effects-on-human-mood-and-social-behaviour.html
  5. Natural light presentation, by kjeld johnso.
  6. Health and Light, by John Nash 1973.
  7. Daylighting controls(http://windows.lbl.gov/comm_perf/daylight/esl321.html)
  8. http;//home.wlv.ac.uk/~in6840/Daylightfactor.htm
  9. daylight factor versusdaylihgt availability in computer-based daylihgting simulations by Karen Kensek and Jae Yong Suk

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