Daylight and sunlight What is light?

Light An electromagnetic wave-the visible

spectrum of radiation from the sun
Colour Wavelengths from 300(violet) to 700(red)
NM determine the colour
Texture White is the sum of the spectrum
The colour of objects is the result of
absorbtion

Sue Wolff Sue Wolff

Solar spectral intensity Units of light

Illuminance: measurement of light received,
measured in lumens per square metre or lux,
Example, indoor, artificial lighting usually
300-500 lux for offices, outdoors varies
from 10 - 30,000 lux
Luminance: measurement of the brightness
of an object in candellas per square metre

Sue Wolff Sue Wolff

Some simple light rules Sunlight and daylight

Light travels in straight lines Sunlight Daylight
The angle of incidence equals the angle of Direct from the sun Light from the whole
reflection Position changes sky
throughout the day In overcast conditions
Light can also be bent by passing through does not vary with
Depend on latitude
narrow slits (diffraction) or through and time of day, year azimuth
materials of different density (refraction) Brightest at the zenith:
In our latitude below

61 degrees 3 times the horizon

Sue Wolff Sue Wolff

1
 Apparent sunpaths

Sue Wolff Sue Wolff

Daily mean
Average radiation
solar intensity
radiation pa throughout
the year for
different
surfaces
Sue Wolff Sue Wolff

Clear sky CIE overcast sky

Brightness at altitude
B
B= Bz (1 +2sin )
3
Brightness at zenith
almost 3 times
brightness at
horizon

Sue Wolff Sue Wolff

2
 Criteria for design
Decide the mood
Functional requirements-usually in lux
Avoidance of glare

Sue Wolff Sue Wolff

Components of daylight
Sky component

Externally reflected
component

Internally reflected
component

Sue Wolff Sue Wolff

Definition of daylight factor Daylight factor

The of light received For a CIE overcast sky , it depends on
at the working plane Geometry of room window and overhangs
as a percentage of the
Reflectance of surfaces
illuminance received
at an unobstructed Transmission of glass
point outside,

Ei x 100
Eo
Sue Wolff Sue Wolff

3
 Adaption
Huge adaption range
At any one time this range is more limited
Depend on the angle of the the source

Sue Wolff Sue Wolff

Simple calculation method for average daylight factor

Colour addition and
DFav= d.T.M d is the angle of visible sky
subtraction A (1-R2) T transmission of glass
A area, R reflectance of internal
Addition:light: red green blue surfaces M maintenance factor

Subtraction: painting and printing: cyan

indigo magenta

Sue Wolff Sue Wolff

What is good daylight? Distribution

View
Minimum and average Maximum ratio between maximum to minimum is
daylight factor 10 to 1
Good distribution Uniformity min/average about 0.2 to 0.4
Avoids glare
(For very uniform enclosures eg sports centres 0.8)

Sue Wolff Sue Wolff

4
 Daylight factor
BS daylighting standards
recomendations 
      

         
             
BS daylight for   

 
housing: ! "" #%$&&' )+*-, )
(&&
Average daylight  " 
$&&.)+*/( )
#
factor 01234 56(&&.)+*/( )
living rooms: 1.5% 73


 " $(& )>-( ?
Kitchens: 2% 8 "
 " 5
9 :;!3< =
Bedrooms 1% 0  5 "   8 " $&&'
1 )>-( )
  
(&&
#@
Sue Wolff Sue Wolff

Daylight factor recommendations European daylight factor

recommendations
Italy glazing 12.5 % of
floor area ,DF > 2%,
schools DF =3 %
France
Germany DIN 5034
Greece windows 10%
of floor area

Sue Wolff Sue Wolff

Modifying light
Light shelf

Sue Wolff Sue Wolff

5
 Sue Wolff Sue Wolff

Uniformity Types of window

Uniformity is the measure of the range of daylight
factors and is defined by the formula
Long slot
Uniformity = minimum DF/AverageDF
High casement
Clerestorey
For reasonable uniformity say in an office or Roof light
classroom attempt to achieve a uniformity of 0.25
to 0.3

Sue Wolff Sue Wolff

Types of glass Daylight Availability London

Clear and low emissivity London
Solar control :
Absorbing
Reflecting
Transparent Insulation materials (TIM)
Thermochromic and electrochromic
Prismatic and holographic

Sue Wolff Sue Wolff

6
 Daylight Availability Azimuth weighting factors
All locations have the same annual hours of daylight per year but northerly locations have more of these hours
outside working hours

Sue Wolff Sue Wolff