TOA

• Luminous Flux


  Indicates the amount of luminous energy emitted per unit of time (1 second) from a source, i.e. its luminous power. For luminous energy it is meant the radiant energy emitted in the range 380 to 780 nm.
• Luminous Intensity


  The intensity of light radiated in a given direction
• Illuminance


  The ratio of the luminous flux received by a surface to the area of the surface itself. It indicates the amount of light that strikes a unit area.
• Luminance


  The ratio of the luminous intensity emitted by a surface in a given direction to the apparent area of that surface. The apparent area is the projection of the surface on a plane normal to the direction considered.
• Luminance indicates the sensation received from a light source, primary or secondary
• Illuminance indicates the amount of light, emitted by a source, which affects the surface considered
• Luminance indicates the sensation of brightness received from a surface
• On two surfaces, one white and the other black, we can have the same value of illuminance, e.g. 500 lux, but the sensation of light received, and then the luminance, will be completely different, since those two surfaces reflect the light differently
• Luminance distribution in the field of vision

  • Determines the state of adaptation, which in turn affects visual performance
  • Affects visual comfort
• Luminance distribution issues to avoid

  • Excessive luminance levels which can cause glare
  • Excessive variations in luminance, which can cause fatigue through permanent re-adaptation
  • Insufficient luminance levels or insufficient luminance variations which create an unattractive, rather dull and thus hardly stimulating work environment
• Extreme variations in luminance in the field of vision mean additional adaptation work when looking at the surroundings, which may cause premature fatigue and lack of concentration, which can lead to visual errors
• In case of serious variations in luminance, impending threats from the environment, eg, approaching vehicles or overhead cranes with loads etc. in Industrial halls may not be recognised in time, rendering timely reactions impossible
• A balanced luminance distribution in the field of vision furthermore supports communication with work environment and people
• The eye cannot adapt simultaneously to two very different levels of luminance
• If areas of very different brightness are both in the central part of the visual field, the concentration on only one of them becomes difficult, if not impossible, and from this situation arises unpleasant eye fatigue due to continual adjustments to which it is forced to adapt to different luminance
• Luminance ratio limits


  • 3:1 - focus area/area immediately surrounding
  • 5:1 - focus area/surrounding area
  • 10:1 - focus area/area more far away
  • 20:1 - Light source/adjacent area
• Workstations must be arranged so that the computer screen is perpendicular to the wall containing the window
• Illuminance


  A measure of the concentration of luminous flux falling upon a surface, expressed in lumens per unit area (lux)
• 1 lux = 1 lumen per square meter
• The original non-metric British unit is the foot-candle, 1 foot-candle = 1 lumen per square foot
• Recommended light levels for different types of work spaces

  • Filing, copying - 300 lux
  • Carry out office work - 500 lux
  • PC work, study by pet, desk work - 500 lux
  • Precision assembly work, fine detail - 750 lux
  • Outdoor light levels - 2 lux to 159,000 lux
• Daylight Factor (DF)


  A measure of the amount of daylight available in a space, defined as the ratio of the illumination of the working plane to the illuminance that would be on a horizontal surface exposed outdoors
• DF [%] = (Eint/Eext) x 100
• Components contributing to the illumination of a point (Eint)

  • Sky Component (SC) - light from the visible sky
  • External Reflected Component (ERC) - light reflected by external objects
  • Internal Reflected Component (IRC) - light that enters through the window and is reflected from internal surfaces
• Eint = SC + ERC + IRC
• A window size based on a day with overcast sky in tropical climate is excessive, causing excessive levels of illumination in most days and, above all, more solar gains: an increase in both investment and operating costs without a counterpart in terms of lighting comfort
• For given glass area and transparency, and external conditions, the daylight factor at a point of the room will be the greater the clearer are the Internal surfaces and the higher the light transmission coefficient of the glass
• DF decreases rapidly moving away from the window. For this reason the ratio of the minimum to the maximum values of illuminance due to the natural light must be maintained above the value 0.16
• Illuminance


  The amount of light entering through the window and being reflected from the internal surfaces; the higher its value the clearer the colours of walls and ceiling
• Daylight Factor (DF)


  • For given glass area and transparency, and external conditions, the daylight factor at a point of the room will be the greater the clearer are the Internal surfaces and the higher the light transmission coefficient of the glass
  • DF decreases rapidly moving away from the window
  • To derive the level of internal illumination in lux from the DF, it is necessary to know the level of external illumination with overcast sky, which is not equal in all parts of the world but decreases with increasing latitude
  • The daylight factor should never be used alone as an indicator for the design of buildings with low energy consumption and high quality of lighting, especially in tropical climates or where the number of annual hours of sunshine is high, higher than in the countries of northern Europe and North America, where this index was developed
• Mean Daylight Factor (DFm)


  The mean value of daylight factors measured at the level of the working plane by a grid of sensors extended to the whole space
• Mean surface reflectance (pm)

  • 0.5 (first approximation)
  • 0.6-0.8 (light colours)
  • 0.3-0.6 (medium colours)
  • 0.1-0.3 (dark colours)
• Daylight Autonomy (DA)


  The percentage of the building occupation hours in which the required minimum level of illuminance can be maintained with the natural lighting alone
• The main advantage of the daylight autonomy with respect to the daylight factor is that it takes into account not only all sky conditions that occur in a given location, but also the orientation and the occupation profile of the space being assessed
• Daylight Autonomy can only be calculated through computer simulations
• Glare


  The visual sensation produced by surfaces which produce high luminance gradients within the field of view
• Types of Glare


  • Discomfort Glare - reduces the ability to perceive details, not necessarily cause visual discomfort
  • Disability Glare - impairs the vision of objects
• Daylight Glare Index (DGI)


  Allows predicting the glare due to the natural light through the index UGR (Unified Glare Rating), which is used also for evaluating the glare due to an artificial light source
• It is important to limit the glare to avoid errors, fatigue and accidents
• Direct Glare


  Depends on the characteristics of the space and of light sources (natural or artificial) directly in the visual field of a person