Problem Set 3

Question 1

What is the difference between radiant flux and luminous flux?

Answer 1

Radiant flux is the power (energy per second) of radiation emitted from a source.

Luminous flux is the light flux per second from a source, adjusted for the luminous efficacy of different wavelengths of light as perceived by the human

Question 2

Explain the terms illuminance, irradiance and luminous efficacy and describe how they relate to the
consideration of lighting energy efficiency.

Answer 2

Irradiance = Flux density (power) per square metre of radiation incident on a surface (W/m2 ).

Watts are units of power (joules/sec).
Illuminance = Lumens of light per square metre incident on a surface (lux).

Lumens measure the power of light as perceived by the human eye.

Luminous efficacy = The amount of light (luminous flux, lumens) produced by radiation as perceived by the human eye per unit of electrical power (Watts) input to a lamp [lm/W].

Some of the electrical power consumed by a lamp will be wasted as heat. Additionally, because the human eye is more sensitive to some wavelengths of light than others, more lumens of light is produced by radiation of some wavelengths. A source that has a higher luminous efficacy will produce more light sensation (perceived by the human eye) per Watt of power. Hence a light source that has a higher luminous efficacy will use less energy per unit of light (and also produce less waste heat) than one with a low luminous efficacy.

Consider diffuse daylight 125 lm/W vs an incandescent light of 25 lm/W. Most of the radiation will become heat within a building. Hence, daylight introduces less heat per lumen into a building than does the electric alternative for the same illumination. The careful use of daylight can reduce cooling loads.

Question 3

List design considerations for acceptable visual performance and visual comfort.

Answer 3

1. sufficient illumination for the task (consult Australian standards)
2. equal brightness of task objects - max ratio of task objects 3:1 (IEA SHC 2000)
3. surrounding lighting correctly related to the task – ratio of illuminance task : surround : background of 1 : 0.5 : 0.2 (Szokolay 2004)
4. prevent glare - no bright objects in the field of view - glare will be caused if luminance ratio > 20:1 (Architectural Energy Corporation 2005)
   - to avoid glare from windows, use shades, drapes, blinds
   - to avoid excessive contrast from windows, paint walls surrounding windows pale colours
   - to avoid possibility of glare/reflections from interior surfaces, use subtle matte colours (not shiny
   white)
   - position lights or use louvres/reflectors so that they don’t shine directly into the eyes

Question 4

What functions should the artificial lighting perform during the day in a room lit by daylight combined with
electric light?

Answer 4

• bring illumination levels up to required levels

- reduce contrast by brightly lighting areas that are not well lit by daylight

Question 5

What is the difference between sunlight and daylight in lighting terminology? Briefly list the benefits and
drawbacks of using natural daylight in architecture and interior spaces.

Answer 5

Sunlight is the light directly from the sun. Daylight is diffuse light from the atmosphere or reflected from
other surfaces.

Benefits:

• save electricity
• continuous spectral distribution
• high luminous efficacy (low heat gain)
• people prefer natural light
• possibility of view

Drawbacks

• possibility of glare from sunlight
• inconsistent & unpredictable lighting
• windows can allow unwanted heat gain/loss
• may be difficult to achieve uniform lighting
• daylight is more horizontal further away from a window

Question 6

Use the tables provided to find the design sky for Sydney exceeded 85% of the time. Why is design sky used in daylighting design?

Answer 6

From the table, the design sky for Sydney (exceeded 85% of the time) is 10.6 x 103 lux.

The design sky is the amount of illumination exceeded 85% of the time in that location (between 9-5). Since
we know that the daylight factor (ratio of indoor to outdoor illuminance) remains constant regardless of
fluctuations in outdoor illuminance, we can determine the indoor illuminance that will be exceeded 85% of the time using design sky x DF. This allows us to design to achieve a minimum acceptable indoor level of illumination (as defined by the Australian standards for a particular task) that will be exceeded most of the time.