Light and Visual comfort (T3)

Question 1

Visual discomfort

Answer 1

• Visual discomfort occurs when the lighting makes it difficult to see what needs to be seen, causes distraction or pushes one the limits of the visual system, all of which are likely to affect visual performance negatively.
• Aspects of lighting that can commonly cause visual discomfort are: insufficient light, excessive light, shadows, veiling reflections, glare and flicker.

Question 2

Human requirements

Answer 2

(Day)lighting for performance/task. 
(Day)lighting for appearance.
(Day)lighting to ‘feel good’.
(Day)lighting for information. 
Daylight to save energy on electric lighting?

Question 3

Visible radiation

Answer 3

We can see visible raidation

Peak at 555nm

Question 4

Halgon

Answer 4

• If you came in from daylight = yellowy (warm)
• Heats up = like a heater
• Peak in the infrared = why they are being fassed out
• inefficent
• lux vaule = office lots of sources of light = containantion from other lights

Question 5

Illuminance

Answer 5

a measure of the flux of light falling onto a surface (from all of the directions ‘above’ the surface). Units of illuminance are lux (depends on brightness)

Question 6

Luminance

Answer 6

a measure of the intensity of light emitted from a surface per unit area in a given direction

Units of luminance are candela per square meter

Question 7

Luminance and brightness

Answer 7

Luminance is an objective measure
Brightness is a subjective attribute of light
The subjective perception of brightness relates to the objective measure of luminance, but not in a simple, direct way

Question 8

Discomfort glare

Answer 8

glare that causes visual discomort but without impairing the ability to see an object

Question 9

Disability glare

Answer 9

glare produced directly by reflection that impairs vision without necessartliy causing discomfort

Question 10

Veiling glare

Answer 10

occurs when a bright source of light ( be it articifcal or natural) causes a reflection on a surface, thus preventing the viewer from seeing the display properly

Question 11

Visual comfort/ performace parameters depends on

Answer 11

the relative and absolute values of brightness for the task and surrounding area including the periphery

Question 12

Why daylight glare studies?

Answer 12

-Increased use of glazing/ daylight
- Productuctivity abd well-being
- Appearance of buildings and spaces
- Reducing lighting energy consumption
Conflict with increased computer based work
- Glare from direct light, reflection and windows brightness

Question 13

Measuring luminace

Answer 13

Luminace meter

imaging photometer

Question 14

High Dynamic Range (HDR) imaging

Answer 14

• Systhesis of a luminace image from a sequence of ‘ordinary’ image captures
• changes contast
  Take several images with a fixed ampture
  Every pixel has different values
  Change the exposure so that each square has vale that is grey

Question 15

How would you begin to evaluate an architectural design in terms of its daylight potential? What factors should be considered?

Answer 15

• Lication / prevailing climate
• Buiding orientation
• Context/ obstructions
• Massing / building envelope
• Aperture size and placement
• Building use / occupancy
• Division of internal spaces
• Glazing properties
• Shading fixtures (external)
• Shading requirements (internal)
• Surface finishes
• Workstation placements
• Electric lighting design
• Electric lighting operations

Question 16

No sky line

Answer 16

• Because the amount is sky visible at the workplace, where obsructions are present it is common to estimate the no-sky line

Question 17

The origin of the measure for daylight in buildings

Answer 17

It appears that the daylight factor was first proposed in 1895 by Alexander Pelham Trotter.
The daylight factor was conceived as a means of rating
daylighting performance independently of the actually occurring, instantaneous sky conditions. Hence it was defined as ratio. However, the external conditions still need to be defined since the luminance distribution of the sky will influence the value of the ratio

Question 18

British standard BS8206

Answer 18

• The average daylight factor used as the measure of general illumination from skylight
• It is considered good practice to ensure that rooms in dwelling and in most other buildings have a predominatly daylit appearance
• In order to achieve this the average daylight factor should be at least 2%
• If the average daylight factor in a space is at least 5% then electric lighting is not normally needed during the daytime, provided the uniformity is satisfactory.
• If the average daylight factor in a space is between 2% and 5% supplementary electric lighting is usually required.
• NOTE: Excessive daylight can cause visual discomfort and be associated with overheating.

Question 19

Methods to predict the daylight factor.

Answer 19

Physical – measuremnts in scale model
Graphical – tabular and analystical methods
Computer simulation – using a program such as radiance