LIGHTING AND GLARE CONTROL Flashcards
What is photometry?
This measures part of the radiant power perceived by the human eye as light
What is Radiometry?
Measures the entire radiant power and the quantities derived from it
What is luminous efficacy (lm/W)?
Type of way that illumination is measured
- Ratio of luminous flux to radiant flux
The ratio of light emitted by the light source within visible spectrum
What is luminous intensity (Candela cd)?
Type of way illumination is measured
- Perceived power emitted by light source in a particular direction per unit solid angle
Amount of light emitted or reflected from a particular area
What is illuminance (lux lx)?
- Total luminous flux incident on surface, per unit area
Amount of light falling on object or surface
What is inverse square of the distance law?
Light intensity decreases with the square of the distance to the source
What is Lambert’s Cosine Law of Illumination?
Light source is not perpendicular to the task surface = illuminance reduced by cosine of the angle with perpendicular plane
Why is light source important in low vision?
People aged 60 years need three times more light to achieve best VA than those in 20s
How can natural light be beneficial?
It needs to be controlled and possible ways to do this are:
- Keep windows clean
- Keep curtains
- Tinting of windows/blinds/curtains
- Work with back to window
How does artificial light help?
- Helps to provide adequate amount of light and look appropriate
- Increase number of lights to give more even spread of illumination
- Add wall, table or floor lights in dark areas
What does direct artificial light do?
Directed to work efficiently with low light loss e.g. downlights
What does semi-direct artificial light do?
60-90% of the light is emitted directly to the work area e.g. surface fluorescent
What does indirect artificial light do?
Reflected from the ceiling and walls (poor efficiency) e.e. pelmet lighting
What is diffuse artificial lighting?
Light diffused with soft shadows e.g. opal glass or a sphere pendant
What are unwanted reflections?
- High luminance reflections which overlay the detail of the task
- Usually reduce contrast and cause discomfort
What are the management strategies for unwanted reflections?
- change relative position of task and lighting/windows
- use additional task lighting to ‘swamp’ the veiling reflections
- screen the task
- screen the source (e.g. window, light)
- Avoid glossy surfaces
- Use a non-reflective screen
What is discomfort glare?
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What is discomfort glare?
- A function of luminance of source, luminance of background, angular size of source and source displacement from the eye
What can discomfort glare be reduced by?
- reducing luminance of source (most effective)
- reducing apparent size of source
-increasing background luminance in line of sight - removing glare source from line of sight
What does a tint do?
- Can reduce both disability and discomfort glare
- can enhance contrast
- can alter colour perception
How do tints work?
- To reduce disability glare, the tint needs to absorb light that is scattered in the eye, whilst ensuring the unscattered light is not reduced
What does a neutral grey tint do?
Will reduce all light hence not change the contrast of the image
What do blue and red lights do?
Blue light is scattered more than red light by the lens and it is also thought to be more harmful to the macula