Lecture 16 - Lighting, Glare and tints Flashcards
Ideal lighting:
• To improve visibility of objects for visually impaired, lighting should be provided in extra areas that distributes evenly, to avoid glare.
• Light coloured walls + ceilings improve light distribution
Local/Task lighting:
• Px with reduced macular function require increase light reading as increases WPM
• Any high luminance bulb will improve performance (doesnt depend on color temp)
Discomfort glare:
• Subjective visual discomfort
• The patient feels visually uncomfortable or fatigued by this type of glare. Symptoms can range from mild to severe, but VA is not affected by and symptoms are usually relieved by tints.
• VA not affected, experiences asthenopia, squinting
• Examples include uveitis, ocular albinism, cone-rod dystrophy, retinitis pigmentosa.
Disability glare:
• Loss of retinal image contrast as a result of intracular light scatter, or straylight.
- Reduces visual performance (VA)
- Severity depends on the angle between task and glare source & relative luminance
- Media opacities lead to intra-ocular light scatter which creates a veiling luminance across all parts of the retinal image. This reduces the retinal image contrast
• Conditions which induce disability glare include ageing, cataract, posterior chamber lOL, posterior capsule pacification, keratoconus, corneal oedema, Radial Keratotomy (RK), vitreous opacities, macular oedema (controversial).
Mechanism of Disability glare:
• If Michelson contrats M=(L max-L Min)/(Lmax + Lmin) = <1.0 then the contrast is reduced, (i.e if 0.67 contrast is 67% therefore reduced) and harder to see
Measurement of discomfort glare:
Discomfort glare can be measured under experimental conditions by having a patient adjust the brightness of a light source until it reaches an ill-defined threshold of unpleasantness. However, it does not seem to have any clinical relevance or applicability.
Measurement of disability glare:
Disability glare can be measured objectively. Tests include devices such as the brightness acuity tester, where the patient views an acuity or low contrast chart under controlled lighting conditions. The instrument allows the simulation of everyday situations like fluorescent lighting, sunlight on a cloudy day or direct overhead sunlight.
Clinical method for assessing disability glare:
- Measure VA or low contrast VA without glare source
- Re-assess with glare source (e.g. penlight)
- Standardise the angle and distance of light to eye
- typically 10cm and 30 degrees.
- Drop in VA of two lines or more; significant degree of glare
What colour temperature is associated with different lightbulbs?
• LED = 8,000°K
• Daylight Fluorescent = 6,500°K
• Halogen = 4,100°K
• Incandescent = 2,800°K
What features can be found on lightbulb packaging?
• Power
• Luminous efficacy
- Light output
- Heat output
• Colour temperature
- Colour rendering
• Light distribution
• Lifespan
Spectral content and colour temperature:
• Contrast sensitivity
• Visual acuity
• Threshold print size
• Critical print size
• Reading rate
• Acuity reserve
Reducing disability glare:
• Glare reduction includes environmental measures discussed before e.g.:
- the avoidance of shiny surfaces,
- use of blinds,
- appropriate task lighting (angle!),
- visors, hats
- tints
• Due to Rayleigh scattering, short wavelengths produce more glare than long wavelengths, therefore yellow tints may reduce glare
Equation for Luminance:
Luminance (I) = Intensity (S) x cos a/d^2
What conditions where low luminance preferable:
• Rod/cone dystrophies
- Achromatopsia
- RP etc.
• Albinism
• Aniridia
• Optic atrophy
- Glare, Performance & Comfort
- Task illuminance
Glare testing includes:
• Glare testers; Brightness acuity tester
• Pen torch (while testing VA + Contrast chart)
- Px’s with opacities will usually display reduced VA with glare
