Light Distribution And Glare Flashcards
is a graphical representation that illustrates the __________________ around the cross- section of a lamp or luminaire.
• It is a curve on a polar graph that shows the relative luminous intensity, in candela,
around the lamp or luminaire
• A cross-sectional map of intensity (candelas) measured at many different angles. It is a _____-dimensional representation and therefore shows data for one plane only.
• The heavy, dark line in the curve represents luminous intensity at various positions beneath the luminaire opening.
• If the distribution of the unit is symmetric:
ꟷ the curve in one plane is sufficient for all calculations.
• If the distribution of the unit is asymmetric:
ꟷ such as with street lighting and fluorescent units, three or more planes are required.
Candlepower ( Candela) Distribution Curve, luminous intensity, two
Categories of Light Distribution Produced By Luminaires:
• ___________
ꟷ ___ to ______% of the light is directed downward for maximum use
• _________
ꟷ ______ to ______% of the light is directed to the ceilings and upper walls and is reflected to all parts of a room or 90 to 100% of the light is directed downward for maximum use.
• Semidirect
ꟷ _____ to _____% of the light is directed downward with the remainder directed upward
• General diffuse/ direct-indirect
ꟷ Equal portions of the light are directed upward and downward
• Highlighting
ꟷ Beam projection distance and focusing ability characterize this luminaire
Direct, 90, 100, indirect, 90, 100, 60, 90
Is excessive brightness in the field of vision that causes discomfort or, in extreme cases, produces a disability from a temporary loss of vision.
ꟷ It is a visual sensation caused by __________ that is sufficiently greater than the luminance to which the eyes are adapted.
ꟷ Glare can be an irritant of or impediment to ______. To prevent the design of lighting installations that produce discomfort, the visual comfort probability is calculated.
Glare, luminance, vision
is a rating of a lighting installation expressed as a percentage of individuals who, when viewing from a specified _________ and in a specified ________, will be expected to find it acceptable in terms of discomfort glare.
• In most spaces, the VCP should exceed %; that is, at least ___% of a statistical group of individuals will rate glare levels acceptable in the space beingmilluminated.
• Guidelines from the ______________________ are used to compute the VCP
location, direction, Visual Comfort Probability, 70, Illuminating Engineering Society of North America
Types of Glare:
• ______________
ꟷ causes visual discomfort without necessarily impairing vision.
• ____________
ꟷ occurs when visibility is impaired from excessive brightness.
• ___________
ꟷ occurs when excessive light enters the eye directly from a light source.
• _____________
ꟷ the result of light entering the eye after reflecting off a glossy
surface. A common type is called a veiling reflection: the reflection
on a glossy surface that impedes seeing such as the reflection on a
computer screen or on a glossy sheet in a magazine.
Discomfort glare
Disability glare
Direct glare
Reflected glAre
- a zone above a 45° angle from the fixture’s vertical
axis.
glare zone
_________ prevent glare and work well in larger spaces such as gymnasiums and conference spaces.
uplights
In small spaces, indirect lighting techniques consisting of________, ________, and _________ lighting installations diffuse light well and prevent glare.
cornice, cove, valance
Properly designed downlights use ________ to prevent light from being thrown into the glare zone.
baffles
Most linear fluorescent fixtures use either a _____ or a ______ to prevent direct viewing of the lamps so direct glare is prevented.
lens , louver
typically made from clear UV-stabilized plastics.
• Clear ______ types include prismatic, batwing, linear batwing, and polarized
lenses
• White translucent diffusers are much less efficient than clear lenses, and they result in relatively low visual comfort probability.
• New low-glare lens materials are available for retrofit and provide high visual comfort (a VCP of 80 or more) and high efficiency.
• Lenses are usually much less expensive but they provide less ______ control than louvered fixtures.
lenses, lens, glare
___________ in a light fixture that create openings for emitting light
• ________ provide superior glare control and high visual comfort compared with lens diffuser
systems.
• The most common application of louvers is to eliminate the fixture glare reflected on ___________.
• Deep-cell parabolic __________ (with 5 to 7 in cell apertures and depths of 2 to 4 in)
provide a good balance between visual comfort and luminaire efficiency
• luminaire efficiency to about ___%. A disadvantage of the deep-cell louver is that it adds 2 to 4 in to the overall depth of a luminaire.
slats, louvers, computer screens, louvers 40
_________ or translucent elements that shield a light source from direct view. They are typically part of the light fixture.
• ________ prevent the light source from throwing light beyond a ___° angle from the fixture’s vertical axis.
• Keeps the light from being broadcast into the glare zone.
opaque, baffles, 30
the amount of light incident on (striking) a surface
illuminance
is the amount of light leaving an object, thus relating to how bright an object appears to the human eye.
ꟷ An object may be illuminated, but the eye cannot see it without visible light leaving (reflecting off) the object in the direction of the viewer.
ꟷ The luminance of a surface is equal to the _____________.
luminance, reflected illuminance
