FACTORS AFFECTING LIGHT LEVEL STANDARDS
Light level standards are affected by light quantity and quality desired, fixture efficiency and other
applicable factors.
Quantity of light or the light output and light levels is measured in lumens, lux and footcandles.
Initial Lumens/ Footcandles reflect the amount of light produced by a lamp when it is installed.
Supply voltage variations, lamp’s interaction with the ballast and dirt build up (Luminaire Dirt
Depreciation) reduce the produced amount of light. Lessening of light output over time, while
continuing to consume the same energy amounts (Lamp Lumen Depreciation) also reduces the
light levels of the lamp and waists energy.
Maintained Footcandles show the light level after light loss factors are taken into account over a
period of time. Mean Lumens show the average light output over the lamp’s lifetime. When
addressing lighting standards a provision for the light quantity depreciation over time due to
multiple factors should be made.
Quality of light depends on the brightness, distribution and light color.
Photometric brightness (Luminance) is the amount of light leaving the lamp or reflecting from a
surface. It is measured in footlamberts, candelas/sq. ft. and candelas/ square meter (metric).
Brightness can produce levels of glare if not contained properly. Glare that comes directly from the
light source is a Direct glare. Reflected glare occurs on the task surface. Discomfort glare does
not blind but creates discomfort. Disability glare prevents vision and blinds. Every fixture has a
Visual Comfort Probability (VCP) rating that reflects its level of visual comfort.
Glare can severely interfere with visual comfort. High brightness ratios produce high contrast and
can also create a visual fatigue during “transient adaptation”, which is the adaptation process of the
eye when brightness changes.
Light Color depends on the visible light spectrum and the wavelength composition of the lamp light
(Spectral power distribution), the color of the light the lamp produces (Color temperature
measured in Kelvin), the way the light source makes the color appear to human eyes and how well
subtle variations in color shades are revealed (Color rendering -Color Rendering Index from 0-
100). The higher the CRI is, the better the color rendition appears. An inappropriate color rendition
can deceive the eye and supply it with wrong information.
Sunday, January 20, 2008
DEFINITIONS/TERMS OF PHOTOMETRY
DEFINITIONS/TERMS OF PHOTOMETRY
FOOTCANDLE (fc)- A unit of measure of illuminance. A unit of illuminance on a surface that is
one foot from a uniform point source of light of one candle and equal to one lumen per square foot.
Footcandle values can be measured directly with handheld incident light meters. One footcandle is
equal to 1 lumen cast per sq. ft. of surface. A typical sunny day can measure between 5,000 fc and
10,000 fc. An average living room measures about 30 fc. A full moon can provide 0.2 fc of
illumination.
ILLUMINANCE (usually 'E' in formulas) is the total amount of visible light illuminating (incident
upon) a point on a surface from all directions above the surface. This "surface" can be a physical
surface or an imaginary plane. Therefore illuminance is equivalent to irradiance weighted with the
response curve of the human eye.
Standard unit for illuminance is Lux (lx), which is lumens per square meter (lm/m2).
1 fc= 10.764 lx
HORIZONTAL ILLUMINANCE: The measure of brightness from a light source, usually
measured in footcandles or lumens, which is taken through a light meter's sensor at a horizontal
position on a horizontal surface.
VERTICAL ILLUMINANCE: The measure of brightness from a light source, usually measured
in footcandles or lumens, which is taken through a light meter's sensor at a vertical position on a
vertical surface.
UNIFORMITY RATIO describes the uniformity of light levels across an area. This may be
expressed as a ratio of average to minimum or it may be expressed as a ratio of maximum to
minimum level of illumination for a given area.
Example: U. ratio max. to min. = 4:1 for the given area, the lowest level of illumination (1) should
be no less than 1/4 or "4 times less" than the maximum (4) level of illumination.
IESNA (state referenced IES): Illuminating Engineering Society of North America (also known as
IES). The society has informed and educated members, the lighting community and consumers on
the aspects of good lighting practice for over ninety years. The members of IESNA include
architects, engineers, designers, educators, students, contractors, manufacturers, and scientists.
IESNA provides technical guidelines and standards, professional journals, and educational training
forums.
IACLEA: International Association of Campus Law Enforcement Administrators. IACLEA’s goal
is to advance public safety of educational institutions. The society represents over 1000 schools in
US, Europe, Australia, Canada and elsewhere. In April 1996, IACLEA’s Board of Directors
adopted the "Recommended Crime Prevention and Campus Protection Practices for Colleges and
Universities". The document included a provision suggesting higher lighting levels than the ones
established by IESNA. These higher illumination standards were developed by IACLEA in order to
provide another standard and to give campus security a justification for higher campus lighting level
standards. IACLEA publishes a bimonthly newsletter and conducts annual conferences.
ICA- The International CPTED Association. ICA is committed to a creating safer and improved
environment though the application of CPTED principles and strategies.
CPTED-Crime Prevention though Environmental Design. CPTED advocates that adequate design
and proper use of the environment can reduce crime and fear of crime, thereby improving the
quality of life.
FOOTCANDLE (fc)- A unit of measure of illuminance. A unit of illuminance on a surface that is
one foot from a uniform point source of light of one candle and equal to one lumen per square foot.
Footcandle values can be measured directly with handheld incident light meters. One footcandle is
equal to 1 lumen cast per sq. ft. of surface. A typical sunny day can measure between 5,000 fc and
10,000 fc. An average living room measures about 30 fc. A full moon can provide 0.2 fc of
illumination.
ILLUMINANCE (usually 'E' in formulas) is the total amount of visible light illuminating (incident
upon) a point on a surface from all directions above the surface. This "surface" can be a physical
surface or an imaginary plane. Therefore illuminance is equivalent to irradiance weighted with the
response curve of the human eye.
Standard unit for illuminance is Lux (lx), which is lumens per square meter (lm/m2).
1 fc= 10.764 lx
HORIZONTAL ILLUMINANCE: The measure of brightness from a light source, usually
measured in footcandles or lumens, which is taken through a light meter's sensor at a horizontal
position on a horizontal surface.
VERTICAL ILLUMINANCE: The measure of brightness from a light source, usually measured
in footcandles or lumens, which is taken through a light meter's sensor at a vertical position on a
vertical surface.
UNIFORMITY RATIO describes the uniformity of light levels across an area. This may be
expressed as a ratio of average to minimum or it may be expressed as a ratio of maximum to
minimum level of illumination for a given area.
Example: U. ratio max. to min. = 4:1 for the given area, the lowest level of illumination (1) should
be no less than 1/4 or "4 times less" than the maximum (4) level of illumination.
IESNA (state referenced IES): Illuminating Engineering Society of North America (also known as
IES). The society has informed and educated members, the lighting community and consumers on
the aspects of good lighting practice for over ninety years. The members of IESNA include
architects, engineers, designers, educators, students, contractors, manufacturers, and scientists.
IESNA provides technical guidelines and standards, professional journals, and educational training
forums.
IACLEA: International Association of Campus Law Enforcement Administrators. IACLEA’s goal
is to advance public safety of educational institutions. The society represents over 1000 schools in
US, Europe, Australia, Canada and elsewhere. In April 1996, IACLEA’s Board of Directors
adopted the "Recommended Crime Prevention and Campus Protection Practices for Colleges and
Universities". The document included a provision suggesting higher lighting levels than the ones
established by IESNA. These higher illumination standards were developed by IACLEA in order to
provide another standard and to give campus security a justification for higher campus lighting level
standards. IACLEA publishes a bimonthly newsletter and conducts annual conferences.
ICA- The International CPTED Association. ICA is committed to a creating safer and improved
environment though the application of CPTED principles and strategies.
CPTED-Crime Prevention though Environmental Design. CPTED advocates that adequate design
and proper use of the environment can reduce crime and fear of crime, thereby improving the
quality of life.
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