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.