Lighting Terms and Terminology
Light and Optics
The electromagnetic spectrum includes all radiant energy from gamma
rays and x-rays, through the ultraviolet and the visible to the
infrared and radio. Electromagnetic radiation has both wave properties
(wavelength, frequency) and particle properties (photons)
The higher the energy, the `bluer' the photon, the shorter the
wavelength, and the higher the frequency.
LIGHT is the radiation capable of producing visual sensation, or
brightness. It is characterized by wavelengths from 380 nm (violet) to
770 nm (red).(atmospheric cutoff is ~320 nm.)
All solid objects radiate a continuous spectrum according to their
temperature - the hotter it is the bluer (but speak of "colder") it
appears. This kind of temperature from a `blackbody' radiator is
called the color temperature
- sun: 5780 K, peak at 502 nm
- incandescent lamp: 2900 K, peak at 999 nm
Lamp color is also rated by the color rendering index (CRI); it
is derived by noting any changes in appearance in eight standard color
samples when illuminated by the lamp as compared with a blackbody of
the same color temperature. The highest CRI rating is 100.
Heated gases produce bright line or emission line spectra as
electrons around the atoms in the gas spit out photons of specific
wavelengths.
- Mercury lines: 365, 404.7, 435.8, 546.1, 577.0, 579.1 nm
- Sodium D lines: 589.0, 589.6 nm
Light spreads out with distance from a point source in such a way that
when you double the distance, the intensity decreases by four. (The
inverse r-squared law.) Light is further diluted if the surface it
strikes is not perpendicular to the direction of light flow.
Light `rays' may be reflected (angle of incidence = angle of
reflection) and so redirected; this principle is used in the design of
reflectors in lighting fixtures.
Reflectance is the ratio of the reflected to the incident light
(usually depends on the geometry and wavelength).
- specular aluminum 95%,
- new snow 74%,
- concrete 40%,
- vegetation (mean) 25%,
- asphalt 7%.
Light rays are refracted when passing from one medium into
another, by an angle that depends on the optical properties of the
medium (and the wavelength of the light). This principle is also used
in lighting fixture optical elements.
Light Measurement Parameters
Light `flow' from a lamp is measured in lumens. A uniform point
source of one candela (luminous intensity in any direction [defined
for 555 nm radiant intensity of 1/683 watt per steradian] ~ one dinner
candle) at the center of sphere of 1 foot radius gives 1 lumen per
square foot. This is also one lumen per steradian. Area of the one
foot sphere is 12.57 sq. ft. so the 1 cd. source produces 12.57
lumens.
- a 60 watt incandescent lamp emits about 1000 lumens
- a 50 watt high pressure sodium lamp emits 3900 lumens
The quantity of light shining on a surface is the
illuminance. It is measured in lumens per square foot (
footcandles, horizontal or vertical), or lumens per
square meter (lux). 1 fc = 10.76 lx.
A few examples of illumination levels
- Sun 11,000 fc
- Full Moon 0.017 fc
- Venus 0.000012 fc (casts shadow!)
- McIntire Park parking lot about 10 fc maxima
- Barracks Road Amoco 80 fc
Luminance is the technical name for what we see coming from an
illuminated surface (e.g. emitted at the surface of a lamp, or
reflected from the ground) with units of candelas per square meter.
- solar disk: 1.6 billion cd/sq.m
- moon: 2,500 cd/sq.m.
- av. clear sky 8,000 cd/sq.m.
- 60 W inside frosted bulb: 120,000 cd/sq.m.
- T8 cool white fluorescent: 11,000 cd/sq.m.
- green electroluminescent source: 27 cd/sq.m.
- darkest sky: 0.0004 cd/sq.m.
Lighting efficacy is indicated by the lumens produced in
relation to power consumed as lumens/watt.
- a 60 watt incandescent produces 17 lumens/watt
- a 50 watt HPS produces 78 lumens/watt
Light Sources
There are six families of conventional lamps commonly used in outdoor
lighting applications: incandescent, fluorescent, mercury vapor (MV),
metal halide (MH), high-pressure sodium (HPS), and low-pressure sodium
(LPS). Except for incandescent, they are all gas discharge sources,
i.e. light is emitted when an electric current passes through the
gas.
MV, MH, and HPS are high-intensity discharge (HID) lamps: there is an
inner quartz envelope (arc tube) tolerant of high temperature (300 -
400 deg F) and pressure (1/4-4 atm.) and containing a mixture of
gases (argon and mercury, argon and sodium, et al..
All gas discharge lamps require BALLASTS
- to provide preheat and ignition voltages
- to provide operating voltage and limit lamp current
- can be solid state electronic (20kHz-60kHz) or magnetic (60Hz)
Incandescent Lamps
- 15-1500 Watts
- 125-33,850 lumens
- 1000 hrs rated life
- 8-23 lm/W; efficiency 10%
- inefficient, short lifetimes
Fluorescent
- UV excites phosphor
- 4-215 W
- 135-15,000
- 34-70 lm/W;
- 6,000-20,000 hrs
Mercury Vapor
- 50-1000 W
- 1580-60,000 lm
- 24,000+ hrs
- 32-60 lm/W; efficiency 12%
- inefficient, high LLD, poor color rendition
Metal Halide
- 39-1800 W
- 3000-150,000 lm
- 6000-10,000 hrs
- 77-83 lm/W; efficiency 21%
- color shifts
High Pressure Sodium
- 35-1000 W
- 2250-140,000 lm
- 24,000 hrs
- 64-140 lm/W; efficiency 26%
Low Pressure Sodium
- pressure ~.00001 atm.
- 18-180 W
- 1800-33,000 lm
- 18,000 hrs
- 100-183 lm/W; efficiency 35%
- high efficiency, instant restrike, low glare, no LLD, poor color rendition.
Lamp Manufacturers; Philips, Osram/Sylvania, GE, Iwasaki, Ushio, et al.
Lamp vendors: www.bulbman.com; www.topbulb.com
Lamp Lumen Depreciation: the output of most lamps drops over the life of
the lamp, especially MV, but not for LPS.
Lighting Organizations
- International Dark-sky Association(IDA)
- Illuminating Engineering Society (IESNA)
- International Association of Lighting Designers (IALD)
- Commission Internationale de l'Eclairage (CIE)
Web Resources
www.darksky.org
www.light-link.com
Luminaire Types
Lamp Distribution Patterns The distribution of light on the ground
and around the luminaire depends on the design of the
reflector/refracting elements. There are several standard
distributions (Type I, Type II, etc); this information is provided by
the manufacturers for their luminaires.
The Coefficient of Utilization (CU) is the ratio of the amount
of light that falls onto the intended area to the total produced by the
lamp.
The cutoff angle is the angle, measured up from the nadir, between the
vertical axis and the line of sight at which the bare source is not
visible.
Glare: when you can see the source directly.
- Discomfort glare: sufficient glare to produce feeling of discomfort.
- Disability glare: glare reducing visual performance and visibility.
Common names usually reflect something about the design style.
Examples include wall packs, shoe boxes, hockey pucks, drop-refractor or
drop-dish cobra heads, acorns, post tops, bed-pan shields, bollards.
p. a. ianna, 1 November 1997