Lighting & Color Science

Start here: ten words every newcomer needs

Overheard on the job

Real-sounding lines from the floor, translated into plain English.

• Same 3000 K on the cut sheet, but the new batch is plus 0.004 Duv and looks green next to the old cans.

  TranslationThe replacement fixtures share the specified color temperature, but their white sits above the blackbody line, so they look greenish beside the existing downlights.

• CRI 92 and R9 is 15, so the produce is going to look tired, ask for the TM-30 report.

  TranslationThe general color rendering score is high but saturated reds render poorly, so request the fuller color rendition data before approving the product.

• We hit 500 lux at the desks with a 0.8 LLF, but UGR is 22, so swap to the low-glare optic.

  TranslationThe design meets the target light level after allowing for aging and dirt, but the glare rating is too high and a lower-glare lens is needed.

• Those cans are a foot off the wall, that's why you're getting scallops, move them out or go to wall wash trims.

  TranslationThe downlights are too close to the wall and are drawing arched patterns on it; move them away or use trims designed to wash the wall evenly.

• Kitchen has pop-on below 20 percent and ghosting when it's off, the smart dimmer has no neutral.

  TranslationThe LEDs will not start at low dimmer settings and glow faintly when switched off, because the dimmer draws its power through the lights.

• Bump 214, the lekos on the box boom need a shutter cut off the proscenium before focus call ends.

  TranslationFlash channel 214 to identify it; the ellipsoidal fixtures on the side position out in the auditorium need their beams trimmed off the stage arch before the focusing session finishes.

• Practical's blowing out, dim it to 30 and add a half CTO, and flag the spill off the back wall.

  TranslationThe visible lamp in the shot is too bright; dim it, warm its color with a correction gel and block the stray light hitting the back wall.

How light is quantified: lumens, candelas, lux, luminance, efficacy and the rules that link them.

• Candelaalso cd
  The unit for how much light a source throws in one direction, and the SI base unit behind lumens and lux.
• Foot-candlealso fc, footcandle
  The North American working unit of light on a surface: one lumen falling evenly on each square foot.
• Illuminancealso light level, E
  The amount of light falling on a surface per unit area, measured in lux or foot-candles.
• Inverse square lawalso inverse-square rule
  The rule that illuminance from a point source falls with the square of distance, so double the distance gives a quarter of the light.
• Light loss factoralso LLF, maintenance factor
  A multiplier, usually 0.7 to 0.9, applied to initial light levels to predict what the system delivers after aging and dirt.
• Lumenalso lm
  The unit of total visible light leaving a source, with each wavelength counted by how bright it looks to the eye.
• Lumen packagealso lumen output option, output package
  Designer and rep shorthand for the delivered lumen output option of a luminaire, chosen from a catalog range. Shop talk
• Luminancealso L, photometric brightness
  The brightness of a surface or source as seen from a particular direction, measured in candelas per square meter.
• Luminous efficacyalso efficacy, lm/W
  How many lumens a light source or luminaire produces for each watt of electric power, in lm/W.
• Luminous fluxalso flux, light output
  The total amount of visible light a source emits in all directions, weighted for the eye's sensitivity and measured in lumens.
• Luminous intensityalso intensity, candlepower
  How much light a source sends out in one specific direction, measured in candelas.
• Luxalso lx, lumens per square meter
  The metric measure of light landing on a surface: one lumen falling evenly on each square meter.
• Nitalso cd/m2, candela per square meter
  An informal but widely used name for one candela per square meter, the unit of luminance.
• Radiometry
  Measurement of optical radiation in physical energy units such as watts, without weighting for human vision.
• Uniformityalso uniformity ratio, max:min
  How evenly light is spread over an area, expressed as a ratio such as minimum to average or maximum to minimum.
• V-lambda curvealso V(lambda), luminous efficiency function
  The standard curve of the eye's daytime sensitivity to each wavelength, peaking in green at 555 nm.

Color temperature, color rendering, chromaticity, color matching and the correction gels that fix color on set.

• Binningalso LED binning
  Sorting LEDs after manufacture into groups with matching color, brightness and voltage.
• CCTalso color temperature, kelvin
  Correlated Color Temperature: the warm-to-cool appearance of white light, stated in kelvin.
• Chromaticityalso chromaticity coordinates, x,y
  Where a light's color sits on a color map once brightness is set aside, given as coordinates such as x, y or u', v'.
• Chromaticity diagramalso CIE diagram, CIE 1931 diagram
  The horseshoe-shaped chart that maps every visible color by its chromaticity coordinates.
• CRIalso Ra, color rendering index
  Color Rendering Index: a score up to 100 for how true surface colors look under a source compared with a reference light.
• CTOalso color temperature orange
  Color Temperature Orange: a correction gel that warms daylight-balanced light toward tungsten color temperature.
• Duvalso delta uv, tint
  How far a white light's color sits above or below the blackbody line, giving it a green or pink tint.
• MacAdam ellipsealso MacAdam ellipses
  A small oval around a target color on a chromaticity chart, inside which color shifts go unnoticed by most viewers.
• Metamerismalso metameric failure
  The effect where two colors match under one light source but not under another, because their spectra differ.
• Minus greenalso minus green gel
  A magenta correction gel that removes the green tint from fluorescent or green-shifted LED light for camera work. Shop talk
• Planckian locusalso blackbody locus, blackbody curve
  The line of whites on a chromaticity diagram that a perfect blackbody passes through as it heats up.
• R9also R9 value, special color rendering index 9
  The color rendering score for a strong saturated red sample, not included in the general CRI average.
• SDCMalso MacAdam step, step
  Standard Deviation of Color Matching: the step size used to state how closely light sources match in color.
• SPDalso spectral power distribution, spectrum
  Spectral Power Distribution: a graph of how much power a light source emits at each wavelength.
• TM-30also IES TM-30
  A color rendition method that rates fidelity and saturation with 99 color samples, giving Rf, Rg and a color vector graphic.
• Tunable whitealso TW, color tuning
  LED lighting whose color temperature can be adjusted, typically between warm and cool whites, by mixing LED channels.
• Warm dimalso dim to warm, dim-to-warm
  LED dimming that also lowers color temperature as light level drops, imitating the glow of a dimmed incandescent lamp. Shop talk

How eyes and bodies respond to light, from night vision to glare, flicker, circadian effects and eye safety.

• Blue light hazardalso BLH, photochemical retinal hazard
  Photochemical retinal damage caused by staring at very bright sources rich in blue light, roughly 400 to 500 nm.
• Flickeralso temporal light modulation, TLM
  Rapid, repeated variation in light output over time, whether directly visible or not.
• Glare
  Visual discomfort or loss of visibility caused by excessive brightness or contrast in the field of view.
• ipRGCalso intrinsically photosensitive retinal ganglion cells, melanopsin cells
  Intrinsically photosensitive retinal ganglion cells: eye cells that sense light for the body clock rather than for vision.
• Melanopic EDIalso mEDI, melanopic equivalent daylight illuminance
  Melanopic Equivalent Daylight Illuminance: a measure, in lux, of how strongly light stimulates the eye's circadian system.
• Mesopicalso mesopic vision, twilight vision
  Describes the in-between vision of twilight and street lighting, when rods and cones both contribute.
• Photopicalso photopic vision, cone vision
  Describes daylight vision, when cones dominate, providing color and sharp detail at luminances above a few cd/m squared.
• Risk groupalso RG
  The photobiological safety class of a lamp or luminaire, from RG0 exempt to RG3 high risk.
• Scotopicalso scotopic vision, rod vision
  Describes night vision at very low light levels, when only rod cells respond and color disappears.
• Stroboscopic effectalso strobing
  The way moving or rotating objects look stopped, slowed or multiplied under light that flickers.
• UGRalso unified glare rating, UGR value
  Unified Glare Rating: a calculated index of discomfort glare from interior luminaires, where lower numbers mean less glare.

Lamps, LEDs, drivers and ballasts, plus the dimming protocols and dimming faults that installers deal with.

• 0-10Valso zero to ten volt, 0-10 V dimming
  An analog dimming method in which a low-voltage DC signal from 0 to 10 volts sets driver output.
• Ballast
  The device that starts and limits current to fluorescent and HID lamps, which would otherwise draw unlimited current.
• COBalso chip-on-board, COB LED
  Chip-on-Board: an LED array with many dies mounted closely under one phosphor layer, acting as a single compact source.
• DALIalso digital addressable lighting interface
  Digital Addressable Lighting Interface: a two-wire digital lighting control bus where each driver has its own address.
• Dimming curvealso dim curve, dimming profile
  The relationship between a dimmer or control signal and the light output it produces.
• DMXalso DMX512
  DMX512: the digital control protocol for stage and architectural lighting, carrying 512 channels per universe.
• Driveralso LED driver, power supply
  The electronic power supply that converts line voltage into the regulated current or voltage an LED needs.
• Droopalso efficiency droop, LED droop
  The fall in an LED's efficiency as drive current or temperature rises, so extra power yields proportionally less light.
• Ghosting
  A faint glow from LED fixtures that are switched off, caused by small leakage currents in the wiring or controls. Shop talk
• HIDalso high-intensity discharge
  High-Intensity Discharge: lamps such as metal halide and high-pressure sodium that make light from an electric arc in a pressurized tube.
• Junction temperaturealso Tj
  The temperature of the active semiconductor region inside an LED, which governs its output, color and lifetime.
• L70also L70 life
  The number of operating hours until an LED product's light output falls to 70 percent of its initial value.
• Lampalso bulb, light source
  The trade word for a replaceable light source, what consumers call a bulb, as distinct from the fixture it goes in.
• LEDalso light-emitting diode
  Light-Emitting Diode: a semiconductor light source, made white for lighting by coating a blue chip with phosphor.
• Pop-onalso pop-on effect
  An LED dimming fault where fixtures stay off at low dimmer settings and then suddenly turn on at a higher level. Shop talk
• Strikealso ignition
  To start the arc in a discharge lamp; restrike is relighting it after it has gone out.
• Tape lightalso LED tape, LED strip
  Flexible LED strip on an adhesive-backed reel, cut to length and used in coves, shelves and under cabinets. Shop talk

Fixture types, beam and cutoff terms, outdoor light control and the visual patterns light leaves on surfaces.

• Beam anglealso beam spread, half-intensity angle
  The angle across a light's beam at which intensity has fallen to 50 percent of its center value.
• BUG ratingalso BUG, backlight uplight glare
  An outdoor luminaire rating for backlight, uplight and glare, each scored 0 to 5, used in light pollution ordinances.
• Cut sheetalso spec sheet, data sheet
  The one- or two-page manufacturer data sheet for a luminaire, with dimensions, photometry and ordering options. Shop talk
• Cutoff
  The angle above which a luminaire emits little or no light, used to classify glare and light pollution control.
• Downlightalso can, can light
  A ceiling luminaire, usually recessed, that directs its light downward, known regionally as a can or pot light.
• Field anglealso field spread
  The angle across a light's beam at which intensity falls to 10 percent of its peak, marking the usable edge of the light.
• Grazingalso wall grazing, graze lighting
  Lighting a surface from a very steep angle, close to the wall, to bring out its texture with strong highlights and shadows.
• Hot spotalso hotspot, bright spot
  A distractingly bright patch in a beam or on a lit surface, brighter than its surroundings. Shop talk
• Light trespassalso trespass
  Unwanted light that crosses a property line or falls where it is not needed, such as into a neighbor's window.
• Luminairealso fixture, light fixture
  The whole light fixture as one product: light source, optics, housing, driver or ballast and power connection.
• Scallopsalso scalloping
  The arched pattern of light and shadow that downlights draw on a wall when mounted close to it. Shop talk
• Spillalso spill light, stray light
  Light that falls outside the area it is aimed at, onto walls, ceilings, audiences or neighbors. Shop talk
• Trofferalso lay-in, 2x4
  The lay-in luminaire of grid ceilings, sized to take the place of a ceiling tile, such as a 2x4 or 2x2.
• Wall washingalso wall wash, washing
  Lighting a wall evenly from top to bottom with asymmetric luminaires set back from it, to make the surface glow uniformly.

Instruments, test methods and photometric files used to prove what a product or a space actually delivers.

• Goniophotometeralso gonio
  An instrument that measures a luminaire's light intensity in every direction by turning the luminaire, or a mirror around it, relative to a detector.
• IES filealso photometric file, IES
  A standard text file of a luminaire's measured candela distribution, loaded into lighting calculation software.
• Illuminance meteralso light meter, lux meter
  A handheld meter that reads the light falling on a surface in lux or foot-candles, commonly called a light meter.
• Integrating spherealso sphere, Ulbricht sphere
  A white-lined hollow ball that spreads a source's light evenly so one detector can read its total lumens and color.
• LM-79also LM-79 report, LM-79 test
  The standard method for measuring a complete LED luminaire's light output, power, efficacy, color and distribution.
• LM-80also LM-80 data
  The test method for tracking how LED packages lose light over time, the basis of lifetime claims.
• Point-by-pointalso photometric plan, photometric calc
  A lighting calculation that predicts illuminance at each point on a grid from every luminaire's photometric data.
• Spectroradiometeralso spectro
  An instrument that measures light's power at each wavelength, from which color and photometric values are calculated.

Fixtures, crew routines and slang from theater, concerts and film sets.

• Board opalso board operator, LX op
  The person who runs the lighting console during performances, executing cues on the stage manager's call. Shop talk
• Bumpalso flash
  To flash a channel or group to full instantly, usually with a console button, to identify it or for effect. Shop talk
• Channel checkalso dimmer check, lamp check
  The preshow routine of bringing up each lighting channel in turn to confirm every fixture works and is aimed correctly. Shop talk
• Cuealso light cue, LQ
  A recorded lighting state and the timed transition into it, triggered during a performance on the stage manager's call.
• Cycalso cyclorama, sky cloth
  Short for cyclorama: a large plain backdrop or curved wall at the back of a stage or studio, lit to fill it with color. Shop talk
• Flagalso solid
  A black fabric panel on a frame used on film sets to block light from hitting part of a scene or the lens. Shop talk
• Flash and trashalso flash-and-trash
  Wry crew slang for busy concert-style lighting full of strobes, chases and fast moving-light effects. Shop talk
• Focus callalso focus, focus session
  The scheduled crew session in which the lighting designer directs electricians to aim and shape every fixture. Shop talk
• Followspotalso follow spot, spot
  A powerful, manually aimed spotlight that an operator keeps on a moving performer throughout a scene.
• Fresnelalso Fresnel spot, Fresnel lantern
  A soft-edged spotlight with a stepped Fresnel lens whose beam can be adjusted from spot to flood.
• Gafferalso chief lighting technician, CLT
  The head of the lighting and electrical department on a film or television set. Shop talk
• Gelalso color, color media
  A sheet of colored or diffusing plastic film placed in front of a lighting fixture to change its color or quality. Shop talk
• Ghost lightalso equity light
  A single bare lamp on a stand left burning center stage whenever a theater is dark and empty. Shop talk
• Goboalso template, pattern
  A thin metal or glass disk with a cut pattern, placed in a spotlight to project shapes, textures or logos. Shop talk
• Hang and focusalso rig and focus
  The work of installing every fixture on the light plot, then aiming and shaping each one for the show. Shop talk
• Haze
  A fine, even mist pumped into the air so that light beams become visible on stage or on set. Shop talk
• Key lightalso key, main light
  The main light source illuminating a subject, setting the direction, exposure and character of the lighting.
• Lekoalso ellipsoidal, ERS
  Theater slang for an ellipsoidal reflector spotlight, the hard-edged fixture with shutters and a gobo slot. Shop talk
• Moveralso moving light, moving head
  An automated moving-head light that pans, tilts and changes color, beam and gobo under remote control. Shop talk
• PAR canalso PAR, PAR64
  A simple cylindrical stage light holding a sealed-beam PAR lamp, the classic rock-and-roll and touring fixture. Shop talk
• Practicalalso practical light, practical lamp
  A working light fixture that appears in the scene itself, such as a table lamp, sconce or neon sign. Shop talk
• Throwalso throw distance
  The distance from a lighting fixture to the surface or subject it lights, which sets beam size and intensity.
• Two-feralso twofer, Y-cable
  A Y-shaped cable that connects two lighting fixtures to a single circuit so they are controlled together. Shop talk
• Wash
  Broad, even light covering a whole stage area or surface, built from overlapping soft-edged fixtures. Shop talk

Frequently asked questions

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