Accent Light:  a light source used in addition to the main and fill lights to enhance separation and dimension in the image.  Examples of accent lights include: hair light, kicker light (a.k.a. skim, rim or edge), and veil light. 

Angle of Incidence = Angle of Reflection:  an oft-quoted law of optics that describes how rays of light reflect off of a plane (flat) surface.  This principle lies at the heart of the understanding of the behavior of directly reflected light. 

Background Light:  a light source used to light the background.  The background light may provide a general wash of light over the background to control tone or may be more localized to create a sense of separation between the subject and background.

Chroma-Key Background:  a background that is dyed or painted with a specific green or blue color.  Image processing software designed for Chroma-Key recognizes the specific color and then manipulates these green or blue areas, usually by dropping in another background.  You can see Chroma-Key in action on TV-based weather reports.  The weather maps behind the broadcaster are generally drop-ins.  Using Chroma-Key, portrait photographers can  provide the customer with a huge selection of backgrounds, but color fidelity at the composite boundaries can make its use problematic.

Fill or Fill Light:  a lighting source used to lighten (fill in) shadow areas created, in general, by the main light.  How fill lighting is provided varies greatly.  Some photographers prefer to use fill in a "global" sense, providing a general wash of light that covers the entire set and sets a "foundation" of light onto which other lighting is added.  Others, prefer to fill "locally", using reflector panels to bounce light from others sources--usually the main light--into specific pockets of shadow.  

Feathering:  an offsetting (and finessing) of a hot-centered-lighting source such that only the "sweet" light surrounding the center is used to light the object. Feathering is usually used with hot-centered parabolics and silvered umbrellas, but feathering can be applied whenever a source has a lighting gradient. 

Flag:  an opaque object, often in the form of a flag-shaped frame covered with black fabric,  used to block light. 

Grid:  Grids are used to limit the spread of light.  Grids used directly on the flash reflector are typically constructed from a fine honeycomb of aluminum which is coated in matte black.  Grids for larger fixtures, such as softboxes, are more commonly made of cloth formed into a grid of squares.

Gobo:  an object, generally placed between the subject or surroundings and a light source.   Gobos may be entirely opaque (flag) or may pass light selectively to create patterns (cucoloris).

High Key:  describes the tonal key of images that are predominately composed of lighter tones and white.  In addition, high key images are often characterized by softer than normal contrast and an ethereal nature. High key is typically used for portraits of women and children.

Inverse Square Law:  a law that states that intensity decreases as the inverse square of the distance.  Many common lighting sources fall off at approximately this rate.  A bare-bulb flash, for instance, follows this law quite closely.  For more on the math and geometry of light fall-off, click here. 

Light Meter:  a device that measures the quantity of light that either falls on the meter (incident reading) or reflects off of a subject (reflective reading).  Some light meters measure only continuous light, while others are capable of measuring both flash and continuous lighting.

Lighting Ratio:  a ratio that expresses either the difference in the light levels illuminating the highlight and shadow areas, or the difference in the actual light output of two sources.  Since these two ratios are not necessarily the same, confusion and argumentation often rule when lighting ratios are involved.  If you'd like to learn more about the calculation of ratios, click here. 

Low Key:  describes the tonal key of images that are composed of predominately darker tones and black.  In addition, low key images are often characterized by higher than normal contrast and a serious or somber mood.  Low key is used for all subject types.   

Main Light (Key Light):  a main source of illumination and the light source that typically defines the contours of the subject.  In facial portraiture, the main light is responsible for lighting the "mask" of the face and creating the shadow areas neighboring such features as the nose, chin, and cheeks.  The main light is typically the strongest light in a multi-light setup, but not always.

Monolight:  a self contained studio flash unit.  A monolight combines the pack and head components into one device.  Monolights tend to be highly adjustable and may be preferable for those who require different and precise output from each lighting fixture.

Muslin:  a thin but durable fabric that is typically dyed and/or painted.  Most muslins are made of cotton, but some, such as those from Botero, are entirely synthetic.

Pack and Head System:  a flash system consisting of a single power source (Pack) and several flash heads that plug into the pack.  The head generally contains a flash tube, a modeling light, some minor circuitry, and often a fan.  Flash heads are generally light in weight and either contain a reflector or can be fitted with a reflector or other light-control device.  The pack contains the control circuitry, the power supply that converts the mains power to the higher voltages used in strobes, and the capacitors that store the charge distributed to the heads.

Parabolic:  a bowl (paraboloid) shaped reflector used to intercept and redirect the light from the flashtube.   The light quality of parabolic reflectors varies widely.  Most parabolic reflectors produce a light with a "hotter" center, which is best feathered. 

Scrim:  a panel or screen used to reduce or soften the light.  Scrims made of black mesh reduce light level without appreciably affecting light quality, while white cloth scrims both diffuse and attenuate light.

Seamless:  a heavyweight background paper, usually on a cardboard core, that comes in a variety of sizes, colors and shades. 

Softbox: a box-like structure that is fitted around the lighting units flashtube and which directs the light through a diffuser on the box side opposite the flashtube.  Like umbrellas, softboxes turn a small source of light into a larger, softer source.  Softboxes, which usually have five opaque faces, and in some instances a recessed diffuser, can provide better control of stray light than umbrellas

Umbrella:  a collapsible reflector that can be used in several configurations to enlarge, soften and broaden a light source.   Photographic umbrellas are constructed much like the handheld umbrellas we use to protect ourselves from the rain, but  with materials appropriate to lighting, and with no handle.  Umbrellas are often overlooked in favor of the more controllable and expensive softbox.  You will find umbrellas in various forms, including: shoot-through, silver, gold, zebra, and satin.     

Watt-Seconds:  a measure of energy that is also known as a Joule.  The Watt-Second rating of a flash unit is a measure of the maximum electrical energy drawn by the unit and is closely related to the amount of energy available for discharge into the flashtube.  The light output of the flash unit is rather loosely tied to the Watt-second rating.  Equally rated units from different manufacturers can deliver significantly different light output.  Things such as, reflector coverage, flashtube treatments, and cable extensions can have a major impact on light output.