Lighting Glossary

Hugo Lighting has developed a Glossary of key lighting terms to assist everyone in lighting terminology.

AC (Alternating Current)

Current which flows in one direction and then the other, alternately.

Accent Lighting

Accent lighting is primarily decorative and consists largely of directional light. Use it to focus attention on artwork, highlight specific features and create mood. It provides a focal point in a room by emphasizing features.

Acoustic rated

Maintains the ceiling acoustic properties in compliance with the Building Regulations Approved Document E – Resistance to the Passage of Sound.

Aluminium Reflector 

A reflective aluminium coating which reflects the heat from the lamp forward, commonly found within GU10 and MR16 lamps. 

Ambient Lighting

Also called general or background lighting. Ambient lighting provides a soft level of light appropriate to such activities as watching television or entertaining. It is also required to allow occupants to enter and exit the room safely. Ambient lighting may come from fixtures that provide a diffused spread of illumination, or strategically placed downlights. This is the basis for the lighting design and can be versatile if used with a dimmer.


Devices used with fluorescent or high intensity discharge (HID) lamps. They provide sufficient voltage to ignite and operate the lamp. Once the lamp is ignited the ballast will limit the current. They are available in electronic and magnetic options. 

Beam cone

Candela (CD)

The measure of luminous intensity of a source in a given direction.

Colour Temperature

Colour Rendering

The ability of a light source to reveal the colours of an object. Measured on the colour rendering index (CRI), and measured in Ra. Ranges from 0 to 100 with the higher the number the better the result.

Unit: colour rendering index = Ra Index: CRI


A component that stores electrical energy. Electronic capacitors are often used for power factor correction and lamp regulation, i.e. in a ballast.

Constant Current

A circuit in which the current remains constant but the voltage may vary. These must be wired in series. Constant current drivers are available in 700mA and 350mA.

Constant Voltage

A circuit in which the voltage remains constant but the current may vary. These must be wired in parallel. Constant voltage drivers are available in 24V DC and 12V DC.

Control Gear

Used to start most artificial lamps. Control gear may include a ballast, igniters or transformers.

DC (Direct Current)

An electric current that moves in one direction with constant strength.

Dichroic Reflector

A reflective lamp that allows the heat of the lamp to pass through the reflector. Dichroic reflector lamps are considered ‘cool beam’ as heat is not reflected forward.


Digital Addressable Lighting Interface: A standardised communication interface to regulate lighting levels and to switch electronic HFD ballasts on and off.


A device in an electrical circuit used for varying the brightness of lamps in a lighting installation.

Direct Lighting

Luminaires with a light distribution which achieves 90 – 100% of luminous flux and reaches the working plane directly.

Discharge Lamps 

Light is produced either directly or by the excitation of phosphors by electric discharge through a mixture of gas and metal vapour, i.e. fluorescent tubes.


A ceiling fitting which projects the illumination forward (down). They are generally recessed but also available in surface mounted options.

Emergency Lighting

Provides lighting when mains supply fails.

Maintained: Unit operates normally and continues to operate when the normal mains power supply fails. (Mains and Emergency lighting)

Non-maintained: The lighting only operates when the normal mains supply fails (Emergency lighting only)

Self test: An internal timer initiates the test function with the emergency fitting to ensure it is working correctly


A tungsten wire which emits light and heat once a current runs through it. Found within incandescent and halogen lamps. In fluorescent lamps the filament is coated with emission mix and emits electrons when heated.


A tungsten wire which emits light and heat once a current runs through it. Found within incandescent and halogen lamps. In fluorescent lamps the filament is coated with emission mix and emits electrons when heated.

Fire Rated

Maintains the ceiling fire barrier integrity in compliance with the Building Regulations B – Fire Safety. 

HID Lamp

High intensity discharge lamps have a longer life and provide more light (lumens) per watt than most other light sources. These are available in mercury vapour, metal halide, high pressure sodium, and low pressure sodium types.

High Bay

When fittings are mounted between 8 – 10 metres or above they are considered High Bay fittings.

High frequency electronic control gear (HF)

Device generally used to operate fluorescent tubes at a high frequency (30-60 KHz) compared to mains supply (50 Hz). This results in a higher quality of light and reduces running costs. These may also be used in conjunction with HID lamps.

High power factor (HPF)

A ballast whose power factor is corrected to 90% or greater by the use of a capacitor.


Required when mains voltage supply cannot start a lamp. Commonly found within fittings which use metal halide and high-pressure discharge lamps.


Refers to the amount of light falling onto an area (lux). 500 lux is required within an office environment. The sun produces 1,200,000 lux and the moon reflects from the sun 3 lux. Unit: lux (lx) = lm/m², Symbol E.

Indirect Lighting 

Light which is reflected onto a secondary source, generally ceilings and walls are the primary source.

Ingress Protection (IP)

This denotes the amount of protection a product has against moisture, and/or foreign objects. It uses 2 digits to denote the rating.

Intumescent material

Material used within Fire Rated Downlights and Gaskets, it expands to form a fire rated barrier once exposed to fire.


Kelvin is a measure of colour that is a simplified way to characterise properties of differing light sources.

1500K Candle Light 
2680-2800K 40W Incandescent Lamp 
3000K 200W Incandescent Lamp 
3200K Sunrise/Sunset 
3500K Tungsten Lamp 
4100K Moonlight 
5000K Daylight 
6500-7500K Overcast Sky 

Kilowatt (kW)

Measure of electrical power which equals 1000 watts.

Lamp Life

This is an average measurement of lamp life. E.g., if 20 lamps are tested, the average lamp life is when the 10th fails.

Light Emitting Diodes (LED)

Small semiconductor which emits light. These are generally available in a variety of colours. The advantages of LED include, low operation costs, size and long life.

Light Output Ratio (LOR)

The ratio of light emitted by a luminaire to the total light output of the lamp it contains. This will always be under 1. Unit: LOR.

Low Voltage

Fittings/Lamps which require a transformer to reduce line voltage to usually 12 or 24 volts.

Low Bay

Fittings which incorporate discharge lamps (mounted horizontally) fitted between 4 – 8m.

Low Power Factor (LPF)

Luminaire without power factor correction (no capacitor).


Refer to Luminous Flux.


A term relating to the efficiency of light output Vs Wattage used. Below are some basic results, this will vary due to different lamps used and also the combination of Lamp and Ballast.

Thomas Edison’s first lamp – 1.4 lm/W

Incandescent lamps – 10-40

Halogen incandescent lamps – 20-45

Fluorescent lamps – 35-105

Luminous Flux

The total light output emitted by a light source or received by a surface. Commonly known as lumens. Unit: lumens lm.


Unit of luminance, equal to one lumen per square metre.

Mains Voltage

Fittings/Lamps which can be powered directly from Mains Power, generally this is 240V AC.

Series Lamp Operation

Requires a single current path, passing through all lamps when operated from a driver.

Task Lighting

Used to illuminate the area where a visual activity or task is performed – like reading, sewing, or preparing food. Task lighting is often achieved with individual fixtures that direct light onto a work surface. The result is usually a fairly strong, directional light displaying the work surface.


Steps voltage up or down. Available in Magnetic or Electronic options.


A measure of electrical power consumed by a lamp or lighting fixture. Watts = Volts x amps, (P = V x I).

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