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Luminous Flux

Luminous flux values – expressed in lumens – describe how much light a source is emitting regardless of direction, correlated colour temperature or colour rendering. The luminaire flux value is an important figure when comparing non-standardised light sources such as LEDs.

Expressed in lumens (lm)

Luminous Intensity (lv)

The luminous intensity of a source can be used to describe the amount of light it is emitting in a particular direction. It is common for luminaires to be high intensity for purposes such as task lighting but less so for horizontal projections, where glare needs to be taken into consideration.

Expressed in candelas (cd) 

Luminance

The luminance of a source can be used to describe how much light it emits in relation to its size. Commonly, up to 3,000 cd/m2 is recommended at 65 degrees (where zero degrees is directly under the luminaire) as the optimal level for commercial spaces using modern computer displays.

Expressed in candelas per square meter (cd/m2)

Illuminance

Illuminance describes how much light is reaching a specified point on a surface. When using a light meter, a series of point measurements need to be taken systematically to get a full picture of the consistency of a light source and where light is being directed.

Expressed in lux (lx)

Reflectance

It’s importance to know the reflectance of the surfaces in a space when considering the design of your lighting system. White painted walls and ceilings can have a reflectance level of 60 to 85 per cent, while a dark carpet will be significantly lower. This can also be affected by furniture and décor, so we always factor this in when considering the absolute reflectance of a space.

Light Output Ratio

The Light Output Ratio of a luminaire describes the amount of light exiting the fitting relative to the amount emitted by the light source within it. This can be further broken down to account for light emitted upward and downwards. High powered LEDs are normally not viable light sources outside of a luminaire. For this reason, LORs are not easily applied to LED-based luminaires and the LOR is often given as 1 (or 100 per cent) for simplicity.

Colour Rendering Index

The specific colours contained with white light affect the way the eye perceives the colour of an object being illuminated. The Sun and incandescent lamps can product light that will cause colours to be reproduced with a CRI of 100 (i.e. maximum accuracy). Light with very few colours will cause colours to be reproduced inaccurately, an issue which is particularly critical in retail environments.

Expressed as Ra

Correlated Colour Temperature

The colour of white light can be compared to the colour of a glowing wire. White light is emitted when the physical temperature of the wire is around 2,700K (equivalent to about 2,400°C). Most sources don’t need to reach such temperatures to generate light, so the CCT is used as a way of describing the apparent temperature.

Expressed in Kelvin (K)