Lighting Terms

 

The role of lighting
 

Lighting plays an important role in our lives.  Good lighting design evokes emotion and feelings, gives form to objects, accentuates colours and textures, creates warmth, gives a greater sense of safety & security and is crucial for our well being.

 
We often take lighting for granted, especially in our everyday lives, but its effects affect us all.  Our memories are formed and reinforced by lighting. We all remember walking through forests with dappled sun playfully breaking through the canopies of trees; warm summer evenings as the sun disappears over the horizon enveloping the rolling landscape in a beautiful crimson light; the intimacy of a candle at dinner; the motion, the sound as flames dance in a log fire on a cold winter’s evening.
 
Good lighting design is not a question of simply analysing technical data and conforming to guidelines, it must look beyond these basic requirements and consider many other aspects.
 
Artists of the renaissance period recognised the importance of light & shadow to give life to a subject.  Ever since, artists have used light to give emotion and focus to their works.  Da Vinci’s use of light in the Mona Lisa is the key element that makes this painting so important.  Light gives shape, depth and dimension to her crossed hands, breasts and face whilst focussing one’s attention on that unforgettable smile.
 
 
How does the eye perceive light?
 
The light visible to the human eye falls between a wavelength of around 380 nanometres to 750 nanometres. Each wavelength is perceived as a different colour.
 
Violet is at the shortest end of the spectrum – around 400nm
 
Red is at the longest end of the spectrum – around 750nm
 
The human eye is most sensitive to colours around the middle of the spectrum which is the yellow-green area (around 550nm) and is least sensitive at the violet and red extremes.
 
 
Daylight
 

Daylight is considered as the optimal form of lighting and the benchmark to which artificial lighting is measured. Natural daylight is perceived as ‘white light’ however the whiteness varies through the day.  It is at midday that the light is whitest.

 

Colour Temperature

The temperature of colour is measured in the units of absolute temperature - Kelvin (K).  Kelvin expresses the hue of the light.  Colour temperatures of around 5000K or higher tend to be bluish/cool whilst temperatures less than 3000K tend to be warm (more like a traditional domestic lamp).

 

Luminous Flux

Luminous flux is the amount of light emitted by a light source.  It is measured in units known as Lumen (lm).

For example a 100W traditional domestic incandescent lamp produces 1250lm whilst an 18W compact fluorescent lamp produces 1200lm.

 

Luminous Intensity

Luminous intensity is the amount of light emitted by a light fitting in a particular direction.  It is measured in units known as Candela (cd).  1cd corresponds to the amount of light emitted by a candle in a single direction.

 

Illuminance

Illuminance is the total amount of light falling on a surface.  It is measured in Lux (lx).  The level of illumination depends on the brightness of the luminaire (light fitting) and the distance from the surface being lit.

 

Bright sunshine                   around 100,000lx

Cloudy day                         around 1000lx

Typical office                       around 300-500lx

Full moon                           around 0.25lx

 

Luminance

Luminance is the reflected light that hits our eyes.  This reflected light is what gives us our visual perception of illuminated surfaces and objects.  It is measured as Candela per metre squared (cd/m2).

It is very important to consider luminance because it is the reflected light we perceive.  For example if the same light is illuminating a black surface and a white surface, the black surface will look much darker than the white but if one were to measure the light hitting each surface with a Lux metre, the light levels would read identically.



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