September 2, 2020

Principles of Lighting Guide

All about the Principles of Lighting, including the definition of light, units of light and colour temperature.


What is light?

Without light, it would be impossible for the world as we know it to exist. Light allows to see and recognise objects and people around us. Natural light causes plants to grow – plants which we use as a food source or more importantly create the biosphere full of oxygen that we breathe. Artificial light allows us to continue working and living when natural light disappears at night. But what exactly is light?

  • Light is energy in the form of electromagnetic radiation. The electromagnetic spectrum includes everything from radio waves to gamma rays. The visible part of the spectrum covers energy in the range of 380nm (violet) to 780nm (red). Visible light consists of energy within the wavelengths covered by this range, which when mixed together appears as white light.
  • Coloured objects only appear coloured if their colours are present in the spectrum of the light source and are reflected to the eye. A broad spectrum will make colours look natural, whilst a narrow spectrum will make colours look dull, or even grey and muddy.
    The eye has 3 sets of light receptors:

    • Cones – to determine colours (red, green and blue)
    • Rods – for monochromatic vision at low light level
    • Melanopsin containing ganglion – detect light/dark to regulate body rhythms
  • The detection of light by Melanopsin containing ganglions helps to regulate the body’s natural rhythm. This is known as the Circadian Rhythm. Darkness stimulates the release of the sleep hormone melatonin, whilst bright light (>1000 lux) particularly in the blue end of the spectrum, stimulates the release of cortisol increasing alertness.
  • Further research has identified that the colour of artificial light and how it mimics natural daylight can promote wellbeing.
The Units of Light
  • Luminous Flux (Light Output) – The total amount of light emitted by a light source.
    Unit: lumen (lm) {cd/steradian}
  • Luminous Intensity (Intensity) – Luminous flux emitted in a specified direction.
    Unit: candela (cd) {lm/steradian} This is the SI Base Unit of light.
  • Illuminance (Light Level) – The amount of luminous flux falling onto a surface area.
    Unit: lux {lm/m²}
  • Luminance (Brightness) – Luminous intensity per projected area of the source.
    Unit: candela per square metre (cd) {cd/m²}
Colour Appearance
  • Definition: Describes the colour a lamp appears to be, or the general ambience of the light it provides.
  • General terms can be used to describe colour appearance, such as warm white or daylight.
  • It can also be defined scientifically as a colour temperature, measured in K (Kelvin). This is derived using a Planckian blackbody radiator material, which glows as it is heated. As the material gets hotter, the colour appears to get colder  e.g. red hot – white hot – blue hot etc.
  • This can be plotted as a blackbody locus, using colour coordinates on a CIE Chromaticity Diagram (1931).
  • Non-Planckian light sources (eg those where the light emitted is not created by heating a black body radiator material) do not lay exactly on the locus. The relationship to Tc is called Correlated Colour Temperature (CCT).
  • However, light sources with the same CCT can look different. Research carried out by David MacAdam found that colour spaces within which the human eye cannot discern a comparative difference can also be plotted, as ellipses. A series of ellipses can be plotted as the human eye detects just noticeable differences. These concentric ellipses can be used to establish the degree of difference that is perceptible. More ellipses, or steps, means an increasing difference. It is generally accepted that the majority of the population cannot discern a difference up to 3 steps. A 5 step deviation is very common for general light sources. This can also be expressed as Standard Deviation of Colour Matching (SDCM).
  • Most Red Arrow luminaires have a SDCM of < 3.
Colour Rendition
  • Definition: Describes the ability of a light source to make objects appear their true colour.
  • The broader the spectral distribution of a light source, the truer the colours of the objects being illuminated will appear.
  • Natural daylight contains all wavelengths of light and therefore exhibits perfect colour rendering.
  • Artificial light sources need a method of comparative measurement to determine how well they render colours. Currently the most widely used method of measurement is the CIE Colour Rendering Index (CRI). This uses 8 standard pastel colours. The rendering ability of a test light is measured for each of these 8 colours against a reference light source. Then the average calculated. This is called ‘Ra’ and is on a scale of 1-100, where 100 is perfect colour rendition.
  • For interior lighting it is generally considered acceptable for a light source to have an average colour rendering of Ra > 80.
  • A further 6 colours are used to index light sources for specific applications – R9 to R14.
  • Work is progressing on an alternative method, the Colour Quality Scale (CQS) to provide better and more accurate indexing of light sources, particularly LED.
Illuminance & Luminance
  • Recommended minimum illuminance levels for indoor work places is provided in BS EN 12464-1:2021.
  • Recommended minimum illuminance levels for outdoor work places is provided in BS EN 12464-2:2014.
  • Guidance can also be found in The Society of Light and Lighting (SLL) Code for Lighting (2022).
  • The recommended light levels affected by the visual task, surface reflectance, surface colours, luminaire design, and maintenance.
  • Illuminance can also be measured on vertical surfaces. Cylindrical illuminance considers vertical illumination in all directions around a notional cylinder. Good, uniform cylindrical illuminance improves facial recognition.
  • Luminance defines the brightness of surfaces, which is what the eye perceives.
  • Luminance is affected by surface reflectance, surface colour, direction of light and creates the sensation of brightness and visual comfort.
  • Good luminance balance is important for applications such as educational establishments. Distribution ratio should not exceed 1:10 for comfort, but some difference is required to prevent a monotonous visual environment.
Glare, Contrast & Modelling
  • Excessive brightness is termed as glare. The main types of glare are:
    • Discomfort Glare – visual discomfort caused by direct view of sources of high brightness, such as sunlight, unshielded windows, high output bare light sources and badly positioned luminaires. This brightness becomes uncomfortable when it is bright relative to the luminance (brightness) of the surroundings, e.g. car headlights on a dark night
    • Disability Glare – impairment of vision whilst carrying out tasks. Caused by reflected light from monitor etc. Sometimes referred to as veiling or reflected glare
  • Discomfort glare can be calculated by Unified Glare Rating (UGR). The index considers the ratio of the brightness of the luminaire to the background luminance from a given observer’s view point. It is the overall installation that is rated, NOT luminaires. Luminaires can be designed to assist in achieving specific UGR ratings, but luminaires in themselves cannot be classified as being UGR compliant.
  • Not all brightness is considered bad. Changes in luminaince and the direction of light can be very beneficial. This is described as:
    • Contrast
    • Modelling
  • Contrast is controlled changes in luminance that creates visual interest, usually using a mix of diffuse lighting and directional lighting. Contrast is perfect for highlighting three dimensional structures.
  • Modelling is controlled directional lighting, which can be used to discern shapes and textures in materials. It creates visual interest by adding depth and shadow. The ratio between cylindrical and horizontal illuminance is called the modelling index.
  • Dramatic display and mood lighting has a modelling index of <0.1 and is characterised by harsh shadows. For offices and retail lighting, some comfortable shadowing is helpful to discern form and has a modelling index of >0.3. Areas where clarity of the objects being viewed is more critical, such as healthcare environments and schools, few shadows are preferable, with a modelling index of >0.5.
LED Light Sources
  • Light Emitting Diodes (LEDs) are the latest technology in light sources and the most efficient artificial white light source available
  • LEDs are semiconductors which emit photons at the p-n junction when current is passed through. This process is termed electroluminescence. Positively and negatively charged doping material (GaN or InGaN) produces blue light. This is converted to other wavelengths – and therefore white light – by a yellow phosphor layer.
  • LEDs can produce virtually any colour appearance of light, but generally the colour temperature is between 2700K (warm white) and 6500K (daylight). Any colour appearance can be specified to suit the application, which is very different to legacy light sources, with the main exception of fluorescent.
  • The performance of LEDs is graded during production, using a process called ‘binning’. The performance criteria are:
    • Colour appearance
    • Luminous flux
    • Forward voltage
  • The highest performance LEDs attract a higher cost, particularly for luminous flux and colour appearance, to ensure consistency of colour in luminaires used together
  • For colour appearance, the best luminaires tend to use LEDs from a ‘quarter bin’ of any specific colour temperature. This is the central 4 sub bins and provides for greater consistency, to help achieve a SDCM < 3 MacAdam steps. This consistency can also be achieved by careful mixing of LEDs from different bins on the same LED module (PCB fitted with one or more LED package, or chip)
  • The combination of driver LED module array has to be carefully designed for each product to ensure optimum performance. In addition, the thermal characteristics of a luminaire have to be carefully controlled, as excess heat can severely reduce the rated life of LEDs.
  • LEDs can exhibit a long rated life as well as being very energy efficient – if the thermal, electrical and optical characteristics of the luminaire are controlled correctly. Life is expressed as the time elapsed when the LEDs reach stated lumen maintenance performance. For example L80B50 – 50,000 hours expresses that after 50,000 operating hours, the LEDs are still achieving 80% lumen maintenance (eg 20% lumen depreciation) for a minimum of 50% of modules. LEDs will continue to provide illumination for an extended period of time, but this measurement provides a method of comparison.
Optical Control

When designing luminaires, two main types of optical control can be deployed.

  • Reflection
    • Specular (Directional)
    • Semi Specular (Mixed)
    • Diffuse
  • Transmission
    • Refraction
    • Diffusion
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