LIF. The Lighting Industry Federation

PRINCIPLE CHARACTERISTICS

When lamp types are being selected for a new installation, the following are the principal characteristics which should be taken into consideration:

1.     COLOUR i.e. Colour Appearance and Colour Rendering

2.     LIGHT i.e. Light Output & Efficiency

3.     LIFE i.e. Service period or frequency of replacement and performance (lumen maintenance)

4.     ENERGY use

Other characteristics include: shape, need for control gear, starting and restarting time, operating position, effect of ambient temperature, relative cost (initial and running), accessibility.

Colour

Colour Appearance (Temperature)

The colour temperature of a lamp is given in Kelvin and this figure indicates whether the lamp has a warm (below 3000K) intermediate (3500K) or cool (above 4000K) appearance and effect. The colour appearance of a lamp (including colour temperature) is no guide to its colour rendering.

The CIE Chromaticity Diagram offers a precise method of defining a particular colour using X-Y coordinates and can be used for a number of reasons including colour mixing. The diagram, sometimes known as the colour triangle, has the 3 primaries red, green and blue at each point of the triangle. Through the centre of the diagram (“white light area”) a full radiator locus can be drawn, the proximity to this locus is used to determine the Correlated Colour Temperature (CCT)

For all fluorescent and discharge lamps the term Correlated Colour Temperature (CCT) is used but this is only an approximation and not an accurate numerical reference. For example two fluorescent lamps may be given the same CCT by different manufacturers but this does not ensure they will appear the same colour.

Colour Rendering

The colour rendering of a light source is an indicator of its ability of realistically reproduce the colour of an object. According to the International Commission on Illumination CIE, colour rendering is given as an index between 0 and 100 Ra, where lower values indicate po or colour rendering and higher ones good colour rendering. The colour rendering of a light source is compared to daylight if its CCT is >5000K and to a black body (i.e. a source that produces a continuous spectrum) otherwise.

Some tasks such as colour matching in the printing industry have high demands in accurate colour rendering and require special attention from the lighting designer. For normal offices, however, the colour rendering group will be 1B or 2, which is easily achieved with normal fluorescent lamps.

>The reason for lamps with a poor colour rendering such as high and low pressure sodium being used at all is their high efficacy. Even though Sodium lamps have a very low CRI, they have been used in street lighting as they offer a greater light output per watt (efficacy) than lamps which provide a good colour rendering.

The colour rendering index (CRI) has been used to compare fluorescent and HID lamps for over 40 years, but the International Commission on Illumination (CIE) does not recommend its use with white light LED. Phosphor-converted (PC) LEDs use broadband phosphors to score relatively high (70- 90+) on the CRI scale. However, RGB LEDs only score 27 on the CRI metric because those particular wavelengths don’t perform like incandescents on the eight sample CRI colours. Regardless of low CRI, the white light generated by commercial RGB LED clusters is usually visually appealing. One possible reason is that they typically tend to increase the perceived saturation (chroma) of most colours without producing objectionable hue shifts.

The following table provides a useful snapshot to summarise CCT and CRI for some popular lamps.

Light

One important factor in determining ‘which lamp’ will be the amount of light emitted. Some lamps such as incandescent or integrated compact fluorescent light sources emit light at 360 degrees, whereas lamps incorporating a reflector, such as halogen direct their light in a given direction at a certain beam angle, such as 25deg. The unit and means of measuring the light emitted is very much dependent on the type of lamp.

Luminous flux

Luminous flux describes the total quantity of light emitted by a light source, both visible and non-visible. The unit of measurement is Lumens (lm). The quantity of light (lumens) emitted by the lamp measured under standardised conditions. For fluorescent and discharge lamps, the initial value is measured after 100 hours of operation. Typically used for measuring non-reflector lamps such as linear fluorescent, compact fluorescent, HiD, Halogen capsules and LEDs.

Luminous Efficiency (Efficacy) 

Luminous Efficiency is the ratio of the luminous flux or light output (lumens) to the electrical power consumed (watts) = lm/W. It is a measure of a lamp’s economic efficiency. The recognised terminology is Efficacy. For lamps operating on separate control gear total circuit power should be included in calculations of overall efficacy.
 

Luminous Intensity

Luminous Intensity describes the quantity of light that is radiated in a particular direction. This is a useful measurement for  reflector lampss, such as in halogen lamps. The unit of measurement is Candela (cd). Typically used for measuring Halogen reflector lamps.

Illuminance

Illuminance describes the quantity of luminous flux falling on a surface. It decreases by the square of the distance (inverse square law). Relevant standards specify the required illuminance (e.g. EN 12464 “Lighting of indoor workplaces”). Typical measurements in the EU would be the lumens/metre² or Lux (lx). This is more likely to be used for measuring the light emitted by a complete luminaire.

Luminance 

The luminance is the only basic lighting parameter that is perceived by the eye. It specifies the brightness of a surface and is essentially dependent on its reflectance (finish and colour). The unit of measurement is Candela/ metre² (cd/m²)

The following table summarises the key lighting units:-

Life

When applied to electric lamps the word ‘life’ can have two distinct meanings. Lamp Life can either be:-

1.     MORTALITY - The time after which lamps cease to operate

2.     LUMEN DEPRECIATION - The time after which the light output is reduced, by normal deterioration, when it may be economic to replace lamps, even though still operating electrically.

The recognised terminology in the industry would tend to be “Service Period”  or Frequency of Replacement

Incandescent (Filament) lamps

The first definition of ‘life’ applies i.e. MORTALITY. The rated life of common types under specified conditions is defined in international standards and is accepted as a practical survival/efficacy compromise. Fig 8 shows a typical survivor curve for a group of filament lamps operated under standard test conditions. For incandescent lamps, the depreciation of light output is less of an issue than practical issues – the lamps are more likely to be affected by heat, vibration and supply voltage variations: see Fig 5.

When lamps are tested, ‘rated life’ is applied to a group (or batch of lamps) and not to individual lamps, and represents the 50% survivor point. Lamps failing earlier are balanced by lamps failing later. Fig 8 depicts a typical survivor curve for filament lamps.
 

LED, Fluorescent, HID and Induction lamps

The first and The second definition of ‘life’ applies i.e. both MORTALITY and LUMEN DEPRECIATION. Present day discharge lamps and fluorescent lamps will survive for many thousands of hours, but during that time the light output steadily depreciates, so that if lamps were operated until electrical failure, the light output could be half or less of what it was initially. In practice, discharge lamps and fluorescent lamps should be group changed at the most economic time.

Recent developments in lamp and phosphor design have yielded greater lives, together with superior lumen maintenance. Refer to LIF lamp manufacturers’ literature for full details.

Planned lamp replacement – an example of fluorescent lamps in an office environment

In all but the smallest installations it is sensible to replace fluorescent lamps as a group at planned intervals. The advantages of planned replacement can be demonstrated in the following example:

1.     Labour costs can be reduced by phasing the replacement cycle to fit the cleaning cycle.

2.     When there would be an interruption to a production process, replacement can be planned for a non-production period.

3.     Lamps will be of matching output and colour initially and over the service period, and will be of the latest technology.

4.     Replacing lamps before electrical wear-out reduces the possibility of failure of control gear.

5.     For design to economic planned maintenance (see latest Society of Light and Lighting Code), fewer lighting points may be required.

For many installations the most economic time for group replacement is when the light output of the lamps has fallen below 80% of the initial value and the lamp failures are becoming significant in the loss of average illuminance. The latest time for group replacement is when the designed ‘maintained illuminance’ has been reached.

If ‘spot’ replacement of individual lamps is used instead of planned bulk replacement, then it is likely that lumen depreciation, except from lamps with good lumen maintenance, may result in low installation efficacy and unacceptable lighting levels. Modern lamps however such as the new generation of triphosphor T5 and T8 tubes has excellent lumen maintenance, so the maintenance schedule for the installation is likely to be determined by the luminaire depreciation rather than a reduction in lamp lumens.

Fig.9 shows a typical curve of depreciation of light output of a group of fluorescent lamps under standard test conditions. (Depreciation in practical installations is usually faster). The rate varies with lamp type, rating and colour. Reference should be made to the technical literature of LIF lamp manufacturers.

Energy Use

Energy consumption [kWh]

Of increasing imporantance now with the high cost of electricity and the need to introduce as efficient light as possible. The amount of electric energy consumed by a lamp over a certain period is expressed in kWh (kilowatt–hours). For example a 100W incandescent lamp consumes 1 kWh in 10 hours (10 hours x 100W = 1000Wh or 1 kWh). The amount of electricity used for lighting is generally based on energy consumption per year (kWh per year).

Carbon Emissions

On 18^th March 2009, the European Commission adopted 2 Ecodesign regulations to improve the energy efficiency of household lamps and of office, street and industrial lighting products. The 2 regulations lay down energy efficiency requirements which will save close to 80 TWh by 2020 (roughly the electricity consumption of Belgium, or of 23 million European households, or the equivalent of the yearly output of 20 power stations of 500 megawatts) and will lead to a reduction of about 32 million tons of CO2 emission per year. Inefficient incandescent light bulbs will be progressively replaced by improved alternatives starting in 2009 and finishing at the end of 2012. As a result of these regulations, 11 billion euros are expected to be saved and re-injected every year into the European economy.

Lamps and Power

All the input electrical power to a lamp is transformed into other forms of power, proportioned in terms of total lamp power as follows:

NB
1.    Light is radiant energy of wavelengths visible to the human eye. It must be included in heat loading calculations
2.    The power loss of ballasts should be added to the conducted/convected heat
3.    All figures are purely representative and intended only as a guideline. For precise/accurate information, please consult manufacturers’ technical data sheets