Thursday, 1 May 2014

A Guide To The Specification of LED Lighting Products- Part 3 of 4

Quality Criteria

The IEC has developed a list of performance requirements for both LED modules and LED luminaires. These provide a set of quality criteria related to the initial specifications of a product and a standardised description on how to measure these quality criteria. 


Following this proposed criteria allows you to easily compare manufacturers claims of initial specifications of LED modules and luminaires. 

Quality Criteria- Rated Input Power

The rated input power shows the amount of energy consumed by a luminaire, including its power supply. 


It is expressed in watts.

Quality Criteria- Rated Luminous Flux

This is the light emitted by the luminaire which is expressed in lumens.

For traditional luminaires it is usual that Relative values are measured and multiplied by the lamp flux. 

For LED luminaires it is recommended that Absolute photometric values are used. Absolute photometry results in a LOR=1.

Quality Criteria- Correlated Colour Temperature (CCT)

Although white light is a mixture of colours, not all whites are the same since they depend on their constituent colours. 

So a white with a higher proportion of red will appear warmer and a white with a higher proportion of blue will appear cooler. 

In order to classify the different types of white light, the concept of colour temperature is applied which is described as the colour impression of a perfect black-body radiator at certain temperatures. 

This concept can be best explained with the help of familiar thermal radiators like the filament of an incandescent lamp or an iron bar.

When these materials are heated to a temperature of 1000k their colour appearance will be red, at 2000-3000k they will look yellow white, at 4000k neutral white and at 5000-7000k cool white. 

The higher the colour temperature, the cooler the perception of the white light becomes.

It is expressed in kelvin.

The initial CCT value classification for the photometric code can be obtained by taking the initial CCT value divided by 100. 

Quality Criteria- Rated Colour Rendering Index (CRI)

Although light source may have the same colour appearance, this doesn’t necessarily mean that coloured surfaces will look the same under them.

Two lights that seem to have the same white colour appearance may be the result of different blends of wavelengths. 

As a result a given material may appear differently since the surface may not reflect the constituent wavelengths by the same extent. 

Its colour appearance will change when it is exposed to one or other light. So, colour rendering is an important criterion when selecting light sources for lighting application solutions. 

With new LED technology coming in, with a narrow spectrum, the CRI index is not in all circumstances giving a fair representation of the colour appearance. 

New definitions and methods for measuring are currently under development in CIE.

The initial CRI value classification for the photometric code can be obtained by using the following intervals:
Code
CRI Range
Colour Rendering Properties
6
57-66
Poor
7
67-76
Moderate
8
77-86
Good
9
87-100
Excellent
Quality Criteria- Lumen Maintenance Code

As the typical life of a LED luminaire is very long, it is time-consuming to measure the actual lumen reduction over life. 

Also the actual LED behaviour with regard to lumen-maintenance may differ considerably by type and manufacturer.

It is not possible to express the lumen-maintenance of all LEDs in simple mathematical relations. A fast initial decrease in lumen output does not automatically imply that a particular LED will not make its rated life. 

In order to validate a life time claim an extrapolation of test data is needed. In the IEC a general method of projecting measurement data beyond limited test time is under consideration. 

Quality Control- Lumen Maintenance Code

In the US an extrapolation based on LM-80 test data be described in IES TM-21. Instead of life time validation, the IEC/PAS has opted for lumen maintenance codes at a defined finite test time. 

Therefore the code number does not imply a prediction of achievable life time. The maintained luminous flux is measured at 25% of rated life time up to a maximum of 6,000 hours and expressed as a percentage of the initial value.

The maintained value determines the lumen maintenance code.
Lumen Maintenance (%)
Code
>90
9
>80
8
>70
7

Quality Control- Photometric Code

A six digit photometric code that displays the important ‘quality of light’ parameters. 


Quality Control- Rated Life (in h) of the LED Module and the Associated Rated Lumen Maintenance

The length of time expressed in hours, during which a population of LED modules provides more than the claimed percentage (x) of the initial luminous flux always published in combination with the failure fraction. 

The recommended series of values for (x) is 70, 80, 90. 

Quality Control- Failure Fraction (Fy), corresponding to the rated life of the LED module in the luminaire

The percentage (y) of a number of LED modules of the same type at their rated life that have failed. 

This failure fraction expresses the combined effect of all componenets of a module including mechanical, as far as the light output is concerned.

The recommended series of values for (y) is 10,50. 

Quality Control- Ambient Temperature (ta) for a Luminaire

The ambient temperature around the luminaire related to the specified performance. 
For a given performance claim the ambient temperature (ta) is a fixed value. 

It is possible to specify performance claims at different ambient temperatures.

If the LED luminaire is to be used at an ambient temperature different to that at which it was tested, correction factors will need to be applied to the performance criteria. 

It is expressed in degrees Celsius.

Quality Control- Power Factor

The power factor should be clearly stated in all cases.

Although product standards may not require this below 26w, it should be noted that some clients, and in particular contractors and local authorities working with un-metered supplies, will require power factor correction of 0.85 or better. 

Quality Control- Intensity Distribution


Photometric data is available in two formats.

Absolute Photometry does not require the use of a separate lumen output for the light source. 

Relative Photometry requires the LED package flux to be quoted.

Both methods produce the same result. For LED luminaires Absolute photometry shall be used. 

Absolute photometry of LED luminaires should be conducted according to IES LM-79-08 Photometric Measurements of Solid-State Lighting Products.

Relative photometry should be conducted according to EN13032-1 (2004) Light and lighting Measurement and presentation of photometric data of lamps and luminaires- Part1: Measurement and file format.

These standards contain advice on measurement uncertainty. 

Photometric results that are calculated by deviation from the tested sample by the use, for example of higher or lower drive currents or dies from bins other than the bin used for the tested device are to be clearly identified as such.

Correction factors used are to be provided with the results.

Quality Control- Drive Current

For proper operation, the power supply and electronics must provide a well-controlled DC drive current. Drive current affects LED operating temperature a thus life and output. 

Normally around 350mA is quoted but this can be higher. The higher the LED is driven the brighter it will be but it may have a shorter operation lifetime and be less efficient. Some of the new multi die LEDs are designed to operate and perform at higher drive currents.

Declaration of the drive current is important when remote drivers are used. 

Quality Control- Optical Risk

The Control of Artificial Optical Radiation at Work Regulations 2010 apply to light emitted from all artificial light emitted from all artificial light sources including LEDs. 

These regulations require employers to protect the eyes and skin of workers from exposure to hazardous sources of artificial optical radiation.

Exposure limits defined in the standard EN-62471 are in European regulation (directive 2006/25/CE). These are a combination of source power and exposure time.
Exposure limits defined in the standard EN-62471 are in European regulation (directive 2006/25/CE). These are a combination of source power and exposure time.

When light sources are placed in a luminaire, the Risk Group classification can change due to the optics used in the luminaire. 
Risk Group 0
Exempt
Risk Group 1
Low
Risk Group 2
Moderate
Risk Group 3
High