In the early days of LED development, signmakers were offered a choice of at best, two colour variants of White illumination, generically described as either Cool White or Warm White. How times have changed! Graham Pritchard, Technical Sales (Internal) at Vink Lighting, advises how to achieve the best results
Colour temperature from module to module was at best patchy and if the sign box or built-up letter did not have a sufficient return depth to allow the light from each module to diffuse, then the result was a very uneven illumination.
During production, LED chips vary in colour, flux and forward voltage and need to be sorted into bins according to their CCT (Correlated Colour Temperature) and the x,y coordinates (or colour point as it appears on the CIE diagram. (See Diagram 1)
These two factors need to be taken into consideration due to the fact that White LEDs with a similar CCT can still have a different colour appearance depending upon the colour point on the chart.
As LED production developed and the major suppliers improved upon their market offering, the need to provide a more consistent product meant that LED chip categorisation (or binning) would need to become more sophisticated. Accurate batch sorting at the production stage and tighter specifications from the module manufacturers required better methods of measuring the colour differences from one chip to the next.
Scientific methods of discerning colour difference had to be used and a specification established by Dr. David MacAdam in 1942, which relied upon visible observation of the Just Noticeable Differences (JND) between two very similar coloured lights provided a useful measure. JND is defined as the colour difference where at least half of the people observing the lights cannot see a difference. These areas of similarity are then plotted on the CIE chart in a series of elliptical zones. These zones are known as ‘MacAdam’s Ellipses’.
A convenient method of expressing the colour difference within a particular bin is the number of ellipse steps the LEDs within that bin fit in to. The lower the number of ellipse steps, the less likely it would be that differences would be noticed. For LEDs used in backlit signage, a minimum acceptable standard should be four ellipse steps. In terms of colour temperature variation, four MacAdam’s Ellipses equates to a tolerance of +/-100 Kelvin.
Being able to specify from a tighter bin ensures that there is no difference in batch-to-batch production. This advantage provides the sign owner with the peace of mind that all signs made from a specified product will maintain a consistent colour throughout the roll-out programme, regardless of time span.
Tighter binning also enables the end user to be more discerning as to the colour warmth chosen, with some LED manufacturers offering as many as eight different shades of white. As well as the obvious aesthetic benefit, certain coloured acrylics are better suited to different white variants allowing for higher Luminance results to be achieved and, in certain circumstances, reducing the quantity of LED modules required to illuminate the sign to the desired level.