Part 1 - Colour
Kevin Stubbs FSLL - Technical Manager at Llumarlite, asks the question that we should all know the answer to.
What do I need from a light source?
Many of us knew where we stood with existing lamps – Tungsten or Halogen at home and for display lighting. Fluorescent lamps in the office. Discharge lamps for factories, warehousing, and outdoor lighting. These had developed to a steady saturation point and we had grown with them, to understand what quality of light we should expect - How much energy they’d use? How often we’d have to clean or replace them? (although despite designing with a suitable maintenance factor, no-one did ever clean them, or ‘bulk’ replace them!).
Colour Quality – this is measured in two ways.
Colour Appearance is how ‘warm and welcoming’ (2700 – 3000K or Colour ’27 - ’30), ‘cool and efficient’ (4000K or Colour ‘40) or ‘cold and stark’ (5,000 – 8000K or Colour ’50 to ’80) looks to an occupant. Although, we humans adapt to the colour once we are in an area, as do modern cameras (which are now mainly smartphones of course). Colour differences are most noticeable going in and out of different areas. This colour can affect our mood, attitude and as we move towards the ‘cold and stark’ blue-ey end of the colour spectrum, we are now finding that can affect our sleep patterns and wellbeing in an indoor environment. So, it is unfortunate that much of the earlier and current ‘cheaper’ LED lighting is often this harsh cold colour that we are being told is ‘daylight’. Natural daylight changes colour from warm (dawn) to cool (noon) back to a warm red glow (sunset), so we do not need to be ‘blasted’ with a poor and unnatural rendition of midday light throughout our working day.
So, to choose a 4000K lamp or LED we are looking for the ‘_40’ part of the colour code.
Personally, my favourite for an office remains “intermediate” 3500K, i.e. ‘_35, but this colour is less readily available. As lamp and LED businesses became globalised - 4000K gradually became the norm in UK offices to fall in line with the larger markets brought by our European colleagues.
Colour Rendering is how the light source ‘Renders’ or reflects light to the eye. If we were trying to compare cloth samples in a tailor’s shop or carpet samples for a home, we would prefer a light source with a full spectrum (all the colours of the rainbow) in it e.g. tungsten or halogen lamps. These have a Colour Rendering Index (CRI) or ‘Ra’ of 100 i.e. 100% of the spectrum will be generated by the lamp, so any colour not reflected by the sample material is simply because that colour is being absorbed by the said material i.e. ‘subtracted’.
At the lower end of lamp quality, maybe an industrial application where colour does not matter to us or what we often see in an LED street lamp application – it does not really matter to us what colours we see, just that we are able to read text on a box label or see obstructions when moving around. In these situation’s the CRI or Ra can be as low as 40 i.e. we can only see 40% of the colours reflected (compared to the 100% we would have seen under the tungsten lamp). Hence in these circumstances, poorer colour rendering means we have lost some of the colours of the spectrum from our light source. Anything reflecting light will give a poor or false colour rendition. This is important if buying clothes. If the colours of a shirt or dress looked bright and vibrant in a shop, we’d want them to look the same wherever we went. So, retail lighting should demand a high CRI or Ra close to 100%. In this situation a compromise maybe required between balancing energy costs and light source production costs. Llumarlite would specify a colour quality ‘CRI’ or ‘Ra’ of 90+ (which means 90 or higher).
The same Ra 90+ is also what Llumarlite would specify for healthcare, where comparing patients’ complexion or appearance is an important way of recognising their ailment.
In an office or other workplace, we need to recognise people and read their expressions (less critically than in healthcare) as well as being able to carry out paperwork tasks and perhaps read coloured text or graphic designs. So, a slightly lower CRI or Ra is acceptable, 80% is normally specified i.e. Ra 80+.
This colour quality makes up the blank referred to above, so the _40 lamp Colour Appearance specified above has an added ‘8’ for an office environment needing a CRI/Ra of 80% – i.e. ‘840’.
There is little written evidence that colour rendering affects our performance, but we can only assume that getting closer to the full colours of the spectrum will be closer to natural light, hence using light sources of CRI/Ra 85 or 90 are likely to make people feel more comfortable in their working environment than the slightly cheaper CRI/Ra 80, but you really want to avoid the lower colour rendering lamps indoors.
The bad news with LED rather than the traditional lamps, is that we can no longer change the lamp colour appearance, rendering or quality by substituting a relatively cheap replacement lamp. We must make the colour choice initially during luminaire selection, and once installed it is likely to stay that way for the light fitting’s lifetime as an integral part of a fixed luminaire. Otherwise there will be (and is) a major programme of recycling and luminaire replacement to carry out.
The good news with LED is that it is becoming cheaper to make luminaires with variable colour, adjustable tuneable white, by building in LED’s at the warm end of the spectrum along with those of a cooler colour appearance, and control equipment to blend and set or vary these colours. Along with flexibility, and the choice to alter the mood of an area, the addition of more specialist controls can allow us to make the lighting more variable or personal, now called “Human Centric”.
Now that we can specify a lamp or LED’s colour, we will want to also understand it’s flicker and lifetime metrics – these subjects will be covered in my next blogs… Part 2 and Part 3.
Where can I get help?
Llumarlite has a team of experts and colour comparator boxes in our office for demonstrations, or we can bring some sample colours and tools to your office to help you compare - what you have now against what you want in the future. Contact us today so that we can help you make the right decisions.