PLS 2018: The elephant in the road
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The elephant in the street relates to the frequently discussed but little addressed topic of glare from LED exterior luminaires and how this affects the visual task through disability glare but in the case of wet road surfaces a loss of uniformity. Industry and user concern exists over the application of Threshold Increment assessment with respect to LED luminaires. Currently threshold increment is based upon traditional light sources which are comparatively large compared to the luminaire. The advent of LEDs with their much smaller size is raising concerns that although these seem to meet the threshold increment requirements, the installations often display a high level of disability glare. So, is the assessment approach right for LED external luminaires? by Kelly Smith MSc BEng(Hons) AMCIBSE MSLL AMILP, WSP
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PLS 2018: The elephant in the road
- 1. The Elephant in the Road Glare, LEDs and Wet Roads Kelly Smith, Principal Engineer
- 2. 2 The Elephant in The Road — CIE workshop — May 24/25 — Berlin — Variables missing in current road lighting calculations. — Looking at what future work the CIE needs to this topic. — Glare, LEDs and wet roads
- 3. 3 The Elephant in The Road — Contents — What is Glare — How is Glare calculated & measured — Threshold Increment — Glare & LEDs — Add Wet Roads — And Stir …….
- 4. 4 What is Glare — Flash Blindness — Paralyzing Glare — Retinal Damage — Distracting Glare — Dazzle — Adaptation Glare — Discomfort Glare — Disability Glare
- 5. 5 What is Glare — Discomfort Glare — Causes discomfort without necessarily impairing the vision of objects and details — Disability Glare — Which impairs the vision of objects without necessarily causing discomfort. — From CIE 112 Reproduced from Lighting Research Centre Reproduced from Autoelecwarehouse
- 6. 6 What is Glare — Discomfort Glare — Depends of four parameters —Source Luminance(s) in direction of observer’s eye. —Solid angle subtended by source(s) at observer’s eye. —Angular displacement of the source(s) from observer’s line of sight. —General field luminance(s) controlling the adaptation level of the observer’s eye Reproduced from Lighting Research Centre
- 7. 7 What is Glare — Disability Glare — Equivalent uniform luminance(s) resulting from the stray light in the eye which superimposes on the location of the vertical shape thus lowering contrast — Depends to two parameters —Illumination of observers eye produced by the glare source(s) in the plane perpendicular to the line of sight —The angle between the centre of the glare source(s) and the line of sight Reproduced from Autoelecwarehouse
- 8. 8 What is Glare — Disability Glare — Equivalent uniform luminance(s) resulting from the stray light in the eye which superimposes on the location of the vertical shape thus lowering contrast — Depends to two parameters —Illumination of observers eye produced by the glare source(s) in the plane perpendicular to the line of sight —The angle between the centre of the glare source(s) and the line of sight Reproduced from Autoelecwarehouse
- 9. 9 What is Glare — Disability Glare — Much easier to quantify —GR (Glare Rating) – mainly for outdoor lighting —From CIE & is difficult to measure in situ. Reproduced from Autoelecwarehouse
- 10. 10 What is Glare — Disability Glare — GR is given as Reproduced from Autoelecwarehouse
- 11. 11 Everyday examples of Glare — Overhead ceiling lights reducing contrast between page and print — Daylight through windows reflecting from computer screens — Using high beam headlights travelling through thick fog — Light reflected from water which obscures what is underneath
- 12. 12 Everyday examples of Glare — Overhead ceiling lights reducing contrast between page and print - headache — Daylight through windows reflecting from computer screens - backache — Using high beam headlights travelling through thick fog - accidents — Light reflected from water which obscures what is underneath - death
- 13. 13 Measurement of Glare — Measurement of glare — Baseline metric for measuring glare is luminance for an observer measured at a particular position and viewing direction (cd/m2). — But luminance must be measured at luminous opening i.e. the smallest point without any breaks that emits light out of the fixture. — But we currently measure LED modules or entire luminaires. — This ignores the individual ‘points of light’ from each LED
- 14. 14 Threshold Increment — TI is ‘measure of disability glare expressed as the percentage increase in contrast required between an object and its background for it to be seen equally well with a source of glare present.
- 16. 16 G Class from EN 13201 — But we have another measure of glare for exterior lighting
- 19. 19 Ways to Reduce Glare — Move the luminaire — Increase mounting height — Decrease or eliminate tilt — Stray Light — Minimise light projected into unintended areas — Prismatic controller / optics — If possible try not to scatter the light but control it.
- 20. 20 Wet Roads Extreme Days of Rainfall
- 23. 23 Wet Roads — More roads being ‘improved’ from SOX / SON to LED. — Colour change sudden rather than gradual. — LED is more closer to a point source. — Reflections from a point source.
- 24. 24 Wet Roads
- 25. 25 Wet Roads
- 26. 26 Wet Roads — Changes in choice of lighting classes to lower levels — More emphasis on uniformity — Vertical Lighting — Refraction/reflection through raindrops
- 27. 27 Further Reading — A STUDY ON THE PERFORMANCE OF HIGH PRESSURE SODIUM AND LED ROAD LIGHTING IN RAINY CONDITIONS — ANA SALMI AHMAD SALMAN — PETER RAYNHAM
- 28. 28 Conclusion – one final stir Metrics LED Climate Humans
- 29. 29 Conclusion – The Challenge
Editor's Notes
- This topic was suggested by the fact that in May there was a conference in Berlin looking at the methodology for lighting roads and the different variable which mean that what we calculate is not a good representation of what we get on the road. The CIE has recognised that there needs to be a lot of research done on this topic and in fact this workshop is just to brainstorm what extra work needs to be done. What we already know is that we are currently having problems with glare, particularly as LEDs seem set to dominate the market. This increase in glare becomes much more obvious as we consider wet roads and the reflectance we get from the miniscule sources which are LEDs.
- The title was chosen for this talk as a way to highlight that we already know there is a problem, we just don’t seem to want to see it – maybe there is too much glare. The bad news is that I’m not going to give you some amazing new revelation, the good news is that I’m only on for 30 minutes.
- According to Peter Boyce in Human Factors for Lighting there are 8 types of glare Flash blindness – temporary state of complete bleaching of retinal photopigment cause by sudden onset of an extremely bright light source such as a nuclear explosion Paralyzing glare – as it says this is when a person suddenly illuminated by a searchlight at night will freeze briefly Retinal damage – either cause by the sun or by artificial light Distracting glare – produced by bright flashing lights in our peripheral vision such as emergency lights Dazzle – this is painful and the instinct is to shield your eyes. This rarely happens indoors and a perfect example would be the glare which causes snow blindness. The glare completely fills your field of view Adaptation glare – caused when the visual system is exposed to a sudden large increase in luminance of the whole visual field – eg when exiting a tunnel. Fortunately for you I’m not that cruel as to go through all of these. The only two we really need to look at are discomfort glare and disability glare.
- In exterior lighting we recognise two types of glare. We call them disability and discomfort glare. This is how they are defined in CIE 112. One causes discomfort without disability The other is disability without discomfort – easy!! Although discomfort glare tends to be psychological & disability glare is physiological, there are not mutually exclusive and in some circumstance you can have both. Glare essentially is when there is a excessive contrast between the background and the light source. If the glare becomes so intense that it completely impairs the vision this is called dazzle.
- So these are the main variables with their official description when looking at discomfort glare. What this essentially tells us is that the glare will be different for every observer position relative to the source and depending on the background – well that’s common sense but it does make it more difficult to calculate. Discomfort glare is mainly subjective. LRC (Lighting Research Centre of Rensselaer Polytechnic Institute) have a method for measuring discomfort glare which is empirical but it requires measurements of a single light at a time which would be impractical for field measurements.
- The first definition is important as to be able to see anything and make sense of it, you need the right level of light, time and contrast. Take one of these away and comprehension of our environment and task becomes more difficult.
- I just want to take a moment and show just how important contrast is to how we see the world. The spirals look like they are pink green & blue – right?? Nope – only pink and green. The stripes going through the green and the pink are orange, the stripes going through the ‘blue’ are the same colour as the pink swirl. This was created by Akiyoshi Kitaoka who is a Japanese professor of psychology. There are many of these available on the internet if you look and most optical illusions of this sort work by playing with contrast. Our eyes might only take in small packages of information as they flicker over an image but the brain interprets it as a whole
- Disability glare is much easier to measure in the lab with empiric date but is calculated from photometric data for standard conditions. It is built into many software packages ie Dialux, AGI21, Relux etc.
- No need to go to sleep just because a formula has appeared. I added this just to point out that Glare Rating is essentially a ratio of equivalent veiling luminance of the luminaires to the equivalent veiling luminance in the eye. The terms are a little misleading as they are based on an illuminance calculation & the other factors convert this to luminance in the formula. The illuminance value may be calculated but this is just a virtual measurement.
- But back to something a little less heavy going. Glare is something we come up against every day and in many cases don’t think about it. We instinctively know that if we change position or angle we can avoid or minimise the glare. But that may not always be possible – especially in a fast moving car which is when glare can become very dangerous and why we MUST give it more of our attention.
- But back to something a little less heavy going. Glare is something we come up against every day and in many cases don’t think about it. We instinctively know that if we change position or angle we can avoid or minimise the glare. But that may not always be possible – especially in a fast moving car which is when glare can become very dangerous and why we MUST give it more of our attention.
- So we know glare can a concern both to health & life but we have ways to measure it and limit it our luminaires I hear you say so what is the problem. So when we had traditional lamp sources we had that behind a diffuser of sorts so we had one big continuous source in the luminaire that we could use for measurement. We treated it as a point source of calculations but it wasn’t really. Outdoor luminaires are visibly a collection of individual point sources and research indicates that the bigger the gap between adjacent LED the more the calculation underestimates the sensation of glare (presumably to a limit of separation when the LED are visually treated as individual luminaires). And now we are finding that many of the metrics we were able to use previously, eg colour rendering, are no longer as effective as they once were. Glare is one of those metrics that we continue to use but don’t really portray what is happening in ‘the real world’
- So right now we can see this gentleman quite clearly – but if he was standing right beside the column you can image that he would not be seen easily – if at all. Threshold increment can be thought of as the amount of extra ambient light you would need to reduce the glare from the streetlight to make the man beside the column visible again. Remember glare is excessive contrast and our lurking gentleman would not have enough contrast against his background to be seen.
- So right now we can see this gentleman quite clearly – but if he was standing right beside the column you can image that he would not be seen easily – if at all. Threshold increment can be thought of as the amount of extra ambient light you would need to reduce the glare from the streetlight to make the man beside the column visible again. Remember glare is excessive contrast and our lurking gentleman would not have enough contrast against his background to be seen.
- So in exterior lighting we have another way we can specify glare and that is the G glass you will find in EN 13201. It states that there are certain levels acceptable at high angles – 70°, 80° & 90° then 90°+ to achieve a certain class and this is also seen as a way of reducing glare.
- And here we can come up against a problem. LEDs can be quite tightly controlled there making them meet the requirements for a high G class. But we have already mentioned that being more of a true point source that LED can produce more glare and the method we have for measuring glare in the lab is flawed. So we are starting to find in some cases – especially with some less scrupulous manufacturers – that G6 luminaires are producing some quite high TI values. And now there is a dilemma for luminaire makers – control your glare properly and risk a lower G class, where you might not even be considered for a tender – or have a G6 rating but end up with a source with higher TI than traditional sources. And remember than even though you can say – we are within the required limits – that the general public are used to traditional sources and will rate LEDs by comparison, not by a computer program.
- These are two luminaires from the same manufacturer – the same physical layout but where the G4 class would make you believe the glare was more tightly controlled you can see that the TI value is much higher. The problem we have is that with LED we have metrics which are no longer meaningful still being used even though the interpretation is no longer logical. This needs some deeper investigation?
- Glare is going to happen when you can see the light source. And the closer you are to the light source then the greater the glare. The best ways to decrease the glare is to make the light source imperceptible and to only light the area under consideration. The current testing methods just don’t seem to be working and disreputable manufacturers are taking advantage – caveat emptor is all well and good as long as we have a level playing field which everyone understands.
- The second part of the problem we have is with wet roads. The graph shows the days we have had which are classed as extreme rainfall. This is a result of climate change and the dreaded greenhouse effect. The worse greenhouse gas is water vapour. The initial introduction of cfc’s and other gasses started the trend which increased the air temperature. With an increase in air temperature, the air can then hold more moisture. We have said that water vapour is the worst and once that started to increase you had a case of positive feedback. More water vapour in the air - the land temperature increases - increases the air temperature – which can hold more water vapour!! All this means that we are getting more rain – and wetter roads.
- We can obviously see that from the time of the industrial revolution that the amount of CO2 in the atmosphere has radically increased and we have an increase in the number of people on the planet due to an increase in health and medical practices – all of might are producing more and pollution which is what is having an effect on the climate of our planet.
- No matter how much some people deny it – humans are having an effect.
- It used to be that we had gradual changes to out street lanterns – sox became son which became metal halide as we discovered more about how the eye works and that ‘white’ light at night didn’t need to be so bright. But now we have situations where old son and even sox lanterns are being swapped out immediately to LED and the most efficient LED are high colour temperature. The gap between efficiency and price is decreasing between the colour temperatures but the damage – at least in the public eye – had already been done. As we have discussed LEDs are more of a point source than our traditional sources and this is causing problems not only with glare but on wet roads where the tightly controlled light from the luminaire become very intense from a wet road.
- This is a picture taken with a luminance camera on a wet road. You can see the difference in the luminance values across the road with the graph at the top showing the measurements along the pink line. Remember glare is a problem of excessive contrast.
- And another picture with again a large spike in luminance as well as a bright field of view.
- We have changes in the standards towards the same geometry giving lower lighting levels which is good because we can all agree that we were historically overlighting. There has also been a trend towards making uniformities more important. We now have an upper limit on horizontal illuminance in the P classes to help maintain a good uniformity. But as you can see in the image with high specular reflections from LEDs on wet roads, the uniformity in our vision is destroyed. We are also starting to see that vertical lighting is more essential than we had thought. The tight control of LEDs which causes the problem with wet roads is also causing a problem with our verticals. We had good vertical lighting with traditional sources without thinking about it but now we don’t and now we realise that it is a requirement for good lighting. And its not only the reflection on we roads we need to consider. With an increase in wet conditions there is also the effect of light refraction through the raindrops to consider. In some cases this will produce a luminance ‘rainbow’ which will increase the amount of light directed to the driver. But we must be careful in designing for wet roads – what will this do to the lighting for dry roads?
- There is a very good paper written at Barlett School of Energy, Environment and Lighting which looks at the problems of wet roads with new light sources
- So we know we have some problems but we tend to consider these problems in isolation. We need to take an holistic approach. We have existing metrics for glare and colour temperature which we have used for quite some time and which we now realise no longer apply to LEDs but without any research we have nothing else to use. And we then have the effect of LEDs – and the tight control of that light when used in an ever increasingly wet environment. That wet environment is caused by humans and the need for the exterior lighting is only for humans and the metrics we have are set for what we understand of human vision. But we are starting to know more about the human visual system and what is important for use to see. We also need to balance human needs with the world and the impact we have. It is no longer just enough to solve one problem without realising the knock-on effect it will have. We need an holistic approach. We light for human needs, if that does not come first then we must ask if lighting is necessary but once that decision is made then we must use the most efficient means we can – and not just economically – but socially and environmentally – the triple bottom line. We need new metrics for a new light source but we also need new lighting methods for this new light source in our new climate – for old human eyes!! In many cases we are using like for like replacements where lamp technology luminaires are being replaced by LED and although the testing to the old metrics are saying the lantern is ok the public are telling us it is too glary. If the luminaire is tested properly – as an LED SHOULD be then the initial testing would tell us there us a problem with the photometry of the luminaire.
- People are turning to LED to save money on energy – and to save more money they are re-heading the existing columns. First problem, LED luminaires can be heavier than the traditional counterparts due to things like heatsinks etc, windage may be different. You are also forcing the LED into a spacing that may not be optimum and into a environment that may not be suitable. The LED luminaires we have now are not completely suitable for the job we are giving them – they need tweaked. And we cannot expect manufacturers to start using a different testing method if 1 – they don’t know how they should be testing, b – if goes against how the official testing says it should be done and third – it leaves them at a disadvantage to less concerned competition. So start at the beginning – the standards and metrics. But to change those takes a long time and it needs a basis in logic and experiment. So to eventually get to a state where we can use energy efficient light to minimise our impact on the world around us, we need to act now.