Light Lines Mar/Apr 20

LIGHT LINES

TRAVELLING LIGHT

A winning concept

BEYOND MEASURE Why lighting metrics don’t add up

SECRETARY

Brendan Keely FSLL bkeely@cibse.org

SLL COORDINATOR

Juliet Rennie Tel: 020 8772 3685 jrennie@cibse.org

EDITOR

Jill Entwistle jillentwistle@yahoo.com

COMMUNICATIONS

COMMITTEE:

Linda Salamoun MSLL (chair)

Iain Carlile FSLL

Jill Entwistle

Chris Fordham MSLL

Rebecca Hodge

Eliot Horsman MSLL

Stewart Langdown FSLL

Bruce Weil

Gethyn Williams

All contributions are the responsibility of the author, and do not necessarily reflect the views of the society. All contributions are personal, except where attributed to an organisation represented by the author.

COPY DATE FOR LL3 2020

IS 13 MARCH

PUBLISHED BY

The Society of Light and Lighting

222 Balham High Road

London SW12 9BS

www.sll.org.uk

ISSN 2632-2838

© 2020 THE SOCIETY OF LIGHT AND LIGHTING

The Society of Light and Lighting is part of the Chartered Institution of Building Services Engineers, 222 Balham High Road, London SW12 9BS. Charity registration no 278104

PRODUCED BY

FROM THE EDITOR

There is no doubt that photometrics are in a state of flux, luminous and otherwise. Challenging the orthodoxy of how we measure lighting is probably long overdue, but the introduction of LEDs, disruptive indeed, and the discovery of the non-visual photoreceptor in the mammalian eye are two developments that have perhaps accelerated the process.

It is always an alarming prospect to throw decades of thinking out the window, but tweaking rather than taking a fresh look at the whole thing can lead to compromise and complication. Committees and camels.

Stephen Cannon-Brookes is one who thinks that a more radical rethink is in order (Tunnel Vision?, p9). The CIE definition of visible light using the photopic spectral sensitivity function known commonly as V λ is under siege, he maintains.

'Our classical photopic-focused understanding of light [is] an increasingly creaking edifice,' says Cannon-Brookes. 'Addressing this, however, is a challenging

prospect for the multiple generations brought up on an orthodoxy that also underpins our understanding of colour.'

Mark Rea proposed the universal luminous efficiency function (U λ) in 2016, and David Loe questioned the validity of V λ in an article two years ago (Does V λ SPD measure up? March/April 2018). Following up with another article based on further experiments (p11), he confirms his challenge of the established approach. 'The results suggested that the V λ [SPD] as a basis of illumination metrology is at the very least questionable,' he says.

Perhaps the outcome will be a measure of boldness as much as anything.

CURRENT SLL LIGHTING GUIDES

SLL Lighting Guide 0: Introduction to Light and Lighting (2017)

SLL Lighting Guide 1: The Industrial Environment (2012)

SLL Lighting Guide 2: Lighting for Healthcare Premises (2019)

SLL Lighting Guide 4: Sports (2006)

SLL Lighting Guide 5: Lighting for Education (2011)

SLL Lighting Guide 6: The Exterior Environment (2016)

SLL Lighting Guide 7: Office Lighting (2015)

SLL Lighting Guide 8: Lighting for Museums and Galleries (2015)

SLL Lighting Guide 9: Lighting for Communal Residential Buildings (2013)

SLL Lighting Guide 10: Daylighting – a guide for designers (2014)

SLL Lighting Guide 11: Surface Reflectance and Colour (2001)

SLL Lighting Guide 12: Emergency Lighting Design Guide (2015)

SLL Lighting Guide 13: Places of Worship (2014)

SLL Lighting Guide 14: Control of Electric Lighting (2016)

SLL Lighting Guide 15: Transport Buildings (2017)

SLL Lighting Guide 16: Lighting for Stairs (2017)

SLL Lighting Guide 17: Lighting for Retail Premises (2018)

SLL Lighting Guide 18: Lighting for Licensed Premises (2018)

SLL Lighting Guide 19: Lighting for Extreme Conditions (2019)

Guide to Limiting Obtrusive Light (2012)

Code for Lighting (2012)

Commissioning Code L (2018)

SLL Lighting Handbook (2018)

In early January we were informed of the passing of Alfred Leung. Alfred had been the SLL regional lighting representative for Merseyside and North Wales since 2009. He was a contributor to the representatives meetings as well as the SLL council. He sat on the CIBSE Merseyside and North Wales regional committee and hosted a great many of the lighting seminars in the region for the benefit of the members and public. He was also close to the Yorkshire regional committee after his work took him to Leeds. We will all miss Alfred, a gentleman, always with a smile.

March sees the biennial visit to Light+Building at Messe Frankfurt. We are very much looking forward to the visit, and both Juliet and I will be there from the evening of Sunday 8 March through to the morning of Wednesday 11 March. The main reason we take this trip every two years is to make contact with the society’s Sustaining Members. They are all there in one place and we do enjoy catching up with them and ensuring that they are enjoying the benefits of their membership. No doubt we will see many of you there in the halls and corridors over the few days of the exhibition. Safe travels to all and we hope your visit is successful.

Ready Steady Light will take place on 24 March. Team places are going fast so we encourage everyone to get their entry form in as soon as possible. Full details can be found on the SLL website under Events.

FROM THE SECRETARY BRENDAN

The Notification of the AGM, SLL Awards and Presidential Address has been sent to all members. The AGM will take place on 21 May at the Ortus Learning and Events Centre, London. Bookings for the Lighting Research and Technology Symposium on Applying Light for Human Health will be open soon. The event will take place on 18 June at University College London. The speaker line-up includes Mariana Figueiro, Mark Rea, Arnold Wilkins, John Mardaljevic, John O’Hagan, Florence Lam,

Russell Foster and Peter Thorns. The presentations and panel discussions will include: an overview of how exposure to light affects human health, what manufacturers and lighting designers need to know about lighting for health, and lighting for better sleep, nightshift work, day work and in schools. Participants will also address lighting for people with dementia, and for sufferers of migraines and other conditions, how lighting necessary for human health can be implemented and the future of lighting for health and wellbeing.

It promises to be a very good day and we hope many of you will join us. We take the opportunity to thank Professor Peter Boyce for his work as Lighting Research and Technology editor-in-chief over many years. His successor is Steve Fotios and we look forward to continuing working with him and the editorial board.

There are still places available at this season's LightBytes focusing on People, Space, Time and Place. The last three events will take place in Bristol (26 March), Glasgow (23 April) and London (4 June). Thank you to all those who have attended the series to date as well as our sponsors: Soraa, Thorlux Lighting, Xicato and Zumtobel. Details of the remaining events can be found on the SLL website.

The Jean Heap Bursary 2020 entry deadline is 27 March and the deadline for the Young Lighter 2020 entry is 15 May. We look forward to receiving your applications and again full details can be found on the SLL website.

Thank you to everyone who has paid their subscriptions for 2020. If anyone has any issues paying their subscriptions, or has questions regarding benefits of membership please do get in touch with us.

SLL Young Lighter 2019

Anna Wawrzyniak outlines her winning proposal for a Light Booster metro car

TUNNEL VISION?

Our traditional photopic-focused understanding of light is increasingly irrelevant and narrow, argues Stephen Cannon-Brookes. Is it time for a radical rethink?

DOES Vλ SPD MEASURE UP (2)?

Following the first pilot experiment on illumination metrology, David Loe continues the investigation by looking at quantifying colour rendering and colour appearance

Iain Carlile finds that plant cultivation is a key topic in the latest Lighting Research and Technology papers

Please note that the January/February cover image of the Elizabeth Line, lighting by GIA Equation, featured bespoke lightboxes by Designplan over the platform doors

© James Turrell, courtesy Kayne Griffin Corcoran

THIRD RECEPTOR PIONEER TO SPEAK AT LR&T SYMPOSIUM

Prof Russell Foster will be among the speakers at the Lighting Research and Technology Symposium in June, the theme of which is Applying Light for Human Health. Foster is professor of Circadian Neuroscience and the head of the Department of Ophthalmology at Oxford University. He is also a Nicholas Kurti senior fellow at Brasenose College. He was instrumental in the discovery of the third non-visual photoreceptor in the eyes of mammals, the intrinsically photosensitive retinal ganglion cells (ipRGC).

Also in the line-up of speakers are Mariana Figueiro, Mark Rea, both of the Lighting Research Center in New York, and Arnold Wilkins, emeritus professor of psychology at Essex University. Other speakers include John Mardaljevic, professor of Building Daylight Modelling at Loughborough University, and John O’Hagan, head of the Laser and Optical Radiation Dosimetry Group at Public Health England. Florence Lam, global lighting design leader for Arup, and Peter Thorns, head of strategic lighting applications at Thorn, will also be in the line-up.

The programme will include both presentations and panel discussions. Topics will include an overview of the relationship of lighting to human health and wellbeing, specific areas such as lighting for dementia sufferers, and an examination of the future of lighting for health.

The symposium will take place at University College London on 18 June. For more details, go to www.sll.org.uk

ON THE LIGHTER SIDE...

University of Minnesota researchers are determining how cuttlefish know how far away their prey is before launching their sudden attacks. It involves getting them to don 3D glasses and playing them 3D movies starring some appetising shrimp.

It was a tricky business, cuttlefish having eight arms to pull off the glasses and all, but the upshot is that cuttlefish, like humans, use stereopsis (computing distance by comparing how each eye sees objects in slightly different positions), but employ different neural circuitry to do so.

The creatures were surprisingly cooperative, though Velcro and glue were alarmingly mentioned. ‘You’ve got to get in the mind of the cuttlefish and make them happy,’ research leader Dr Trevor Wardill told The Guardian.

DARK SKY PARK OFFERS GRANTS TO IMPROVE LOCAL LIGHTING

Northumberland National Park, part of the Northumberland International Dark Sky Park, is offering grants to local residents and businesses to improve their lighting and cut light pollution. The project is funded by energy snack company Clif, and forms part of its first round of conservation grants to national parks in the UK.

Businesses and residents living in the park can apply for funding to help them adapt or replace any outside lighting that might be unshielded or too bright, and could be contributing to light pollution beyond their boundary. ‘It’s estimated that over £1bn is wasted in the UK through poorly installed, badly designed and inefficient outdoor lighting every year,’ said Duncan Wise, visitor development and marketing manager at NNP.

The Northumberland International Dark Skies Park has recently been credited with contributing more than £25m of dark skies tourism to the region.

• SLL members Kristina Allison and Katerina Konsta have both been awarded CEng

• Current SLL president Jim Shove has become the 100th elected fellow of the SLL

• Application is now open for the 2020 SLL Jean Heap Bursary. The latest progress report of 2019 recipient Karen van Creveld is now available online:

www.cibse.org/society-of-lightand-lighting-sll/education

TRAIN OF THOUGHT

Young Lighter 2019 Anna Wawrzyniak outlines her winning proposal for a Light Booster metro car

We are an indoor generation, in many cases spending up to 90 per cent of the day in artificially lit spaces. This results in low levels of exposure to natural light, even though the fact that significant exposure to daylight is crucial to entrain the human’s inner clock, or circadian rhythm, which regulates the sleep-wake cycle and most internal body functions. As we now know, a non-visual receptor (intrinsically photosensitive retinal ganglion cells, or ipRGC) found in the retinas of mammals records the phases of light and darkness, forwarding the information directly to the biological master clock (the suprachiasmatic nuclei, or SCN) rather than to the visual cortex.

Through experiments such as those conducted by the Max Planck Institute for Behavioural Physiology from the 1960s to 1980s, it has been established that without

exposure to daylight the circadian rhythm drifts from 24 hours to an average 24.18 hours, manifested by eleven minutes’ longer sleep every day. To avoid this steady shift humans need external entrainment to synchronise with the 24-hour rotation of the earth.

Employees who have no exposure to daylight in the morning or during the lunch break, and whose workplace may be equipped with a static lighting system as opposed to some form of ‘human-centric lighting’ (HCL), may only have the chance to experience ‘circadian lighting’ in the time they spend commuting.

The mega trend of urbanisation has led to the growth of cities. This expansion has in turn seen an increased duration in the time spent commuting, with an average 2060 minutes spent daily on public transport in European cities. By introducing HCL to public transport, particularly in the daylight-

Anna Wawrzyniak studied interior architecture at the University of Arts and Design Halle (2012-2017) and graduated with a master’s degree in architectural lighting design 2019 at the Royal Institute of Technology Stockholm. As well as winning the SLL’s Young Lighter 2019, she was awarded the Luxi Lighting Design Prize for Best Lighting Design Concept, newcomer category, for her idea of a Light Booster metro car. She is currently working at the architectural lighting design consultancy Peter Andres Lichtplanung, Hamburg

excluded area of underground metro carriages, this time could be used to provide the commuting workforce with ‘circadian lighting’ while on the way to the workplace.

Commute in public transportation –circadian light can be proposed Artificial office lighting –not sufficient to boost circadian rhythm

PROPOSED LIGHT-BOOSTING APPLICATION

The proposal is for a Light Booster metro car, which would involve installing special lighting in a single metro carriage, allowing passengers to experience a ‘circadian light boost’ with the aim of supporting their health and wellbeing. The lighting system would need to have the appropriate qualitative characteristics, including the right intensity, spectral distribution, directionality and timing.

The proposed lighting concept has been based on a particular underground metro car, type DT5 of Hamburger Hochbahn AG, the local transport company in Hamburg, Germany. This ensured realistic dimensions for the proposed lighting system. As only a single car is refurbished according to this proposal, the likelihood of acceptance, for practical and economic reasons, is increased.

To address the potential users’ needs precisely and to create an efficient solution for the comparatively short duration of light exposure, the study involved a literature review, user observation, and lighting measurements of an existing metro car and bright artificial sky dome. The time of light exposure, the duration of exposure, the directionality of the luminaire, the spectral composition and the intensity of light were all considered crucial elements if a positive result is to be achieved.

The average daily commute on public transport (bus, tram, S-Bahn, ferry included) in Hamburg lasts 29 minutes one way. The literature shows that even short exposure of artificial light can contribute to better health and wellbeing, and therefore this length of time would be sufficient for light treatment. It is proposed that the lighting design should be applied in the morning and afternoon

‘The Light Booster metro car could also offer a colourful artistic light experience at the weekends’

rush hours, between 6am-9am and 4pm7pm, in order to benefit the broadest range of people. The hours in between could be bridged with default lighting. As 87 per cent of passengers were seated during the observation of actual commuters, the conclusion was that the optimal focus for the lighting system was on vertical illuminance at eye height in a sitting position (1.3m).

Dealing with artificial light, energy consumption is also an important factor to consider, and it was important that the design should aim for creative and scientifically proven ways to use resources responsibly. The fact that intermittent light pulses have almost the same effect as continuous light exposure could therefore work to advantage in more than one respect.

The reason might be that ipRGCs are responsible for long-duration pupil size adaptation. The visual system is sensitive to changes rather than continuity in the visual field, and dynamic light levels trigger the ipRGCs more often and raise attention. Monochromatic light at low intensities (8-40 lux) affects the circadian system more effectively than white light. These factors could be integrated into the design both to save energy and make the concept more holistic.

So in summary, implementing HCL in a metro car aims for entrainment of circadian rhythm, a better subjective mood, an increase of sleep quality and a desensitisation to blueish light in the evening. This is achieved while also creating an efficient, aesthetic and sustainable solution.

Considering the results of the literature research, the following design principles were developed. The application time of the lighting would be divided into two scenes. For the morning session, the metro car would be illuminated with bright, white light. The white light would have slowly changing intensities to better stimulate the ipRGCells. The literature suggests that a good benchmark light level would be a vertical illuminance above 1000 lux.

 Measurements made during user observation

Position Direction of Glance

 Measurements taken for metro car and artificial sky dome

Metro Car Lighting

Artificial Sky Dome

Additional areas with vertical illuminance of around 40 lux in tones of blue (470nm) light would supplement the scene. The evening session would feature low levels of white light, illuminance below 100 lux, and red (630nm) light at a level of 40 lux. The introduction of the red light as an energising element, preparing for afterwork activities, is based on research by Figueiro et al: ‘...studies have shown that long-wavelength (red) light exposure increases objective and subjective measures of alertness at night, without suppressing nocturnal melatonin’ (Light at night and measures of alertness and performance:

implications for shift workers, 2015). Both scenes are provided by the same light system, mounted on the ceiling and vertical sides of the metro car to create a high vertical illuminance. Luminance distribution is balanced to allow pupil opening for maximum light input. Between the rush-hour periods, the carriage would be illuminated by default lighting, using only linear parts of the luminous surface. The lighting design shown here is suitable for normal weekdays. However, the Light Booster metro car could offer a colourful artistic light experience at the weekends, addressing a broad range of users and raising awareness of the impact of light.

CONCLUSION

In conclusion, the Light Booster metro car involves a creative and holistic approach, which aims to take HCL to the next level. The implementation of the proposal can lead to better health and wellbeing. This would arguably increase productivity among employees which can be translated directly into commercial benefit. But an investment in the concept is not only an investment in society, it promotes public transportation, which is beneficial for the environment.

The proposed design has an exemplary usage of space based on the number of passengers and frequency. Standard HCL applications in an office environment count one person per 15 square metres, whereas the lighting in a metro carriage supplies four people per square metre. Of course, the costs for energy consumption will be higher than for the lighting of an average metro car. However, in total the yearly energy consumption of one Light Booster metro car roughly equals the yearly consumption of a usual German single household at 2.256 kWh. This result is arguably moderate in relation to the benefits.

However, the strongest argument for the concept might be its recognisable and unique nature which could function as a tool to educate people about the crucial exposure to qualitative light. It might start a discussion among city dwellers as to whether urban lifestyles have shifted too far from nature – in particular exposure to daylight – in people’s daily routines, and at what cost. The design raises awareness of the role light plays in ensuring health and wellbeing.

When discussing ‘human-centric lighting’ in general one should proceed with caution, a proviso that has been expressed by a number of researchers and academic bodies: ‘(...), it is not yet possible to predict the nonimage-forming impact of a given illuminant based upon its intensity and spectral composition’ (Measuring and using light in the melanopsin age, Lucas et al, 2014).

Lighting design proposals must consider the context of a space, its users and their activities. More research and applied research findings in practice is needed to prove the theoretical results. Implementing the idea of a Light Booster metro car in practice, under scientific control (for example, a postoccupant evaluation) could develop new insights in the field of ‘circadian lighting’.

It has to be acknowledged that the concept

consumes resources which might not be necessary if people’s behaviour changed – 30 minutes of daylight in the first part of the day, whether the bike ride to work or a walk in the lunch break, would be equally if not more beneficial for health and wellbeing.

HCL concepts try to mimic the advantages of daylight. They are still predicated on artificial lighting, a symptom of an indoor generation, which lacks light, air and movement in nature. They are indicative of

an economy-centric, rather than humancentric, urban environment. While they are at least better than average artificial light, they do not address the root of the problem. Perhaps we should rethink how to build communities to enable an even larger fraction of the population to enjoy the benefits of micro commutes, making walking or cycling to work more feasible. Ultimately, greater exposure to natural light remains the best solution.

TUNNEL VISION?

Our traditional photopic-focused understanding of light is increasingly irrelevant and narrow, argues Stephen Cannon-Brookes. Is it time for a radical rethink?

As we struggle to accommodate the emerging goals generated by rising concern for human wellbeing, siege is about to be laid to many of the core tenets that underpin our current understanding of light. It is easy to underestimate the influence that has been exerted by the definition of visible light using

the photopic spectral sensitivity function known commonly as V λ . It has given us a tool to measure and deliver light, but it has also encouraged us to regard the visible spectrum of light independently of its close neighbours, ultraviolet and infrared radiation, encouraging the implied disconnection of vision from the wider sensory system. At the

risk of mixing metaphors, future generations will regard this period as one of tunnel vision. The discovery of the blue-light-stimulated melanopsin generators in the retina has rendered our classical photopic-focused understanding of light an increasingly creaking edifice. Addressing this, however, is a challenging prospect for the multiple generations brought up on an orthodoxy that also unpins our understanding of colour. However, in doing so we may be able to appreciate what has made it so difficult to understand a range of phenomena and cultural interpretations or beliefs that seem alien or inexplicable. Examples of the latter can be seen in relating colours to different energy centres or chakras in the body, and our still rudimentary engagement with a new, or perhaps rather old, awareness of the natural environment known as biophilia.

Lighting metrics

Our photopic world has also been conditioned by an evolutionary and technological addition, namely fire or, to the lighting world, incandescence. This has literally coloured our perspective of lighting and how we address artificial light sources in far-reaching ways, for example, the initial standard being a candle and hence our use of candela. While this is not surprising given that nearly all artificial light sources were incandescent until the arrival of discharge tubes, it has in practical terms bestowed a benchmark for lighting technology and raised a number of challenges – a good example being the apparent paradox that a red-rich light source may have the same quality of colour rendition as one with a continuous and evenly distributed spectrum. It takes almost an exercise of faith to believe that there can be a smooth transition in correlated colour temperature (CCT) of a light source on the black body locus from low to high values (in kelvin). Much effort has been devoted to papering over the dislocation. If we step back for a second, there is something innately nonsensical in trying to define the CCT of (cool white) sources against the temperature of a notional radiator that doesn’t exist. In the practical world we inhabit the answer has been to use a function based on the spectral distribution of sunlight or overcast skylight and thus render CCT measurements as purely mathematically derived values, helpfully overlooking the reality that no one has ever made a light source with this spectrum to act as a comparison. At the risk of making a plea, language really does count since the way metrics and units are named and/or defined has conditioned how we approach the subject.

A range of proposals has been offered to try to reconcile the increasing complexity of how we address light and lighting. These mostly focus on the assimilation of the sensitivity function derived for melanopsin production and range from use of direct measurement, a ‘new’ lumen, through to comparative measurements

against V λ , and a number equivalence measurements, some permitting the direct use of photopic illuminance. All are aimed at users answering the question, is there enough?

This bottom upwards approach has been a feature of artificial light/modern photometry, based on the dialogue between task performance and the economics of delivering light. Both are brakes on extending our perception of lighting, but they are starting to loose their stranglehold. Lighting designers are beginning to address the wider visual field, escaping the flat lands of horizontal task illuminance targets so beloved of professional bodies and governments, while the price of a lumen (definition at the choice of the reader) has dropped to a point where energy saving is no longer a primary issue, especially when maintenance is properly costed.

These warm thoughts should not distract from the prospect of the rug being pulled from underneath the whole edifice. We are being propelled towards an increasingly compromised situation where different metrics and units will serve to confuse the wider audience and quite possibly participants in the lighting world. Many are being adopted so swiftly that they have had marginal validation, mostly in clinical contexts, and yet are being adopted in building guidelines that promote targets as aspirational standards.

Is it time to redefine what we know? Clearly, we should encompass our understanding of our various responses to light (though this now sounds a little human-centric). The implications are now being addressed, but once again we are seeing an incremental process, largely in response to discoveries. The best-known example is U λ , proposed by Mark Rea, which encompasses all the currently known spectral sensitivity curves and as a consequence the majority of the visible spectrum (400-700nm). This looks tidy, but begs a bigger question, namely is it not time to make the leap and once again acknowledge that our (and most animals and plants)

‘We should acknowledge that in evolutionary terms light from the sun is our benchmark source’

evolutionary light source is daylight, either direct or diffused sunlight?

Time for a modest proposal, hopefully less monstrous than Swift’s?

First, we need to acknowledge how little we actually know. The discovery of iPRGCs in the retina is humbling because it suggests that we should expect to discover more spectral sensitive curves

and these are unlikely to be directly related to vision. Beyond this, we should expect that the way the body interrogates stimulus from these spectral sensitive responses is not compartmented. This would help to explain why sunlight exposure is (largely) beneficial: it stimulates vitamin D production and helps to reduce symptoms of depression, while healing a range of diseases. Let's remind ourselves that much of our understanding of the medical aspects of sunlight were shelved following the introduction of antibiotics.

Secondly, we should acknowledge that in evolutionary terms light from the sun is our benchmark source and, as Nick Baker sought to remind us, we are 'outdoor animals'. It is reasonable to expect that we make use of the whole spectrum of sunlight (radiation), even if our understanding of the processes involved is still fragmentary.

It is thus a simple step to propose that we reconsider our definition of light, effectively back to one before the adoption of the lumen. Does the word sunlight come as a surprise?

Stephen Cannon-Brookes is a past president of the SLL, and works both as a private lighting consultant and an associate professor leading UCL's module on lighting and wellbeing

DOES Vλ SPD MEASURE UP?

Following the first pilot experiment on illumination metrology, David Loe continues the investigation by looking at quantifying colour rendering and colour appearance

The second part of the pilot experiment (see p14 for summary of the first) considered whether a daylighttype spectral distribution would make a better basis for illumination metrology. For example, daylight is always seen as the best illumination for making critical colour assessments. With this in mind the following distribution is proposed (see Fig 1). The area proposed ranges from 400-700nm, described as the human spectral range and approximately following the daylight spectral distribution. It is a regular mathematical shape, a polygon; this means that any point within the shape can be described mathematically. This could be useful when comparing it with the spectral distribution of an electric light source. The test distribution SPD is labelled DV λ . (D for daylight and V for human vision). Also, it has been divided into six equal bands to aid mathematical ratings across the human spectral range.1

PILOT EXPERIMENT 2

Following the first experiment, there remained the question of how the above could be used to quantify colour rendering quality and colour appearance rating. It was decided to test the accuracy of human colour vision under three different light sources: natural daylight, warm white and cold white LEDs (Fig 2). The Farnsworth-Munsell colour test was used, involving 100 colour samples covering a wide range of shades, all of a similar reflectance value. The colour range was divided into four sets. Initially the individual samples in each set were mixed up, with the subject being asked to place the samples in colour order between the two fixed ends (Fig 4).

An experiment was carried out by 16 UCL MSc Light and Lighting students with the individual results for all four boxes combined, and for each of the three light sources. The individual results were combined into the average for the group and adjusted where necessary for any illuminance level differences. The averages for the group results for the three sources were compared and indicated similar results. This was surprising since the LED light sources had very different SPDs but both have erratic distributions suggesting that they may be similarly poor. This suggests that the Farnsworth-Munsell 100 hue test is inappropriate in this situation. A further

 Fig 1: suggested DVλ SPD as an approximate for daylight spectral distribution, together with the area covering the human visual colour range (400–700nm) divided into six equal band widths. It also shows how warm and cool lamps might be accommodated

thought is that if the observer is looking at, say, three adjacent samples to make their judgements over the normal reading distance, this equates to a conical zone of approximately 5 degrees, a zone covered by foveal vision, and an SPD V λ which the earlier experiment showed to be, in terms of colour vision, limited. This again suggests that the FarnsworthMunsell test is probably inappropriate.

However, by visually studying the SPDs of the two LED sources similar to those used in the experiment (Fig 2), it can be seen that for the cold white source the radiation is predominantly in the blue part of the visual spectrum and reducing rapidly into the red part of the spectrum. For the warm white lamp the reverse is the case, in other words, showing a peak around 620nm in the warm part of the visual SPD. This suggests that neither is very good considering their effectiveness for the whole visual colour range. This is similar to the early fluorescent lamps which also had poor colour performances. It was only when multi-phosphor coatings were introduced, with radiation covering more evenly across the whole visual range, that good colour performance was achieved.

Comparing them both with daylight,

using the suggested DV λ as the basis for illumination metrology, although not uniform over the visual range, there is only a difference between the two ends of the

visual range of around 30 per cent. This is probably the reason why daylight scores so highly in colour recognition and matching.

COLOUR RENDERING AND COLOUR APPEARANCE

It would seem that since daylight is recognised as the preferred light source for assessing colours and colour matches, this should form the basis by which to classify human colour assessment of light source illumination. Also, that it should be the DV λ SPD that is used as the standard on which to base a measurement system, at least until a more accurate understanding of the relationship between light and vision is found.

Regarding colour rendering, it would seem that a way forward would be to compare the spectral distribution of a light source in question with the distribution DV λ divided into a number of wavelength bands. Six has been suggested here, each of the same wavelength width and spread equally across the human spectral range 400-700nm (Fig 1). The experience of the Farnsworth-Munsell colour test pilot experiment, described above, indicates that a single number is unlikely to be the solution. However, a system that compares the closeness of the SPD of a light source to the DV λ distribution in terms of the percentage difference for each of the six

Summary of the thinking behind the study

The human visual system allows us to see fine detail, and gives us the ability to see all colours across the visual spectrum (400-700nm). It also allows us to see volume and texture through patterns of light and shade, and probably much more. Early humans had these abilities, but as the only illumination many thousands of years ago was from daylight, it would be reasonable to expect that human sight would have evolved under a daylight-type spectral distribution. Early in the development of electric lighting the need arose for a standard spectral distribution, to be agreed internationally, by which light measurements would be made. In 1924 the CIE Vλ distribution was agreed and remains the standard by which all photometric measurements are made.

However, because the measurements were made by observers using an optical instrument rather like a telescope, the field of view would have been limited, probably to the human visual region known as the fovea, where the retina has the highest density of sensors, enabling sight of very fine discrimination. However, the field only encompasses a cone of around 2-5 degrees in diameter, hence it does not represent all of human sight, which encompasses a cone of around a 40-degree diameter. Nonetheless it has been checked a number of times with similar results.

The Vλ distribution is a bell-shaped graph centred approximately on 555nm and encompasses the yellow-orange part of the human spectral range. This suggests that the ends of the human spectral range, the purple and red parts, are seriously reduced or discounted.

 Fig 2: spectral distributions of warm white LED and cool white LED light sources

 Fig 3: spectral distributions of flame and incandescent light sources

wavelength bands seems possible.

For example, if a light source’s radiation for each of the bands was within say 10 per cent (positive or negative) the lamp would be classified to be of very high CRI. Or if each of the differences were within, say, 30 per cent it would have a lesser colour rendering quality. Further work would be necessary to determine the actual percentage values and so on.

Regarding colour appearance identification, since daylight is not described as either warm or cool, the suggestion is to provide different test SPDs as shown by red and blue dashed

lines (Fig 1). This indicates that reddish light is increased or decreased for the two light appearances, but the whole colour range is still included.

SUBCONSCIOUS EFFECTS

A subconscious effect of light on humans has long been known as a process within the human brain caused by radiation of around 460nm. However, the actual effect and the process have not yet been properly explained, and whether it is present or not, except that it affects the body clock.

‘More studies will be needed both to confirm the requirement for a new SPD as a basis of illumination metrology, and to test the suggestion of DVλ’

The position is shown in Fig 1 by a vertical purple dashed line. This indicates that the radiation is prominent in daylight. Further, it has been suggested by a research team at Rensselaer Polytechnic Institute, Lighting Research Center, New York, led by MG Figueiro et al, that when this radiation is present human performance is enhanced. 2

This would mean that having daylight, or electric light with a similar SPD, in a working environment could provide performance benefits, though the work so far has not indicated the amount or the nature of the benefits, or whether there are any detrimental effects.

Counter to this is that if a light source does not have daylight-type SPD, but that of a flame source or incandescent lamp, which have similar SPDs where there is no, or very little radiation at 460nm, then they might create a more relaxed human condition. This has been suggested in care homes where red light sources have been used at night-time to help poor sleep quality in the elderly. 3 A further possibility is whether exposure to daylight has any effect on health or on the recovery from surgery. Some readers may remember the work of Roger S Ulrich, who compared the recovery rate of hospital patients who had a view out of their window of sky and trees with patients

Courtesy of NPL Courtesy of NPL

whose window looked on to a brick wall. The result showed benefits for the former.4

So far there has been little research into the mechanics of this process. It was reported in the 1960s that there was a gland within the human brain, the pineal gland, which acts as a switch suppressing the amount of melatonin in the brain and replacing it with a stimulant – or is it the reverse? If this is the case then it would account for the results reported above. But a further consideration is the route by which it gets to the brain. For example, is it possible that it is via the rod retinal sensors? These have an SPD with a peak of around 500nm, and appear to do little during daylight hours.

WHERE NEXT?

The above has been an attempt to indicate the possible or even likely shortcomings of the V λ SPD as a basis of illumination metrology. Ways in which it might be improved have also been suggested. But more studies will be needed both to confirm the requirement for a new SPD as a basis of illumination metrology, and to test the suggestion of DV λ . This will require further scientific research effort into the operation of human sight and its effects. We should remember that human sight can automatically adjust for extreme luminance variations to avoid glare in an attempt to improve sight, and can automatically adapt

Summary of pilot experiment 1

To test the validity of the Vλ distribution the study involved taking a number of M&S socks of different colours from across the visual range, and photographing them under daylight illumination. They were then photographed again with a filter over the lens, claimed to have a spectral transmission similar to Vλ. It shows a yellowish tone over the whole photograph. Predictably, it also shows colour samples in the low wavelength end of the human spectral range (purplish) and the high wavelength end of human spectral range (reddish) to be downgraded.

From the photographic images, luminance measurements were made of the different coloured socks from the two light sources (daylight and daylight + Vλ filter) and adjusted to correct for any differences in illuminance. The ratio between the two different images for each of the coloured socks ranged from 1.42-2.38, a difference that indicates that some coloured items are downgraded. It also means that using the Vλ distribution as the basis of illumination provides a distortion of the effectiveness of some parts of a light source’s radiation depending on the particular colours.

The results suggested that the Vλ spectral distribution as a basis of illumination metrology is at the very least questionable.

It was concluded that it would be useful for the experiment to be repeated with more exacting conditions. For example, it would be useful to know how accurate is the SPD of the Vλ filter. It would be helpful to use a better range of colour samples. Also, if a camera is used to measure the effects described, to test that the spectral distribution response is at least similar to that of daylight.

See SLL Newsletter March/April 2018 for full details of the first pilot study

to a colour wash which at least distorts colours. But these mechanisms have not yet been considered. All of this will require extra funding from both industry and governments. It will also require international collaboration through the CIE for anything to happen. In which case we may continue to make poor quality lamps and

inaccurate illumination measurements.

But there is no reason why the LED source manufacturers can’t explore ways of developing lamps with improved colour performances as suggested. In effect to produce ‘deluxe’ colour performance lamps to surpass the often poor performance experienced.

References:

1. Does Vλ SPD still measure up? Society of Light and Lighting Newsletter Vol.11, Issue 2, March/April 2018. https:// issuu.com/matrixprint/docs/ sll_mar_apr_18

2. MG Figueiro et al, Circadianeffectiveness light and its impact on alertness in office workers, LR&T, 2019; 51: 171-183.

3. Mercier K. Maximising health and sleep in the elderly. Lighting Design and Application. October 2012; 42-47.

4. Roger S Ulrich, View through a window may influence recovery from surgery, Science 27, April 1984.

 Fig 4: Farnsworth-Munsell 100 Hue Colour test showing the colour range with the four boxes

SEED OF AN IDEA

Iain Carlile finds that plant cultivation is among the key topics in the latest Lighting Research and Technology papers

Han et al’s paper examines the use of LED lighting in plant growth and development, specifically, wheat sprouts. Experiments were undertaken using varying photosynthetic photon flux densities with a continuous spectrum of light. The result of the experiments was that an optimum photosynthetic photon flux density was identified. This may enable a reduction of irradiance during early development of cultivated plants with no harm caused to the plant, while lowering the energy consumption required for cultivation.

Also considering the growth of plants, Degni et al’s paper looks at the impact of the light spectrum and photosynthetic photon flux density of LEDs on the germination and seedling emergence of okra. The light spectra used in the experiments had peaks in the red, blue or green regions of the visible spectrum.

The authors noted that there were significant interactions between light spectrum and photosynthetic photon flux density on the mean germination time, germination rate, uncertainty of germination, and seedling emergence height. The spectrum peaking in the red showed a better germination rate and less uncertainty than the spectra peaking in the blue or green parts of the visible spectrum. The varying photosynthetic photon flux also affected the seedling emergence height.

Investigating the influences of light on the circadian system, Truong et al developed a computational model of circadian stimulus, using photometric and colourimetric quantities, rather than spectral power distribution (SPD).

The computational model uses simple parameters of illuminance and the chromaticity coordinates of the white light source. Validations of the computational model were undertaken, demonstrating a good fit with Rea’s quantity CS 2018 results for circadian-effective light on humans, with acceptable errors for white light sources, in

the illuminance range of 10-10,000 lux. This therefore provides a relatively simple formula.

Bao et al conducted an experiment on 24 young adults with myopia in both eyes and equal refractive power. The subjects undertook a reading task of Chinese text under varying illuminance conditions (3, 30, 300 and 600 lux) and text contrast (95 per cent and 45 per cent). During the experiment measurements were taken of the subjects' reading distance, angle of head tilt, and reading speed.

The results showed that the subjects tended to shorten their reading distance and increase their head tilt angle when reading at low illuminance, and further, reading speed was notably slower at low illuminance and low contrast. The observed effects were found under both of the text contrast levels that were tested, but were more pronounced at the lower contrast. It was found that an illuminance of 300 lux or greater, coupled with high contrast text, facilitated good reading



Photomorphogenesisofwheatsprouts

posture and rapid reading of the Chinese text among all of the subjects.

Iain Carlile FSLL is the immediate past president of the SLL and a senior associate at dpa lighting consultants

Lighting Research and Technology: OnlineFirst

In advance of being published in the print version of Lighting Research and Technology (LR&T), all papers accepted for publishing are available online. SLL members can gain access to these papers via the SLL website (www.sll.org.uk)

Circadian stimulus –A computation model with photometric and colorimetric quantities

W Truong, V Trinh and TQ Khanh

Photomorphogenesis of wheat sprouts with LED irradiation of different intensities

T Han, T Astafurova, S Turanov, A Burenina, A Butenkova, E Surnina and D Valiev

Impact of light spectrum and photosynthetic photon flux density on the germination and seedling emergence of Okra

BF Degni, CT Haba, WG Dibi, YA Gbogbo and NU Niangoran

Effects of illuminance and contrast on the reading of Chinese text by myopes

J Bao, R Tan, J Huo, B Drobe and H Chen

Figure1. (a)Blockdiagramofthecontrolsystem;(b)pictureofthe‘‘Phytotron’’greenhouse;(c)specializedirradiation unit;(d)spectrumoftheLEDirradiationsystem

Events

2020

8-13 MARCH

Light and Building

Venue: Messe Frankfurt

https://light-building.messefrankfurt.com

17 MARCH

SLL and CIBSE Scotland region: Spring lecture: Lighting beyond the borders of standards

Speaker: Henrik Clausen

Venue: University of Strathclyde, Technology and Innovation Centre, Glasgow sll@cibse.org

21 MARCH

Junior Ready Steady Light (SLL event)

Venue: Rose Bruford College, Sidcup sll@cibse.org

24 MARCH

Ready Steady Light (3 hours CPD)

Venue: Rose Bruford College, Sidcup sll@cibse.org

26 MARCH

SLL LightBytes: light and wellness

Venue: The Engine Shed, Bristol sll@cibse.org

16-17 APRIL

CIBSE Technical Symposium: Engineering Buildings, Systems and Environments for Effective Operation

Venue: University of Strathclyde, Technology and Innovation Centre, Glasgow www.cibse.org/technicalsymposium-2020

22 APRIL

CIBSE West Midlands region: Lighting for Retail Premises

SLL LG17 CPD seminar

Venue: Birmingham Chamber of Commerce sll@cibse.org

8-13 MARCH: LIGHT AND BUILDING,

23 APRIL

SLL LightBytes: light and wellness

Venue: Royal College of Physicians, Glasgow sll@cibse.org

3-7 MAY

Lightfair International Trade Fair and Conference

Venue: Las Vegas Convention Center www.lightfair.com

16 MAY

International Day of Light

Unesco global initiative

https://spie.org/about-spie/ international-day-of-light

21 MAY

SLL AGM, Awards and Presidential Address

Venue: Connect Room, Ortus Building, Maudsley Hospital, London SE5 sll@cibse.org

4 JUNE

SLL and CIBSE North West region: Technical and publications discussion

Venue: Arup Manchester sll@cibse.org

LightBytes

The LightBytes Series is kindly sponsored by Soraa, Thorlux Lighting, Xicato and Zumtobel. For venues and booking details: www.sll.org.uk

LET Diploma: advanced qualification by distance learning. Details from www.lightingeducationtrust.org or email LET@cibse.org

CIBSE Training: various courses across the whole spectrum of lighting and at sites across the UK. Full details at cibse.org/training-events/cibse-training

FRANKFURT