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High Efficiency and Rendered Colour Using LED Solid State Lighting

Final Report Summary - HERCULES (High Efficiency and Rendered Colour Using LED Solid State Lighting)

Executive Summary:
LEDs have seen rapid uptake across many industry sectors, as they offer improved energy efficiency and rapid start-up at reasonable cost. However efficient LEDs often produce a blue-rich light output, which in the medical industry, can make it difficult to see differences in skin, tissue and organs. In this safety critical application, there is a need for improved colour rendering of surgical lights. Patient environment is also beginning to be recognised as a key factor in patient recovery, particularly the availability of natural lighting.

The HERCULES project has sought to address these challenges, by producing an improved LED-based illumination system for an extended range of applications. Within a medical luminaire, the key elements which impact light efficiency are LED selection, the efficiency of the electronic driver, and the thermal design. The optical design dictates the brightness and colour mixing of the final beam. From a customer perspective, the luminaire must also be compact, easy to use, and easy to clean. These key elements formed the basis of the project objectives, which are summarised as:
• Develop improved colour rendition and uniformity through LED selection and mixing, and optical design
• Improve energy efficiency through electronic driver design, thermal management and materials selection.
• Design a robust and disinfectable luminaire casing consistent with the lighting and thermal demands

The HERCULES technology is based on combining the light from different LED sources to produce near-perfect colour rendition and variable colour temperatures, with high energy efficiency. LEDs were selected by developing a model. An electronic driver was designed which operated at 95% efficiency. Optical models were developed to simulate reflector shapes, and choose the best colour mixing solution, giving optimal colour rendition. A thermal model was designed and used to simulate the examination lamp, and select the most appropriate materials. Finally two prototypes were designed to demonstrate the technology, a bed light and examination lamp.
The bed light can be used for room illumination, reading and for examination. The light has been designed with extremely high colour rendition across the full visible spectrum, with controllable colour temperature that matches the circadian rhythm cycle of natural daylight. It can be programmed either from a dedicated programming unit, or from a smartphone.
The examination light can switch between five fixed light settings, ranging from blue to red. On the red light setting, tissue is resolved more accurately, allowing the surgeon to operate in comfort and with confidence. The cold light setting is useful for general inspection, and is particularly good for showing veins. The unit has a sterile handle, which can be removed for sterilisation, and a grab handle for initial positioning. A control panel is fixed to the front of the unit for colour change, beam intensity and spot size change. The skin of the examination light has been designed without crevices, and so is easy to clean.
The HERCULES project has successfully developed technology which has met targets for colour rendition, energy efficiency and luminaire design, and been demonstrated through the production of two prototypes. Results have been disseminated extensively using conferences, trade shows, newletters and articles. The SMEs have been helped to develop a realistic exploitation plan for the technology, and a clear route to market is apparent.

Project Context and Objectives:
HD LED refers to a patented technology developed by Brandon Medical for use in surgical lighting systems. The technology is specifically aimed at the efficient generation of high colour rendered white light using state of the art solid state light emitting diodes. The technique enables enhanced colour rendered light to be produced by mixing different white and coloured (usually red) LEDs with the addition of a filter element. Optimisation of the white/red/filter mix enables the generation of high colour rendered light to be achieved over a range of different colour temperatures. The technique is the subject of a granted US patent (US patent no. 8201966, issued on 19th June 2012).
The technology has been successfully integrated into number of medical products including Brandon Medical’s Astralite, Galaxy Ultra, Astramax and Quaser products. In addition the technology is also used in the Derrungs examination light HD LED products. Outside of medical applications, HD LED technology has also been used for refrigeration retail display through an agreement with Nualight Ltd.

The aim of HERCULES was to advance the HD LED technology further both in the medical lighting market area but also into other general lighting products. To achieve this, the project focused on a number of key technology areas.
1. HD LED light source selection and optimisation
HD LED relies on the careful selection and colour mixing of the core LED devices. Enhanced performance can also be obtained by using specific colour shift filters.
To date, many of the LED configurations which have been developed using the HD LED technique have been optimised by experimental techniques. Within the HERCULES project, a combination of theoretical analysis, mathematical modelling and testing was used to develop a comprehensive understanding of the capabilities of the HD LED technique.

2. Optical design
The light distribution which is generated by the source LEDs in an LED luminaire does not normally match the light distribution which the luminaire is required to produce. As a result, optical elements are required to achieve the correct transformation from source to field distribution. Generally the optics used in broad beam luminaires are relatively straightforward to design and source – several manufacturers (e.g. Carclo) offer standard off the shelf components. However narrow beam luminaires – i.e. task lighting – generally requires more careful design. Ensuring high efficiency delivery of the light from the luminaire to the lighting field with a well-controlled, focused beam usually requires custom optical design specific to the selected LED source and luminaire geometry. Current HD LED implementations employ custom elliptical reflectors and aspheric lenses to achieve this. In HERCULES, the optical design approaches for HD LED technology were explored and developed, to establish techniques for ensuring good, uniform colour mixing from arrays of LED devices.

3. Electronic design
A critical part of any LED luminaire is the electronics which are used to drive the LEDs. Numerous approaches have been developed for LEDs and in early implementations of the HD LED technology constant current drive configurations have been used. Whilst this approach is able to deliver reasonable efficiency (typically ~80%) there is scope for further efficiency improvements using alternative strategies. In addition, simple LED drive circuits generally tend to have poor power factors. For low wattage lighting there is no current requirement for achieving high power factor, however achieving a high power factor, whilst maintaining efficiency in a cost effective manner would represent a considerable technology improvement over existing solutions. In HERCULES, alternative electronic drive strategies for HD LED technology were examined, with the potential to improve on efficiency, power factor and cost of implementation.

4. Thermal management
Unlike conventional incandescent light sources, the light generated by LEDs, increases with decreasing junction temperature. Visible LEDs do not emit infra-red light. As a result, the heat generated by the device due to inefficiencies in the conversion of electrical power into optical power needs to be conducted away from the dice into an appropriate heat sink, to avoid heating of the device and reduction in light output.
Much of the improvement in high brightness LEDs over the last decade, which has enabled LED technology (including HD LED) to be able to provide sufficient light levels for specialist and general lighting applications, has been around improving the thermal management within the LED devices and ensuring maintenance of high luminous output at elevated junction temperatures. As LED technology moves from specialist lighting applications to more general lighting, there is an increasing need for more effective, low cost, thermal management solutions.
Improving the thermal management of an LED light engine enables less LEDs to be used in the fixture (reducing cost – less LEDs). Alternatively, with improved thermal management more light can be generated with the same number of LEDs (again reducing cost).
Within project HERCULES, the approach and technologies used for LED thermal management were analysed and further developed, within the context of the HD LED technology. Specific areas which were examined were the use of lower cost LED PCB substrates (e.g. FR4 vs. MCPCB), more effective thermal design of the LED enclosures, more effective heat sinking approaches and optimising the heat sink materials and shapes.

5. Integration
In order for an LED light engine to function optimally, it is important that the key functional components are integrated effectively. Most of the functional elements (electronics, optics, LEDs, thermal management, enclosure) are interdependent and the optimal functioning of any one component will be dependent on the behaviour and function of the others. Also, from a cost perspective, significant cost savings can be achieved by integrating the functionality of the various components within an LED luminaire – for example the optics can form part of the enclosure, the heatsink can be integrated (partly) into the optics and enclosure etc.
Within the HERCULES project, a work package was dedicated to integration. Specific light engine integration solutions were developed applicable to HD LED with a focus on both effectiveness and cost.

Summary of project objectives
These key elements formed the basis of the project objectives, which are summarised as:
• Develop improved colour rendition and uniformity through LED selection and mixing, and optical design
• Improve energy efficiency through electronic driver design, thermal management and materials selection.
• Design a robust and disinfectable luminaire casing consistent with the lighting and thermal demands

Project Results:
Foreground intellectual property has been developed in the form of four project results: LED Subassembly, Light Engine, Optical System, and Luminaire.

1. HERCULES high CRI, efficient LED subassembly
HD LED relies on the careful selection and colour mixing of the core LED devices. Enhanced performance can also be obtained by using specific colour shift filters.
To date, many of the LED configurations which have been developed using the HD LED technique have been optimised by experimental techniques. Within the HERCULES project, the aim was to develop a more comprehensive understanding of the capabilities of the HD LED technique, through a combination of theoretical analysis, mathematical modelling and testing.
A market review was carried out to investigate the available white and colour LEDs. A provisional selection was made based on previous experience of mixing results.
A mathematical model was created in MatLab to help refine the selection of LEDs and filter technology. Modelling results were validated against experimental data for some of the LEDs modelled, and good correlation has been seen. The model enabled the HERCULES team to better understand the relationship between mixing coloured and white LEDs in combination with filters. Using the model, techniques were investigated for improving the overall performance of the HD LED technique. The model allowed LEDs to be evaluated quickly and easily after first measuring the spectral characteristics to assess suitability for HD LED applications. The model was extended to enable further (multi-colour) mixing approaches to be evaluated.
The model was used to interpolate between LEDs in the chromaticity diagram and to understand and identify the correct LED bins to be selected for optimal performance. Using details about the LED sources, their spectra were mixed by the model and a Correlated Colour Temperature was generated.
A final selection of LEDs was made for the Bed Light and the Examination light. For the HERCULES bed light prototype a Source Mixing optical design was used, with a combination of cold white, warm white and red LEDs. For the examination light, a field mixing solution was used, with cold white and red LEDs.

2. HERCULES tunable CRI light engine
The electronic driver currently used in the Brandon Medical products was reviewed, and recommendations made to improve the design. Electronic drive strategies were investigated with the HD LED technology to improve on efficiency, power factor and cost of implementation.
The HERCULES project investigated the use of three resonant mode techniques for LED drive.
• Single Switch Resonant Buck
• Resonant Soft Switching Flyback Converter
• LLC Resonant Converter
The modelling and testing enabled recommendations which resulted in a reduction in size, a decrease in cost, and an increase in efficiency.
The LLC Resonant convertor was built and tested. While measured efficiencies did not meet modelled values, opportunities for improvement were identified and a patent was drafted to protect the technology.

2. HERCULES optical system for LED light collection and homogenisation
The light distribution which is generated by the source LEDs in an LED luminaire does not normally match the light distribution which the luminaire is required to produce. As a result, optical elements are required to achieve the correct transformation from source to field distribution. Generally the optics used in broad beam luminaires are relatively straightforward to design and source – several manufacturers offer standard off the shelf components. However narrow beam luminaires – i.e. task lighting – generally requires more careful design. Ensuring high efficiency delivery of the light from the luminaire to the lighting field with a well-controlled, focused beam usually requires custom optical design specific to the selected LED source and luminaire geometry. Current HD LED implementations employ custom elliptical reflectors and aspheric lenses to achieve this.
In addition to producing the correct light field distribution, it is important to ensure good, uniform illumination and to avoid beam artefacts – including colour shadowing and beam break-out. Source mixing and field mixing were explored as potential solutions to achieve this.
Field mixing involves taking a number of individual source and optical beam forming elements – each of which produces its own distinct beam – and overlapping the beams at the field position. This technique has the advantage that the optical design can take advantage of the relatively small size of the high brightness source LEDs which enables compact, high efficiency optical elements to be used. The main disadvantage is that it is difficult to ensure good spatial overlap over a range of field distances which leads to beam and colour separation.
Source mixing involves combining the light from several LEDs to produce a secondary mixed source of light which can then subsequently be manipulated by a secondary optic. The advantage of this approach is that there is no apparent source or colour separation visible in the beam to the user. The disadvantage is that this sort of design tends to be less efficient (it includes multiple optical elements often involving diffusers) and also the effective source size is always going to be larger than the individual light sources. In addition, clustering LEDs together into arrays introduces additional complexity in manufacture and also requires effective thermal management.
A range of optical concepts suitable for next generation HD LED light engine implementations were generated and analysed. Theoretical analysis was performed using Zemax and indicated a number of promising approaches for both field and source mixing. Source mixing was the most challenging, although the reflective mixing chamber and chip array solutions which were subject to initial analysis and optimisation looked promising.
Studies into field mixing optics showed that by adding higher order surface profile terms to an elliptical reflector profile, optimised beam profiles could be developed. These profiles exhibited a high D50/D10 ratio, could be relatively compact and had high optical efficiency. Prototypes of these designs were developed, fabricated and tested. The test results confirmed that these designs performed according to the Zemax model and gave confidence for integration into the HERCULES demonstrator models.
The principles generated in this research were then applied to design the optics for the two demonstrators, the Bed Light and the Examination Light.
The HERCULES bed light prototype incorporated a Source Mixing optical design which enabled highly efficient, high colour rendition light to be produced using the core HD LED technology. This was achieved by using a combination of cold white, warm white and red LEDs
The Examination Light retained a Field Mixing design, but used the models developed to improve the reflector design to that shown in Figure. The reflector cones were supported on a reflector plate and angled to give a uniform distribution of light.
Testing revealed that the optical performance targets were met by both the bed light and the examination light.

3. HERCULES intelligent, tunable CRI luminaire

Examination Light
A spreadsheet based thermal design tool, HITCAL, was created to model thermal transfer from the LED board through the examination light, to assist with thermal design. Using the model to investigate various scenarios, recommendations were made on the PCB design, assembly of the light engine, materials for heat dissipation and the geometry of the luminaire casing.
The HITCAL spreadsheet was used to generate thermal design constraints for the examination light demonstrator.. A finite element model was built to validate some of the assumptions in HITCAL. When the prototype was built, experimental measurements were made and correlated with the HITCAL model. Experimental results were close to the temperatures predicted in the HITCAL model, and the spreadsheet was distributed to the SMEs for their use.
The casing was designed to provide a functional and attractive luminaire. On the top side a control panel can be used to adjust colour, beam intensity and spot size. On the underside the light is projected through a transparent window. A grab panel is incorporated into the lower clam shell, and the yoke arms can be used as an additional handle if required. A sterile handle (with a removable outer cover) is provided for use in sterile operating conditions. Smooth edges were used throughout, to prevent trapping of dirt and provide a surface which was easy to clean.
Materials for manufacture were also investigated for the examination light. Four polymers, Pocan S7020, Sabic Cycoloy C2950, Valox 357U, and Lexan 943A, were assessed for resistance to cleaning agents, Alcohol gel, Virusolve, Chlorclean and Tec Care.
Filter materials for manufacture were also investigated. The prototypes produced in HERCULES had very good colour performance and met the current project requirements without using additional colour filters. However future developments, for example using even brighter new LEDs, may require high performance filters for optimal efficiency. Although commercial filters are available they are either too unstable and short-lived, or expensive coatings, or solid glass elements, and not suitable for low cost or ease of application within a luminaire. During the HERCULES project, an optical filter material for a polycarbonate lens was identified and sourced. One liquid based route for low cost and ease of application was investigated and gave good adhesion and durability characteristics but not the requisite optical properties. The second route showed good promise from an optical perspective and was available from a commercial PVD route coating company.

Bed Light
The bed light was developed to be modular so that different configurations of the lamp can be developed in future. The lamp was also designed to be simple to assemble with a minimum of parts and assembly operations. Several of the parts were designed from standard extrusions with simple plastic mouldings and stamped metal parts making up most of the other components. The bed light was demonstrated with transparent side walls, and with side walls removed. This feature – which is a unique aspect of the HD LED technology – enables a luminaire to be designed suitable for bed head use which has a clear air flow through the body of the luminaire. Not only does this help improve the luminaire’s thermal performance, it also prevents the accumulation of dirt and dust on the luminaire surfaces which is important in healthcare environments.

The Bed light and the Examination light were tested and found to meet the targets set out at the beginning of the project.

Potential Impact:
4.1 Socio-economic Impact and the wider societal implications of the project so far
LEDs are low cost, and increased adoption of this technology will have a knock-on effect in cost reductions across the consumer and professional industry.

The improvements made in the quality of the lighting available for patients may positively influence patient recovery. Positive effects made on the time and quality of the patient recovery may enable improvements in the cost effective use of beds in the hospital environment. Studies have shown (Ulrich, 1984) that patients who stay in rooms with windows have shorter post-operative stays and give fewer negative comments regarding patient care. It may not be possible to ensure all patients are able to have window views, but the addition of ‘natural’ lighting may make a valuable contribution to patient wellbeing.

Improvements in the quality of colour rendition and tunable systems relating to the medical examination light may reduce eye strain and improve surgical procedures and non-surgical examinations enabling faster diagnosis and improved accuracy, enabling better care in the medical sector.

The improvements to the thermal management system will enable reductions in size of unit, enabling space in the operating theatre to be freed up for other assistive technology.

LEDs offer a low voltage solution to light production. They can be connected directly to low-voltage systems consisting of photovoltaic cells and batteries. In this way good quality, affordable lighting will become available to people in poor socio-economic groups.

Reductions in the size of units and improvements in efficiency of lighting may reduce the environmental impacts of freight and energy usage.

4.2 Dissemination Activities

The results of the project were disseminated to relevant audiences, keeping in mind the Intellectual Property to be protected. Project dissemination activities were managed by the Dissemination Manager, Andy Morland of DTM Cumsa, with the support of TWI. A project website was created and is available online at www.hercules-project.eu.

Dissemination in the project took place through the project website, relevant conferences, magazine articles, posters, networking and scientific journals. A project video was also produced. A full list of dissemination activities is given in Table A2. After the project finishes, several further activities are planned, to ensure wide dissemination of the project results. A brochure has already been developed by Brandon Medical to promote the examination light.

4.3 Exploitation of results

Exploitable Results

The exploitable results from the project were:

• HERCULES high CRI, efficient LED sub assembly.
• HERCULES tunable CRI light engine.
• HERCULES optical system for LED light collection and homogenisation.
• HERCULES intelligent, tunable CRI luminaire.

Although these could be separated and licenced individually, the SMEs in the project intend to create products which will incorporate some or all of these exploitable results. These products are:

• Examination Light – Brandon Medical
• Bed Light – Brandon Medical
• Light Engine – Brandon Medical
• Design and Manufacture solution for Luminaires – DTM Cumsa
• Advanced Integrated Lighting Solutions for the Medical Industry – Synergy Medical

The bed light and examination light were built as demonstrators in the project. The bed light is shown in Figure 1 and the examination light is shown in Figure 2. These will be used as the basis for a new range of Brandon Medical products, which will be sold by Synergy Medical as part of their Advanced Integrated Lighting Solution. The light engines will be sold separately to luminaire manufacturers. DTM Cumsa will use the design knowledge gained in the project to stimulate growth of their Design and Manufacture solution.
Unique Selling Points
High performance lighting in a medical environment is essential for diagnosis, surgery and for the general well-being of patients. Current LED products suffer from:

• Blue-rich colour output (R9<50% and colour rendition index 70 – 85%).
• Low energy efficiency for the electronic driver (80 – 85%).
• Variable thermal performance.
• Bulky design.
The HERCULES products offer:

• Excellent colour rendition good uniformity of illumination.
• Low energy usage
• A compact form factor with materials which can be easily cleaned.
• Ergonomic and elegant design

The HERCULES examination light offers a CRI of 96, an R9 value of 97, electrical drive efficiency of 95%, a D50/D10 of 0.65 and good colour uniformity and thermal performance. The HERCULES bed light offers a CRI of 97, an R9 value of 98, electrical drive efficiency of 95%, and excellent colour uniformity and thermal performance. The combination of these specifications puts the HERCULES products at the top of technical performance, and provides a unique selling point for the SMEs in this project.

The high Colour Reflective Index (CRI) light and colour adjustment of the Light Engine are useful for both medical and general lighting applications. While some tunable light engines are available on the market today, they typically have very high price points. Materials and components have been selected which are suitable for high volume production, which will enable the HERCULES product to compete on price as well as technology.

Business Plan

The demonstrators in the HERCULES project will be refined to make them suitable for production, and then will be sold through as two new product lines for Brandon Medical. The light engine will also be sold as a standalone unit. Through these new products, Brandon Medical will have an increase in turnover of 25%. The technology developed in the HERCULES project will allow DTM Cumsa to strengthen its offering to the automotive and domestic lighting sector, and should result in a 50% increase in turnover. Synergy Medical hope to triple their market share in Northern Ireland through the HERCULES technology.

List of Websites:

http://www.hercules-project.eu

Contact Graeme Hall, Brandon Medical Co Ltd, Elmfield Road, Morley, Leeds, LS27 0EL,UK Tel: +44 (0)113 277 7393