Whilst attractions like award-winning Muncaster Castle in the England's Lake District overflow with visitors in the summer season, extending that popularity as the nights draw in is at the same time a challenge and a commercial necessity for operators.

Country houses like Muncaster have a great history of pioneering new technology. Peter Frost-Pennington, whose family have occupied the castle for 800 years, was thus maintaining a proud tradition when he turned to Marl for innovative LED lighting solutions that would not only make access safe and practical for visitors, but could become an attraction in themselves.

Major attraction

Muncaster currently attracts over 90,000 visitors a year, and the imagination and energy devoted to its development is reflected in the array of awards it has gleaned. In 2006 for example it gained the Large Visitor Attraction of the Year Award 2006, in the Cumbria for Excellence Awards, and in 2003 it won an Excellence in England award for the best visitor attraction.

One of the best ways of growing this number is to extend the season beyond the peak May – September months right out into the autumn. Usually Muncaster closes its gates at dusk, and visitors only see the splendid gardens during daylight hours. However as night time approaches the gardens take on a completely different persona, particularly as there is little residual light from anywhere else to disturb the sheer darkness of the rural Lake District.

Muncaster has much to offer the nocturnal visitor, not least reputedly England's finest array of ghosts, but there are some specific access issues associated with hosting of visitors after dark.

The most obvious is the 800m walk from the main gate to the castle, which would certainly need to be lit. Equally important is the need for emergency lighting, to maintain safety should the mains supply fail for any reason. The most significant challenge, however, is to give visitors a reason to come.

Metal Halide trial

In 2003, Peter Frost-Pennington started to experiment with floodlights around the gardens – placing 150W metal halide lights directed onto the trees and smaller 50W halogen lights as a fill-in along the walk from the car park. This "Darkest Muncaster" concept was established with the aid of grants from the Millennium Commission and Rural Regeneration Cumbria. The synchronised combination of lights, music, sound and special effects transforms Muncaster's famous gardens into an experience which tingles the senses and entertains the entire family.

Around the Castle itself, Peter created 2 separate lighting "shows" one using the front face of the Castle as a massive screen for computer projections and the other using programmed lighting units in the small valley or ghyll bordering 2 other sides of the Castle. For this ever changing "Ghyll Show" he used a combination of 500W Tungsten Halogen lights and the smaller 50W halogen lights since the standard metal halide lights take several seconds to reach full strength, and need a ten-minute cooling down period before restarting.

Although effective, these types of light are costly to operate. Lifetimes for both the Halogen and the metal halide lights are in the region of 1000 hours use, and their impact on the Castle energy bill was quite noticeable.

LEDs in Darkest Muncaster

As he developed and extended the Darkest Muncaster concept, Frost-Pennington identified Marl as a source of LED lighting, suitable for outdoor use that would match the light output of his existing units but using much lower energy – and with bulb life of 70,000 to 100,000 hours. Peter Frost-Pennington has added Marl LED lamps extensively into the Ghyll Show. Aztec 5W light fittings were installed in as many windows as possible on that side of the Castle, replacing existing 50W fittings. This gives a 90% reduction in energy use.

Marl RGB light engines are particularly effective in this kind of application, as they mix light colour at source – eliminating annoying effects such as triple shadows and fringes. They can fade elegantly from colour to colour – giving the impression that the building is ablaze for example, or providing a 'Mexican wave' effect. With over 16 million colours available for each light, the only constraint on the effects achievable is really the imagination of the designer.

Marl RGB colour change lighting solutions include built in DMX drivers, allowing direct addressing by control consoles. Working closely with lighting designers and Marl's technical staff, it is now possible to easily programme, change and improve shows. In the future, Frost-Pennington hopes to attract other artists and lighting designers to Muncaster to play with the system, creating bespoke effects and eventually co-ordinating these effects with music.

Protecting the environment

The environmental side of this operation is close to Peter Frost-Pennington's heart, living and working in one of England's most beautiful and isolated country areas.

"We're very aware of our surroundings," he explained. "Visitors are always surprised at how dark Muncaster is at night – and it was important to me that our effect lighting is contained to our premises, and doesn't cause unnecessary light pollution in the surrounding area."

Noise considerations ruled out fireworks. Muncaster is the Headquarters of the World Owl Trust – and hosts one of the finest collections of these birds. "Clearly we didn't want to stress our captive birds – let alone the wildlife of the area, of which there is plenty," explained Frost-Pennington. "Darkest Muncaster provides visitors with a breathtaking visual experience without the loud bangs of fireworks."

Finally, energy consumption is always an issue, both from a cost and a carbon emissions perspective. Frost-Pennington said "In one case, we were able to use a single 5W LED lamp to replace two 500W halogen units gelled with different colours. The energy savings achieved by moving to LED lighting are very significant."

Safety

Health and safety is always a top priority for Peter Frost-Pennington, and providing sufficient light to allow visitors to evacuate safely in the event of a power failure was an important issue.

"Normally, safety lighting requires a back-up generator to operate, but LED lights are so efficient that strings of them can be run from a normal UPS for the twenty minutes it takes for visitors to return to their cars," he explained. "This means that the safety lights come on instantly, avoiding the start up delay you would unavoidably get if we were using a generator."

The Future

LED lighting is advancing rapidly, with new and more powerful solutions emerging all of the time. As suitable solutions emerge, the 150W metal halide lamps lighting the path from the car park are obvious targets for replacement. LED lamps able to match the output of the 500W Tungsten Halogen lights are also on the cards. There are four projectors illuminating the front of the castle, and eight 250W up lighters covering the aspect facing the Ghyll. All of these could poten- tially be fulfilled with LED lamps.

England's country houses were amongst the first to embrace many technologies that are now commonplace in every home. Electric light is just one example. Refrigeration is another. Now again, Muncaster is leading the way with a new lighting technology.

For more information, please contact us.

by David Moorhouse, Sales and Marketing Director
Marl International Limited

For the last architectural LED contract we secured, the final meeting was in a room containing 12 different people from five different disciplines, each of which had an input into the lighting fixtures to be used. Architectural lighting has been a Marl success story – we’ve trebled our sales in this market in the last year, but achieving this success has involved us in building the right team including the right channel partners.

The decision making process on these sales is quite unique. For a large project, the client who is paying for the building will be advised by a lead architect or consultant. They may involve consulting architect(s) as well as specialist mechanical, electrical and civil consulting engineers, and a quantity surveyor or cost consultant. The occupier of the building, who may or may not also be the client, will involve their facilities management, health and safety and personnel departments. For retail premises, pubs and clubs, marketing will certainly want an input, as the lighting is an integral part of the customer experience. Having completed the design, the project will be put out to tender, usually to several potential contractors. The actual lighting may then be installed by a subcontractor who will have an account with an electrical wholesaler. That’s who you need to supply the product to.

Succeeding in this market is through working with people with the right specific industry knowledge. It takes a unique combination of skills, to be sufficiently aware of the drivers of these different disciplines to address all of their needs, and to understand LED technology well enough to represent it effectively. Marl has a team with the right commercial and technical background, and specifically, has partnerships with two distributors that come to this market with different strengths. Architectural FX, our exclusive distributor for the London region, has a long track record in the supply of lighting products to the commercial building industry. Foremost Components has a history in the supply of MMI components to the electronics industry, and has built a focussed team that can take Marl products to this market effectively.

Marl LEDs go into a host of applications: trains, planes, automobiles, defence hardware of various kinds, white goods, industrial electronics and consumer goods. Each of these markets has its own specialist know how and language, but in each one the sales process is essentially the same. The architectural market stands alone, and we owe our success in this domain to our knowledgeable team in house, partnering with Architectural FX and Foremost.

by Julian Cooper, Marketing Manager
Marl International Limited

Introduction

LED lamps are relatively simple to work with – requiring no ignition voltage to start, and generating no nasty spikes or surges. Observing some simple rules of thumb, however, will improve the efficiency of the lamp and prolong its life. Applications with exacting requirements in terms of light wavelength or other performance can also be readily addressed, providing some specific characteristics of LED operation are recognised by the designer.

Driving LED Light Sources

Light emitting diodes (LEDs) are semiconductors with light emitting junctions designed to use low-voltage, constant current DC power to produce light. LEDs have polarity and, therefore, current only flows in one direction. Driving LEDs is relatively simple and, unlike fluorescent or discharge lamps, they do not require an ignition voltage to start. Too little current and voltage will result in little or no light, and too much current and voltage can damage the light emitting junction of the LED diode.



A typical LED forward voltage vs. forward current profile is given in Figure 1. From this figure it can be seen that for a given temperature a small change in forward voltage produces a disproportionately large change in forward current. In addition, the forward voltage required to achieve a desired light output can vary with LED die size, LED die material, LED die lot variations, and temperature. As LEDs heat up, the forward voltage drops (Figure 2) and the current passing through the LED increases. The increased current generates additional heating of the junction. If nothing limits the current, the junction will fail due to the heat. This phenomenon is referred to as thermal runaway. By driving LED light sources with a regulated constant current power supply the light output variation and lifetime issues resulting from voltage variation and voltage changes can be eliminated. Therefore, constant current drivers are generally recommended for powering LED light sources.



For some applications current limiting devices, such as resistors, can be an inexpensive alternative to constant current drivers for restricting current flow. However, there are many trade-offs. First, resistors generate heat and, therefore, waste power. The heat generated by resistors needs to be dissipated. In addition, voltage changes from supply voltage variations will translate into changes in light output, and with resistors alone there is no protection for the LEDs to prevent damage from high voltage. A few applications, such as portable lighting, may tolerate these trade-offs but, for most applications resistors are not recommended.

Light Output

Light output of LED light sources increases with increasing drive current. However the efficiency, expressed in lumens per watt, is adversely affected. Figure 3 illustrates this relationship. LED lamps normally have a “Test” current listed on the product data sheets. This Test current is provided as a reference point for other technical information provided. Drive currents may be chosen at any current up to the maximum recommended current for the specific LED light source used. Driving LED light sources above the maximum recommended currents may result in lower lumen maintenance or, with excessive currents, catastrophic failure.

Temperature Effects

Performance characteristics of LED light sources are specified for a rated current and 25°C LED die junction temperatures. Since most LEDs operate well above 25°C, these values should be considered for reference only and the light output should be based on the anticipated operating temperatures.



The light output from LED light sources decrease with increasing LED die junction temperature. Higher LED die junction temperatures resulting from increased power dissipation or changes in ambient temperature can have a significant effect on light output. Red and Amber die manufactured from AlInGaP (Aluminium Indium Gallium Phosphorus) are more sensitive to temperature effects than Blue and Green InGaN (Indium Gallium Nitride). Therefore, it is important to consider the effects of temperature when designing for specific light output or efficacy levels, and to maximize the thermal management of the system. Figure 4 shows the changes in light output for LED die versus LED die junction temperature.

 In addition to affecting light output, temperature also has an effect on the dominant and peak wavelength. LED die wavelength characteristics are commonly reported at 25°C junction temperatures. With increasing LED die junction temperatures resulting from higher drive currents or ambient conditions, wavelengths typically increase in from 0.03 to 0.13nm/°C, depending on die type. Temperature variation can also cause slight shifts in colour temperature for LED white light sources. Applications requiring specific wavelengths or colour temperature should take this effect into account when designing drive conditions and heat sinking.

Electrical Design

Driving single LED light sources in non-dimming applications is relatively simple. A constant current driver is chosen to deliver the desired current, with enough forward voltage output to accommodate the maximum input voltage of the LED source. LED light sources are not designed to be driven with a reverse voltage. Driving multiple LED light sources with one driver is generally done with the LEDs arranged in series strings to avoid uneven light levels resulting from voltage variations. When selecting a series string driver, the output voltage should be high enough to accommodate the sum of the maximum input voltages of LED light sources.

Dimming

Dimming LEDs is most commonly done either by lowering the current, or through a technique called Pulsed Width Modulation (PWM). LEDs have a very quick response time (~20nS), and instantaneously reach full light output. Therefore, many of the undesirable effects resulting from varying current levels, such as wavelength shift or forward voltage changes, can be minimized by driving the light engine at its rated current and rapidly switching that current on and off. This is known as Pulse Width Modulation. Pulse Width Modulation is the best way to achieve stable results for applications which require dimming to less than 40% of rated current. By keeping the current at the rated level and varying the ratio of the pulse “on” time versus the time from pulse to pulse (commonly referred to as the duty cycle), the brightness can be lowered. The human eye can not detect individual light pulses at a rate greater than 200 cycles per second and averages the light intensity thereby perceiving a lower level of light.

Thermal Design

With increasing power there is increased thermal load and more heat to dissipate. Higher temperatures of the LED light sources can result in reduced lumen maintenance and shorten useful life. When designing a new system a heat sink should be selected with sufficient cooling capacity to keep the die junction below 125ºC. If designing around an existing heat sink the maximum operating current for a given heat sink design is the lower of: 1. The maximum rated current for the LED light source, or 2. The current to maintain the LED die junction temperature below the maximum specified temperature. LEDs generally must be operated at or below a junction temperature of 125°C.

The junction temperature (Tj) can be calculated from the array power dissipation (Pd), the array thermal resistance (Ra), the thermal resistance of the material used to attach the array to the heat sink (Rb), the ambient temperature (Ta), and the thermal resistance of the heat sink (Rh). The following formula gives the array junction temperature:

Tj = Ta + Pd (Rh + Rb + Ra)

Conclusion

LEDs offer an energy-efficient, low maintenance form of indication or illumination. The foregoing discussion demonstrates that their characteristics also make them very easy to integrate successfully into a circuit design. Providing some simple design rules are observed, these components will provide long term, reliable service.

Protection is key to making the most of an LED's lifetime potential

by Nathan Orton, Design and Quality Manager
Marl International Limited

Service lifetimes of 70,000 – 100,000 hours, and power savings of up to 85% are leading to growing adoption of LEDs in place of other forms of indication and illumination. Incandescent bulbs, halogen lamps and even fluorescent tubes are being replaced in control panels and ambient lighting applications alike.

Since the rated life of the LED can be longer than the service life of the system in which it sits, the component is potentially maintenance free. Not only does this eliminate the direct cost of having a fitter replace the component at intervals, but it also reduces or eliminates the cost of a supporting stock of spares, as well as the need for preventative maintenance inspections.

However, the surrounding environment contains many hazards that can bring about the premature demise of an LED component. Given that an LED has twenty times the operational life of an incandescent bulb, it is also twenty times more likely to encounter abuse or accident. Environmental factors – shock, vibration, moisture ingress - can be to blame. An electrical malfunction or error during installation can expose it to a spike or a reverse current. A silent ‘killer’ is heat. Junction over-temperature can slowly but surely eat away at the life of the component – and impair its efficiency in the process.

A well designed LED fixture provides appropriate protection against such issues, and makes the difference between a component that is essentially maintenance-free, and one that still requires inspection and replacement at intervals.

The environment

In the industrial or transportation environment, specifying an IP67 sealed component that is fully potted for maximum resistance to vibration and shock is a no-brainer. In truth, there is no application where there is zero risk of the component being bathed in coffee or coke, or falling to the floor from an overburdened pair of arms. Marl, for this reason, fully pots almost every LED lamp providing outstanding protection against fluid ingress, shock and vibration. The cost overhead of doing so really isn’t that great, and is infinitely smaller than the cost to the customer than replacing a damaged device in the field.

Marl pots all its LED components to provide protection against damage by fluids, shock and vibration.

To ensure the fixture survives should the equipment be dropped, or worse physically attacked, the front needs to be toughened as well. Precautions that can be taken include the use of shatterproof glass and polycarbonate lenses and metal housings. Where there is serious risk of vandalism, tamperproof fixings can be provided.

Electrical malfunction

In electrical environments that are ‘noisy’ no engineer would dream of fitting an LED without providing appropriate over-voltage and reverse voltage protection. But in the end over-voltages and reversed polarities can come the way of an LED in any circuit. One can never entirely discount the risk of the power supply being connected backwards or adjusted incorrectly.

Fitting a diode or bridge rectifier is straightforward, and need not occupy much space. These simple precautions give reverse voltage protection or allow AC operation, making life easier for the designer, the production engineer and the service manager alike.

This bulb replacement LED incorporates a bridge rectifier to simplify electrical connection – but still fits into the same physical form factor as the bulb it replaces.

Similarly, some LEDs will survive higher levels of overvoltage than others. A 24V LED should not be exposed to more than 24V, but a quality device may survive exposure to as much as 32V. A lesser component will blow at 28V or less. If this sounds a fine distinction, the issue should be considered in the context of our expectations of the component. For an incandescent bulb with a service life of 5,000 hours, the expectation is that it will require frequent replacement. For an LED with a rated life of 100,000 hours, the expectation is that it will not, and it is much more noticeable when it does.

Thermal factors

LEDs are moving from indication to illumination – from the control panel to building and vehicle lighting applications. Although an LED generates minimal heat by comparison with other lighting technologies, it does still generate some. At 1W or above this thermal output can be enough to create issues, especially during prolonged use. Management of the heat output is critical to the longevity for LED components.

The key factor here is the junction temperature of the diode. Keeping the junction cool improves not only the service life but also the efficiency of the LED. The Lamina light engine used by Marl can be used at a junction temperature of 125°C – but is much more efficient if its junction temperature is kept to 55-60°C.

Marl uses a simulation driven design process in order to achieve a thermal design for its products that balances good heat removal with maintaining the minimum size and weight of heat sink. Thermal analysis using SolidWorks COSMOS 2007 software gives the temperature distribution, temperature gradient and heat flowing in the product – as well as the heat exchanged between the product and its environment. The movement of heat through the component and heat sink into the environment can be pictured by plotting heat flux vectors. The investment in software is amply repaid by the results it achieves. All prototypes are subjected to tests in a temperature chamber, and the measured temperatures are normally within 3-4° of the simulation result. Using this process, Marl has never had to redesign a product following the building of the prototype.

A heat distribution image from Marl’s thermal modelling software.

Fit and forget

LED panel indicators and lighting are revolutionising the industry by offering fit and forget illumination that can be all but eliminated from the maintenance schedule. A well designed fixture around the diode itself is needed to allow the LED component to achieve its full potential, in the face of the environmental, electrical and thermal issues it is likely to encounter during its operational life.