LEDs in Real Lighting Applications: from Niche Markets to General Lighting

Matthias Wendt - Philips Research Aachen, Germany

Jan-Willem Andriesse - Philips Lighting Eindhoven, The Netherlands

Abstract—Due to the enhancements in the field of Solid State Lighting, being in price per lumens or lumens per watt applications for LEDs in lighting products migrate from niche to general lighting. A number of shortcomings today are discussed as well as concepts for improvements.

LED lamps; LED illumination; general lighting; multi-LED light sources

I. THE OUTLOOK OF LED LIGHTING Light Emitting Diodes in lighting applications allow for

more ways to express users creativity. This redefines the lighting possibilities through saturated colors and creating imaginative effects. LEDs feed designers and engineers because of the environmental resistance, LED lamps new form factors and very long lifetimes.

An important feature is the spectral contents of LED lamps where the beam does neither contain IR nor UV light as required in a number of display applications. Especially the availability of saturated colors make thinking in LED lighting much more thinking in atmospheres than in applying bulbs!

In various markets this will have different impact but already now we see may applications and design studies for e.g. retail, architectural lighting, hospital and home.

II. DRIVING IS INVOATIONS IN LEDS In SSL there is a well-known statement similar to Moore's

law for silicon integrated circuits, which claims that LEDs will increase in brightness by a factor of about 30 every decade whilst their costs will decrease by a factor of about 10. This statement proposed by Roland Haitz [1,2] of Hewlett Packard has held true since the 1960’s with the exception that the light output from the latest generation of LEDs has exceed the long term.

LEDs, until only a few years ago used mainly as simple indicator lamps in electronics and toys, have become as bright and even more efficient than known light sources like incandescent bulbs or even fluorescent Lamps (see figure 1). These have already begun to replace incandescent bulbs in many applications, particularly those requiring durability, compactness, cool operation and/or directionality (e.g., traffic, automotive, display, and architectural directed-area lighting).

Moreover, further major improvements are achievable. Electrical-to-optical energy conversion efficiencies over 50% have been achieved in infrared light emitting devices. If similar

efficiencies were achieved in visible light emitting devices, the result would be well exceeding 150 lm/W for a white light source. This would be nearly two times more efficient than fluorescent lamps, and ten times more efficient than incandescent lamps. So energy savings possible when using Solid State Lighting devices for general lighting applications are enormous [3,4].

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Figure 1. LED performance relative to known lamp types [5]

(Compiled by JY Tsao / Sandia Labs using [1,2, 6,7,8,9])

But after focusing on costs and efficiency there are other challenges to face before LEDs get fit for the high volume general lighting applications [10].

III. FOR LIGHTING APPLICATIONS FURTHER IMPROVEMENTS ARE NEEDED

Whereas the expectations are high we do see a number of dissatisfiers with current LED technology. Breaking efficiency records does not come along with production control records. Production spread in LED devices is challenging lamp design

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for colored and color variable applications. Dependant on color and the lighting application the human eye shows high sensitivity for color differences. So deviations in emitted wavelength can give customers the impression of low quality lamps. For wavelength as well as intensity spread driving and control has only limited possibilities to compensate. Feedback color control systems drive system costs up. This makes it hard to reach the requirement on initial color point (for white color temperature) variance and variance over time and operation conditions like temperature.

The flux per package is still not at a level that allows for “real” lighting applications. In comparison to halogen or fluorescent lamps LED devices are available mostly in low wattage. Making lamps from these devices asks for mounting of multiple LED devices as well as optical solutions to get good light beam homogeneity.

Looking into the requirements for lamp manufacturing possibilities for lead free assembly will be necessary. These have to be compatible with mainstream high throughput assembly lines (e.g. for reflow soldering) to go into the mass manufacturing of LED based lamps.

In addition the reliability of devices as well as final products has to get up to the level expected in “Lighting” markets. To reach this it is also important looking at lifetime expectations not only for the LED devices but also for the whole lamp system. It is the common understanding that LED devices are extremely rugged and allow for lifetime sealed applications exceeding 50000 hours. But can that anyhow be reached for mounted device that are sealed together with optics, cooling and driving means?

Finally the system cost per lumen is still threatening in many markets because total cost of ownership (TCO) is often not the strongest selling point. Surely expected energy savings give good arguments in areas where energy supply is a hard constraint as it is in solar powered island operation. But the initial lamp costs are often limiting penetration of LED based lamps.

In the lamp system optical requirements are very high. As there is the request for high beam homogeneity for each lamp and also between different lamps and for color consistency a stringent binning solution on system level is necessary.

For white applications using red, green, and blue LEDs a good color rendition as measured with the color-rendering index CRI1 is not easy to reach [13]. Further limitations are due to lack of standardized thermal as well as control driver interfaces. Also innovative user-interfaces supporting variable lighting still have to be established as well. Finally robust control and driver architectures that guarantee high quality light with the typical production spread of LED devices will need attention.

1 The technical CRI report was prepared by the

CIE (Commission Internationale de l'Eclairage) Division 1 Vision and Colour, see also [11,12]

IV. MODULAR APPROACH FOR A VARIABLE WHITE LIGHT SOURCE

A modular approach with standardized interfaces on optical as well as on cooling sides can help out there. As an example for this modular concept a module is shown that offers on the optical side a multifaceted reflector for light mixing and beam forming. This module addresses spot lamp applications and has integrated cooling means for convection cooling.

Also the electrical driver and control interfaces would need to be standardized making the described modules easy to use. Such modules limit the design effort for lamps to a minimum and allow the integrator to make good use of the very impressive properties of LED devices as light source.

One of the very nice features of the saturated color capability of LEDs could be used for color mixing lamps. For these getting homogeneous light distribution in the beam and repeatable colors over lifetime and operating conditions are the most interesting challenges.

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Ideal peak wavelengths for CRI>80 in a target color temperature range of 2700K < Tc < 6500K is blue 460nm, green 540 nm and orange 615 nm. With this OGB mixing however a strong sensitivity for amber-red peak wavelength over temperature is experienced. Because the required orange high power LEDs are not available nearest red and amber dice have been used.

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Peak wavelength bins @ 25°C (nm): B 457-462; G 532-537; R 608-613

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As shown in figure 3 a CRI above 80 can be demonstrated over nearly all color temperatures, die temperatures and spread when using the OGB wavelength combination discussed above. For color mixing a segmented reflector has been designed that is optimized to promote a round beam shape. The spot module comprises additionally sensors and ESD protection. Drivers and controls are kept separately. The housing is designed as a cooler that runs at about 70°C at room temperature and 10W electrical power.

V. SSL LIGHTING INNOVATION - A DEVELOPMENT TOWARDS GENERAL LIGHTING

In the beginning LEDs started in special lighting applications like traffic lights and automotive rear lights. This was enabled by the high power LEDs mid 1990th. During start of the new century some projects were done e.g. in outdoor lighting and on stage. [14]

Today we see many new applications showing up. That was again visible on the Light & Building Fair Spring 2006 in Frankfurt. Many LED applications from decorative lights to display like applications were shown there. In real lighting applications e.g. for wall washing it was shown how lamps are designed as integral part of buildings and furniture. Until end of the decade we can expect more applications becoming feasible that had not been tried with the old light sources. A number of examples will be shown during the presentations.

VI. CONCLUSIONS LED technology moves forward rapidly, but the devices are

still to dim and too expensive to have a favorable total cost of

ownership. LED industry is currently pubescent and the adolescence will follow. As discussed a modular approach is essential to deliver quality and reliability in the system. LEDs are the perfect choice for colored lighting. This will fuel architectural lighting as well as atmospheres in shops, office and finally homes. Humans have a strong disposition for natural, daylight-like white and phosphor-converted LEDs come closer. So it is clear that LEDs in Lighting will grow from a “Designers’ Delight” to a great variety of lighting solutions.

REFERENCES

[1] R Haitz, F. Kish, J.Y. Tsao, J. Nelson, “The Case for a National Research Program on Semiconductor Lighting”, 1999 Optoelectronics Industry Development Association (OIDA) forum, Washington D.C.,

[2] A. Bergh, G. Craford, A. Duggal, .R. Haitz, “The Promise and Challenge of Solid-State Lighting”, Physics Today, December 2001 Volume 54, Number 12, pp. 42-47

[3] M. Kendall, M. Scholand, “Energy Savings Potential of Solid State Lighting in General Lighting Applications”, US Department of Energy, Washington, DC, April 2001

[4] D. A. Steigerwald, J. C. Bhat, D. Collins, R. M. Fletcher, M. Ochiai Holcomb, M. J. Ludowise, P. S. Martin, and S. L. Rudaz, “Illumination With Solid State Lighting Technology”, IEEE Journal On Selected Topics In Quantum Electronics, Vol. 8, No. 2, March/April 2002, pp310-320

[5] J.Y. Tsao, “Solid-State Lighting: Lamps, Chips And Materials For Tomorrow”, IEEE Circuits & Devices Vol 20 No 3, May/June 2004, pp 28-37

[6] J.Y. Tsao, Ed., Light Emitting Diodes (LEDs) for General Illumination Update 2002 (OIDA, Sep 2002)

[7] N. Nakayama, S. Kijima, S. Itoh, T. Ohata, A. Ishibashi, and Y. Mori, “High-Efficiency ZnCdSe/ZnSSe/ZnMgSSe Green Light-Emitting-Diodes”, Optical Review 2, 1995, pp. 167-170

[8] J. Edmond, H. Kong, and C. Carter, “Blue LEDs, UV Photodiodes and High-Temperature Rectifiers in 6H-SiC”, Physica B: 7th Trieste ICTP-IUPAP Semiconductor Symposium 185, 1993, pp. 453-460

[9] W.D. Nordhaus, "Do Real Output and Real Wage Measures Capture Reality? The History of Lighting Suggests Not", Reprinted in Timothy F. Bresnahan and Robert J. Gordon, eds., The Economics of New Goods, Vol. 58, Chicago: University of Chicago Press, 1997, pp. 29-66

[10] M.G. Craford, “LEDs a challenge for lighting,” in Light Sources 2004, Proceedings of the 10th International Symposium on the Science and Technology of Light Sources, Toulouse, 18-22 July 2004, G. Zissis, Ed. Bristol: Institute of Physics Publishing, 2004, pp. 3–13.

[11] W.A.Thornton, Luminosity and Color-Rendering Capability of White Light”, Journal of the Optical Society of America, Vol. 61, No. 9, September 1971, pp. 1155-1163

[12] Y. Ohno, “CIE Fundamentals for Color Measurements”, IS&T NIP16 Conference, Vancouver, Canada, Oct. 16-20, 2000

[13] Y. Ohno, “Color Issues of White LEDs”, Illumination", OIDA Workshop Preliminary Report, October 26-27, 2000

[14] M.G. Craford, N. Holonyak, F.A. Kish, “In pursuit of the ultimate lamp”, Scientific American, (Feb. 2001)

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