A Critical Advancement in the MR16

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While most of the world’s politicians and environmentalists are focused on the demise of the common incandescent lamp and the rise of the A-lamp LED for existing lamp sockets worldwide, designers are anxious for the LED MR16 lamp. In a wide range of applications in homes, hotels, stores, offices, and outdoor locations, the MR16 halogen lamp remains in high demand due to its small size, style, crisp-warm tone and highly directional light quality. For 30+ years, the MR16 has been synonymous with “lighting design” more than any other light source.

The lighting community is frustrated that an LED replacement for the MR16 halogen lamp has been too long in coming. Common knowledge about LED lamps, particularly their preference for low-voltage power, seems to suggest that MR16 LED lamps should be easy to develop and would be among the most important products for early adopters. But despite a number of product attempts (including a lamp with an internal fan), until recently none have posed a reasonable challenge to the performance of the halogen lamps they might replace.

With a fundamental technical breakthrough, a product has been developed that can meet or exceed the expectations of the marketplace. This white paper review indicates that the first MR16 lamp family with performance comparable to generic MR16 halogen lamps appears to have been achieved.

This paper has been developed to critically review the new technology introduced by Soraa and to assess the accuracy of its Vivid product claims. While the intended primary audience is the professional engineer or lighting designer, this paper will be useful to many in architecture, interior design, product design, engineering, manufacturing, and others directly engaged in the design, construction, and management of facilities, as well as those particularly concerned with light, lighting, and color.

Understanding GaN on GaN
Light-emitting diodes (LEDs) are a special type of diode, which is a common electronic circuit component used in all electronic devices. Like all diodes, an LED passes electric current in one direction only. All diodes dissipate some energy when the electric current passes through; common diodes convert this energy to heat, but LEDs convert the energy substantially to light. The color of light emitted is determined by the type of semiconductor material that is used in the active region of the device, and by the thickness of the individual layers within the active region. All LEDs that generate white light for architectural lighting use Gallium Nitride (GaN) as the semiconductor material. The forward voltage drop (measured in volts) and the current through the diode, (measured in amps or milliamps) measure the wattage of the diode. These are typically regulated by a driver – an electronic circuit between the LED and main power – that maintains stable voltage and current in order to prevent the LED from fluctuating or burning up.

To make an LED, crystal layers of GaN are grown on a substrate material. The substrate material must have certain qualities, and the most commonly used for LEDs today are sapphire and silicon carbide. Due to differences in material properties between GaN and these materials, the GaN crystal grows imperfectly on such foreign substrates, and produces a high incidence of imperfections which reduce the light generation efficiency of the LED.

The primary scientific breakthrough of this product is the ability to grow GaN crystals on its native GaN substrate (“GaN on GaN”). The crystal grows much more perfectly, can accommodate much higher power densities, and allows the LED to emit 5-10 times more light from the same crystal area. This is called “light density” and it’s the reason that GaN on GaN devices exhibit far more point source-like qualities. As an added benefit, the GaN on GaN technology is more heat-tolerant than other substrate types and allows higher energy conversion in small form factors.

To further increase efficiency compared to other diode types, this family of products uses a design that mitigates LED “droop,” a phenomenon observed in GaN-based LEDs wherein efficiency drops as power density is increased. This design allows the LEDs to maintain high efficiency at high operating power densities and produces a very bright, point source-like light source.

Common Colorimetry
MR16 lamps are often selected for their ability to render objects vividly and make colors appear natural. Part of the original appeal of the MR16 was its higher Correlated Color Temperature (CCT) in comparison to common incandescent lamps. Objects with higher color temperature than the ambient environment appear brighter and call attention to themselves (hence the old theater lighting axiom of “fill warm and key cool”). The halogen MR16 lamp is the perfect addition to regular tungsten lighting and adds highlights and sparkle to a conventional design in an energy-efficient and economical product.

In addition, the MR16 has other important qualities. It is small, attractive, and adds sparkle and drama to ordinary spaces. It is often used for the general lighting of sophisticated spaces because of its size and the interesting contrast it creates. This legitimizes the concept of 2700K MR16s for fill lighting as well as 3000K MR16s for key lighting. In fact, a commonly applied professional accessory for the MR16 lamp is a 2700K filter, used for precisely this purpose.

LED lamps produce light differently than halogen models. Halogen MR16 lamps tend to emit light almost perfectly relative to the reference standard “blackbody.” LED lighting is more like fluorescent lighting, using blended phosphors to fluoresce and emit light of various wavelengths that, working together, synthesize white light. The relative accuracy of this synthesis among different MR16 LEDs is a crucial criterion when specifying sources for lighting design.

(See Spectral Power Density Chart above)

Professional evaluation of a light source often begins by reviewing its color spectrum. Especially as light sources become more complex and use quantum physics to create light (rather than heat), a spectral power density diagram is an intuitive way to better understand what to expect of the light source. In figure 1-1 above, the spectra of several lamps are demonstrated.

The conventional halogen MR16 lamp emits light very closely matching the blackbody curve, as expected. But note several key observations (keyed to the figure):

1. In the UV region, all LED sources have no emission, but halogen lamps do. While it seems small, UV from most light sources is the principal cause of photo-degradation and must be controlled.
Figure 1-1 Spectral Power Density of MR16 Sources
2. Violet pumped LED lamps overshoot violet and other near-UV wavelengths, which can improve whiteness rendering. But even short wave visible light is a source of concern with respect to museum lighting.
3. Violet pumped lamps undershoot the blackbody in blue, but not by much.
4. Blue pumped LED lamps strongly overshoot blue at approximately the human circadian response peak, with both positive and negative potential in health and light situations.
5. Blue pumped LED lamps undershoot cyan, reducing color rendering throughout the green-blue range.
6. Most sources are about the same in the green-yellow-orange range.
7. The lower CRI LED products roll off in medium and long wave red, hence the poor R9 numbers.
8. Pure red is centered at about 687 nm. The high CRI LED lamps approach the response of the halogen IR lamps. While the LED lamps then roll off fairly quickly towards long wave red, the halogen IR lamps roll off more gently.
9. Only the conventional halogen lamp follows the blackbody curve into the IR region.
It is generally recommended that analyzing the SPD of any candidate light source should be the first step in color review. Because human color vision varies due to chromatic adaptation, cognition and other factors, it often helps to identify potential issues from the SPD chart before relying on other metrics or visual inspection.

Light Quality
The lumens emitted by a source are a historically significant measure of its output and efficacy, measured in lumens per watt. Luminous flux is less valuable when evaluating a directional light source, because the principal interest in the source is the candlepower. Moreover, the lumens of the source contained within the assembly of lamp, reflector/lens cannot be used for conventional calculations. For these reasons, many directional light sources do not have published lumen ratings – and for those that do, the information is of little practical value.

For most accent and display lighting, the Center Beam Candle Power (CBCP) and beam angle are used to make source selections. Being directional light sources, MR16 lamps are specified by their CBCP and beam angle in degrees. CBCP is the measure of peak intensity and is measured in Candela (Cd). Beam angle is defined by the angle at which the intensity is half of the peak intensity.

MR16 LED Lamps
LED MR16 lamps have generally been designed for direct replacement in MR16 luminaires. This means that the lamp contains a driver suitable for 12-volt AC operation. Most LED MR16 lamps work on existing transformers and dimmers, and in general, the MR16 will use less energy and create less heat than the halogen lamp it replaces, with benefits that include lower energy costs and longer equipment life.

The dimension of MR16 halogen lamps has been reasonably standardized throughout the industry, but the design of some LED sources has resulted in compromises in form factor. For instance, most LED lamps require convection cooling, and the use of any glass lens (normally needed for safety with halogen lamps) will damage most LED lamps. Some LED lamps even have protruding lenses to prevent the glass lens from being used. Others require internal fans that extend the height of the lamp beyond standard dimensions, and also can draw extra power when dimmed to 0 percent. Therefore, even though LED lamps are referred to as “MR16,” they may have dimensions that deviate from the standardized size envelope in a non-standard manner.

Beam Quality & Management
The optics of Soraa’s new LED products are not unusual, but are unique in 35W+ LED MR16 lamps; because of the single small high-intensity source, the optical system performs exceptionally well. While this LED is fundamentally a directional source, a lens is added to redirect the light energy. This lens is particularly effective at controlling beam, field, and spill – essentially eliminating spill. The LED is all but free of spill light, making glare shielding easier and more effective.

Shadowing
Shadows play an important role in creating a dramatic and effect-rich lighting scene, and in interpreting textures. Especially if the texture structure is fine, small differences in shadow definition can have a big impact on the visual impression.

An infinitesimally small source or (a true “point”) source creates a shadow with a crisp, immediate transition between light and dark. All practical light sources have some size and will have some transition between the dark of the full shadow and the unobscured illuminated area. The larger the source is, the larger the transition area. The transition area also increases as the light source moves closer to the object. The transition zone is further increased with the introduction of multiple sources.

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Another advantage of a single source is that duplicate or mushy shadows are avoided.

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Beam Management Accessories
The artistry of using MR16s has historically involved external light quality management, notably the use of lenses to alter the beam pattern, and baffles and louvers to shield the spill light from sight. With this generation of products, new methods will need to evolve because:
• The source is relatively spill light free, making the shielding accessories less important
• The beam is relatively halation and striation free, making smoothing lenses mostly unnecessary
• The beam of this LED is “flat,” meaning that the angle of equal intensity is greater than with most halogen lamps, resulting in a steeper run-back near beam edge. Beam edge modification may be needed for some art display and cannot be performed using the standard borosilicate lenses such as solite.

These issues are not generic. Compared to other LED MR16 products, the GaN on GaN family will probably require an accessory holder that maintains free airflow around the lamp (except for the totally enclosed version), and a unique set of accessories, in order to realize its full potential as a high-performance display lighting system.

Many MR16 applications require the use of IR and UV filters that alter the color of the light and reduce light output. The violet peak of the GaN on GaN lamp may be of some concern, but compared to halogen lamps, both this family and conventional LEDs exhibit complete attenuation of UV and short wave indigo, whereas halogen lamps still emit measureable light well in the UV-A region.

Visual Acceptance
The challenge of the MR16 LED is to be visually acceptable in comparison to halogen MR16s. Taking into account the Hunt Effect (a phenomena in which color quality becomes more important as colors are more brightly illuminated) and chromatic adaptation, the majority of existing MR16 applications will look best when using either 2700K or 3000K lamps, which will closely match an existing halogen installation. After all, halogen lamps can be dimmed to change color temperature, and the so-called “dim warm” behavior remains a challenge for LED systems. But in a space where the lights are usually dimmed, the 2700K LED can be used to approximate the typical dimmed color temperature.

But even among halogen MR16 lamps, it is common for the designer to seek a warmer color temperature. Often, color temperature warming lenses such as “cosmetic peach” are installed on halogen MR16 lamps to diminish the harsh cool edge of a halogen lamp operating at 3000-3100K. The use of UV filtering lenses for museum lighting applications has a more or less similar effect. In both cases, a 2700K LED would likely be best accepted.

LED Considerations
Some LED emitters are specifically designed to emit UV, and others to emit IR, but these emitters are not used for architectural lighting. Architectural lighting LEDs emit neither UV nor IR.

However, one of the principal considerations involving MR16 LED is its use in the display of fine art and collectibles. Photo-degradation occurs when paintings, drawings, apparel, and furnishings are exposed to light or UV, causing color fade and fabrics to fall apart. As a rule, LED MR16s are free of UV, but cannot be considered to be absolutely safe. In both the VP3P and BP2P systems, short wave visible light peaks are reason for concern. Filtering to reduce the peaks may be required for critical applications.

Powering and Dimming Behavior
The MR16 LED is generally intended as a replacement lamp to be used in existing MR16 halogen sockets. In addition to some type of transformer, a dimmer also controls many MR16 applications. Since the combination of LED, driver, transformer, and dimmer were never designed to operate together, unpredictable interactions can occur. In almost all cases, the low end of the dimming range is still fairly bright compared to a halogen lamp, which can fade to black. In addition, some combinations of lamp and other electrical components will cause the LED to flicker, drop out, or sometimes, not operate at all.
The design of the driver inside the LED is the key to how the device will operate. Without any real standardization, each LED product will most likely operate differently. A list of known compatibilities [should be] listed on the product cut sheet and company Web site.

Energy Efficiency and Economics
The basis of this calculation is the original halogen lamp. If replacing a 50-watt generic MR16 with a 12-watt LED, the approximate savings (including reduced transformer loss) will be about 40 watts per luminaire. At 10 cents per kilowatt-hour, the energy savings in a year of typical commercial use (4,000 hours) will be about $16.00. This simple payback alone may be adequate justification for use of the lamp.

Dramatically reduced relamping costs are another advantage of the LED. A quality brand generic MR16 lasting 2,000 hours costs about $4.00. By warrantying the LED to three years, these LEDs will outlast generic lamps 6 times over, saving $24.00 in lamp costs as well as the cost of relamping.

Conclusion
For a wide variety of current MR16 applications, these GaN on GaN lamps are respectable alternatives to, and perhaps even an improvement on, halogen. They provide benefits of lower energy use and longer lamp life without sacrificing color or candlepower performance.

However, lighting design professionals will quickly realize that while this first generation GaN on GaN MR16 LED lamp is a worthy replacement of generic 50W lamps, their output is still less than some state-of-the-art halogen lamps, especially the 30- to 50-watt IR lamps. Further improvements in the technology will overcome the heat challenges and will provide higher output product in the future. Those designers needing performance and accessories for professional applications will find these LED MR16 products quite useful, but will probably need to continue to rely on light engine technology for a complete product family of more powerful products and proven accessories.

To read additional articles on inside lighting please see:

Art Donovan: Steampunk Gallery

Theodore Alexander: Handcrafted Lighting Video

Donna Frasca, Reveals Which Colors Will be Strong in 2012

 





Comments

  1. Dennis McCarthy says:

    Jim -

    A very good read on the form factor subset , the right amount of info irrespective
    of SSL acumen. One question – the MR 16 also can come in the AC/LED design
    package – Not one word for the alternative- WE all recognise the challenges of
    getting quality from a LED luminiare – There is good reason to include an approach
    that isn’t bogged down by electrolytic capacitors and drivers . The many MR-16s
    that I see , use ,dissect and sell are flawed by the thermal dynamics taking place
    on small patches of real estate . It’s the challenge of making SSL consumers aware
    that the SSL realm is not completely reliant on heat sinks and drivers -

    Dennis M- renown LED expert and OLED advocate

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  1. [...] ceiling from stepped coves and soffits. To illuminate the display shelves, the design team combined LED MR16 lamps with LED striplights. Low-voltage, recessed adjustable fixtures provide both general illumination [...]

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