Remote phosphors yield better light bulbs
Separating the phosphor from the blue-emitting chip is a great way to improve the performance of LED light bulbs, according to Mitch Jansen from Intematix. With this architecture bulbs are brighter, their emission more uniform, and they last far longer, thanks to superior thermal management.
To capture the lucrative general illumination market with solid-state devices, manufacturers of lighting products must generate white emission with LEDs. There are two ways to do this: Colour mixing three or four monochromatic LEDs spanning the visible spectrum; or combining a blue-emitting chip with longer-wavelength phosphors.
Although the former approach has several strengths, including the promise of very high efficacies and a spectral output that is similar to that produced by fluorescent sources, the combination of a blue LED and phosphor is more attractive, thanks to its superior spectral coverage and greater simplicity. This simplification occurs because it is easier to drive and control one type of LED while maintaining a colour balance, rather than driving different types of devices – such as red, green and blue sources . Characteristics of individual LEDs change with time and their driving conditions must be monitored and adjusted periodically in order to enable the lighting product to deliver the same colour over its lifetime.
Figure 1. Phosphor materials are widely used in direct white conversion at the LED level (1a) and can also be used in remote phosphor configurations (1b and c)
When phosphors are used, they determine all aspects of the quality of light, from colour temperature (CCT) to colour rendering index (CRI). Today, phosphors are generally applied directly to the top of the blue-emitting chips, and in many cases this is followed by further encapsulation in a silicone dome to improve light extraction (see Figure 1 (a)). However, it is possible to use a remote phosphor, which can be coated on two-dimensional shapes, or formed into three-dimensional ones (See Figure 1 (b) and 1 (c)). Going down this route, designers of lighting system must consider optics, thermal management and electronics for building a complete system.
Figure 2. Intematix ChromaLit lighting systems use blue LEDs in a mixing chamber to provide higher system performance and simplification of the optical interface
At Intematix, a phosphor manufacturer based in Fremont, CA, we have been investigating the challenges and benefits of building lighting systems using remote phosphors (see Figure 2). This is pumped with a blue LED, using what can be described as a mixing chamber configuration. Surfaces around the LEDs and around the remote phosphor are coated with highly reflective materials, enabling the ‘recycling’ of photons, leading to greater light extraction from the system, greater efficacy and lower costs.
Recycling can be highly beneficial, because phosphors are lambertian emitters. This means that photons are emitted from the phosphor in an isotropic manner, rather than just a narrow beam. This causes half of them to initially travel back towards the LED chips which act as absorbers. The reflective surfaces in the mixing chambers significantly improve the ultimate photon extraction.
Our experiments with a standard 2700 K remote phosphor show that reality tallies with this theory. Regardless of whether we use a standard mixing chamber producing about 450 lumens, or a highly absorptive mixing chamber delivering about 235 lumens, almost 50 percent of the light output travels back to the source.
Reflective materials are not just used in mixing chambers – they also line a traditional white LED package. However, in most cases the die reflectivity is much lower than that of the reflective materials that line up a mixing chamber, and the ratio of the reflective area to the absorbing area is much smaller in an LED package than in a remote phosphor system.
Another benefit that stems from separating blue LEDs from phosphors is a lower operating temperature, which results from distributing thermal energy over a larger space. This ultimately improves reliability at the system level. Thanks to distribution of heat and light over far larger areas, flux and thermal densities at the phosphor level are much lower than those found in conventional white LED systems. In both traditional and remote phosphor architectures, heating not only occurs in the chip, due to light-generation in the LED; it also occurs in the phosphor, due to absorption and down-conversion of blue light. When remote phosphors are used, heating in this material can be removed through convection and radiation.
The combination of efficient mixing chamber designs and better thermal distribution at the system level can result in system level efficiency improvements as high as 30 percent compared to white LED systems.
Other optical benefits can be achieved from designs incorporating remote phosphors. This material acts as a single lambertian source, enabling excellent colour distribution. Driving a system with multiple blue LEDs results in very little colour separation in the emitted light. This effect, also known as “colour over angle”, is a significant issue in white LEDs and is usually solved at the system level by mixing the emission of multiple white LEDs. In addition, remote phosphor systems achieve a colour distribution of better than 3 SDCM (standard deviation of colour matching). This is good enough to eliminate the need for an optical diffuser, which is often employed to mix and homogenize emission from white LEDs. Removing the diffuser trims the bill of materials for the lighting system, and also leads to higher efficacies, because diffusers reduce overall light output.
High-volume manufacture of white LEDs doesn’t yield devices with identical characteristics, so products are separated into bins. This presents significant logistical challenges. To enable high quality light at the system level – a high CRI, a consistent colour-over-angle and a value for SCDM of less than 3 – LEDs must be binned for colour and colour uniformity. This issue disappears when LED-based lighting systems are built from blue LEDs and remote phosphor elements. All the necessary CCTs and CRIs can be achieved with a reduced number of remote phosphor parts, resulting in a simpler supply chain. Product obsolescence can also be addressed in a simpler way, because as the performance of blue LED chips improve, one can maintain the same performance and system footprint while using fewer blue LEDs.
One attractive feature in remote phosphor systems is the ability to swap remote phosphor elements. This means that the same basic systems can deliver a variety of illumination conditions, an attractive feature in some instances, such as on movie and television sets where filming requires different ambient illumination conditions for different scenes.
When remote phosphors are deposited on a planar surface, they offer glare-free, consistent light. This is not possible with the point source of single LEDs, unless a heavy diffuser is used, which has an unwanted side effect – lower efficacy. Thanks to the benefits of remote phosphor technology, it is starting to see adoption in under cabinet lighting, television and film lighting, and downlights. These designs benefit greatly from the superior quality of light achieved with remote phosphor systems, which reduces variations between fixtures and enables uniform and consistent lighting.
Three-dimensional remote phosphors are gaining acceptance in high-power LED replacement lamps, which will be the next ‘killer’ application for this type of solid-state device. One example of this is our novel 75-W equivalent reference design (see Figures 3 and 4), which meets Energy Star requirements, including intensity over angle. This lamp features our ChromaLit Contour remote phosphor and an efficient, flow-through heatsink delivering superior thermal performance over that of LED-based lamps using conventional white-emitting devices. Other components include eight Philips Lumileds Luxeon ES 1000 mW blue LEDs, Furukawa reflective material for the mixing chamber and an iWatt driver rated at 85 percent efficiency. This bulb produces a minimum output of 1100 lumen, has an efficacy in excess of 70 lumens-per-watt, and its AC power consumption is under 16 W.
Figure 3. The Intematix 75W lamp reference design combines the benefits of a remote phosphor with an efficient flow-through heat sink design
Figure 4. A 75 W equivalent lamp operating in power-off condition (a) and power-on condition (b). The thermal profile of the lamp (c) shows that the remote phosphor is just 82OC
Lamps that qualify for Energy Star status must deliver omni-directional emission characteristics. Meeting this is tough with traditional LEDs. It requires additional optical elements, because the emission from this type of LED is much more directional than that of remote phosphor. Strong diffusers can counter this by shaping the beam and reducing glare, but it is still tough to create a lamp that delivers wide-angle, uniform emission. Mounting LEDs vertically can help to address some of these issues, but such designs often run into thermal and cost issues.
In stark contrast, lamps built with remote phosphors do not need an external diffuser to deliver omnidirectional, glare-free emission that meets the Energy Star requirements in flux distribution (see Figure 5). This is partly due to the uniquely designed remote phosphor shape, which combines uniform directionality with uniform diffuse emission that is free from hot spots.
Figure 5. Intematix 75 W equivalent reference design produces more than 10 percent of its total flux in the 1350 to 1800 angular distribution zone, which is considerably more than the 5 percent requirement stipulated by Energy Star. Another requirement decreed by this US body is an intensity distribution tolerance, measured from the mean, of less than 20 percent between 0O and 135O - in this design it is just over 18 percent
Conventional LEDs lamps run hot, often reaching temperatures in excess of 90 °C at the heat sink, with heat removal only occurring via conduction through the LED heat sink. Turning to remote phosphors makes a massive difference, and ultimately enables higher efficacies and a longer operating life because heat is shared between the heat sink and the remote phosphor surfaces. Measurements with thermocouples and thermal imaging cameras show that when our 75 W-equivalent, remote-phosphor bulb produces 1150 lumens at an ambient temperature of 25 °C, its heat sink and remote phosphor are at 60 °C and 82 °C, respectively.
Assembly of bulbs using remote phosphors is straightforward, and there are lots of CCT and CRI options to choose from. Our products cover colour temperatures from 2700 K to 5000 K and deliver CRIs above 80. Output of up to 1600 lumens is possible, which is sufficient for making a 100 W equivalent lamp. Our products are attracting significant interest; expect to see volume production of lamps before the end of 2012.
What’s more, the benefits of remote phosphors are not restricted to one shape of bulb. In candle-shaped bulbs they can create spectacular chandelier sources, and even in mundane applications, such as outdoor camping lanterns, they can deliver great improvements in the quality of light produced. Remote phosphors can also deliver incredibly high CRI values in downlights used for hospitality and retail, enabling tomatoes and meats to look ready to eat. And they also have the potential to create a pleasant office environment, by placing them in LED-based products with a linear design. In short, remote phosphors are here to stay, and how widely they are used will only be limited by the imagination of lighting designers.