When it comes to creating white light using LEDs, there are many different methods, all of which have their advantages and disadvantages. First, the most common and traditional is chip-level conversion, where the converting material is integrated directly onto a blue LED die or fills up the LED package volumetrically. A second method converts the blue light of a diode far away from the chip, which utilizes a mixing chamber and remote optic integrating a converting phosphor, fluorescent dye, quantum dots, or other converting material. Yet another method is color mixing using LEDs of different colors such as red, green, and blue, where each LED’s intensity is varied additively to create the desired color temperature. Each method presents different challenges and brings to the designer different advantages from a system performance perspective with regards to system efficacy, CRI, optical efficiency, energy efficiency, heat sinking and other design considerations. It is important for designers to understand the nuances of each method when deciding on which method to use, whether one is selecting the light engine to be used in design of a new fixture, or selecting the type of fixture to be used in a given application.
The first method to generate white light using LEDs, which is also the oldest and most common, is chiplevel (or package level) conversion. There are essentially two methods of package level conversion. In what I call true chip-level conversion, the active converting material (such as a phosphor) is deposited directly on the blue LED die and the exiting blue light is converted almost immediately, as in the case of the OSRAM Golden Dragon Plus (Figure 1).The other most widely used method for phosphor application and conversion is known as volumetric conversion such as in the Nichia 183A (Figure 2). In volumetric conversion, the phosphor effectively floods the chip package and the blue die rests underneath a sort of pool or film of phosphor.
While either method can be argued to be equally efficient and effective at converting blue light to white light as phosphors are intended to do, each presents a different set of challenges for secondary optic design.
An LED utilizing true chip-level conversion better approximates a point source of light compared to an LED which uses the volumetric method. This has a major influence in optical design applications where a point source is essential to light quality and efficiency. For example, most parabolic optics will require a point light source to maintain efficient beam control. When using a volumetrically converted chip, shadowing and dark/light rings are often observed due to its diffuse nature, where the larger emitting area creates superfluous reflections within the optic. Another example of the importance of focal point location occurs in refractory optics, such as batwing style lenses, which perform very extreme light bending and have a very low tolerance on focal point location, such that much of the light escaping from a volumetrically converted chip enters the optic at incorrect incident angles, resulting in color separation and undesirable Fresnel scattering losses which can result in lower optical efficiency.
There are indeed times when a volumetrically converted LED is desirable, however. In light guides, the larger diffuse surface of the LED widens the area of injection, and effectively narrows the spacing between LEDs which will promote a shallower mixing area, thus enabling larger emitting areas for the light guide. This can lead to thinner and more attractive bezel design in, for example, LCD panels. Also, volumetric conversion can be a reason to choose a particular LED when the LEDs are placed very closely behind a diffuser lens that requires higher backlighting uniformity. As can be expected, the extra few millimeters of emitting surface area provided by a volumetric conversion can help with uniformity at the surface, especially in the case of very high clarity diffusers.
In the case of reflective optics where flat and angled reflectors are used to direct and control the light, the chip-level phosphor conversion method perhaps matters a little less. A key point to remember in a situation such as this is that highly specular surfaces can be glary and distracting to people in the space. However, when flat reflectors are used, the concept of a focal point usually does not come into play and for the most part light exiting from an LED, or array of LEDs, has already achieved a far-field characteristic by the time it first reflects off a flat optic.
Designing for high optical efficiency is one of the most cost-effective ways of maximizing the efficacy of an LED lighting system. By paying attention to the type of optics needed for the application, and selecting the right LEDs to be used with those optics based on knowledge of the phosphor conversion method, designers can realize easy and significant gains in system efficiency.
In a remote wavelength conversion optic, discrete blue LEDs are mounted in an array and directly illuminate the inside surface of a lens which contains the conversion material. As previously noted, there are many different types of conversion methods available on the market today including phosphors, fluorescent dyes, quantum dots, or other conversion technology. Typically, the lens is mounted as part of a mixing chamber several centimeters away from the LEDs to provide uniform light at the surface. This method results in a number of optical and thermal effects designers should be aware of.
Thermally, there will be heat generated on the lens from the wavelength conversion. Designers should be sure to select materials with high conversion efficacies to minimize this heat, which can cause rapid deterioration of the lens substrate or the conversion material itself. This results in the need for creative heat sinking of the lens, which is often a challenge since lenses are not often made from highly thermal conductive material, and they do not frequently have a very large or tight thermal interface with heat sinking materials.