Gallium nitride (GaN) LEDs based on sapphire and silicon-carbide substrates have advanced over the last 20 years but generally still fail to meet the light quality aspects we associate with natural lighting (daylight and incandescence). The shortcoming is partly due the led high bay light fixture wholesale focus on increasing luminosity at the expense of aspects such as color and whiteness rendering, beam pattern, and total light output. A different approach would be to develop high-efficiency LEDs and leverage the performance to provide the quality aspects of natural lighting. Let's discuss ways to characterize light quality and how it can be delivered by LEDs built on homogeneous substrates.
The latest GaN-on-GaN LEDs will afford the headroom from a performance perspective to allow a focus on better light. The third-generation devices exhibit peak wall-plug efficiency of 80% (at 85 ° C), and 65% at 150 A / cm2, a current density five (or more) times higher than for LEDs based on foreign substrates. The practical benefits of this recent performance leap are twofold. First, it enables a compelling LED-based solution in systems running at high temperature and high current densities, where conventional LED technology has struggled. And the headroom provided by increased efficiency can be spent on light quality.
Indeed, Quality of Light (QoL) must be considered as an essential property of light sources. QoL has many aspects, but perhaps the best known is color fidelity, measured by the color rendering index (CRI) Ra and the special index R9. The latter is important for deep reds but also, crucially, the tones of human skin. Most LED products aim moderately for Ra in the range of 80 and R9 above 0. This target is based on the often-held belief that such values are sufficient. On the other hand, higher-end options are also available with values as high as Ra = 95 and R9 = 95. Such higher color rendering requires photons at the ends of the spectrum (violet and deep red) and therefore comes at cost of efficiency . The superior efficiency of GaN-on-GaN technology will be crucial in supporting high CRI, as the efficiency headroom more than makes up for the penalties inherent at the high QoL levels.
An equally important and often disregarded measure of light quality is whiteness rendering. While rendering of the color white may seem trivial, this is not the case due to the decades-old and near-ubiquitous use of optical brightening agents (OBAs) in manufactured white products. OBAs absorb short-wavelength light present in natural daylight, as well as in halogen and incandescent lamps, and convert the energy to blue light. The OBA essentially causes a color shift that is perceived as a bright and clean white. Many white objects including fabrics, paper, plastics, and others contain these whiteness-enhancing OBAs.
The OBA trick does not play well with many LEDs. Conventional LEDs use primarily blue emission and do not deliver short-wavelength radiation. They are thus unable to excite OBAs, making white objects appear dull and yellowish. A solution is the use of high -efficiency violet pump LEDs that correctly excite OBAs and render whites accurately. The amount of violet light can be precisely tuned to emulate the whiteness rendering of natural light sources.
FIG. 1. Optical brightening agents (OBA) create additional blue energy in reflected light.
FIG. 1. Optical brightening agents (OBA) create additional blue energy in reflected light. A halogen lamp, and LED # 2 based on a violet emitter, result in white paper with OBA appearing bright white. Conventional LEDs based on a blue emitter such as LED # 1 result in the paper appearing cream colored.
Fig. 1 depicts the color science behind the use of OBAs. The material in question converts the short-wavelength energy as shown in the graph on the left. (For background information on color science see the multipart series that concluded in LEDs Magazine last year .) The figure compares how the human visual system would perceive the color of commercial white paper illuminated by different light sources with identical 3000K CCTs. The example compares a halogen source, a conventional LED (# 1) with no OBA excitation, and a high -QoL LED (# 2) with OBA excitation. The violet energy in LED # 2 is tuned to match the halogen lamp and the pair result in color uniformity within a 1-step MacAdam ellipse (also called SDCM or standard deviation of color matching) . led high bay light for sale results in a 7-step color error - meaning that the paper appears cream colored rather than bright white.
LED comparison
In fact, not all high-CRI LEDs perform similarly when you consider all aspects of QoL. Table 1 compares GaN-on-GaN, -sapphire, and -silicon-carbide LEDs side-by-side for similar 800-lm flux levels. The data came directly from the manufacturer datasheets in the case of the Cree and Nichia LEDs. We see that for similar input power and efficacy levels, the QoL for the violet-primary GaN-on-GaN LED is much higher. The comparison shows that the raw performance of the GaN-on-GaN LED can overcome the inherent efficiency penalties of using a primary violet emitter resulting in Stokes' loss and the use of deep red phosphor that radiates some energy outside the human visual sensitivity range. The GaN-on -GaN LED delivers the QoL aspects we associate with natural lighting.
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