Advantages and Defect Analysis of Laser White Light and Vehicle Lighting
FAIZ RAHMAN of the University of Ohio, Ohio, believes switching from LEDs to lasers for phosphor-pumped white light sources increases efficiency, extends lifetime and produces highly collimated beams.
Currently, laser sources are based on semi-polar GaN laser diodes and incorporate advanced phosphor technology. As the laser focuses on a tiny spot on the phosphor that emits light and converts it to white light, the light source can output safe and highly collimated white light. Laser-based methods, whose properties include a much higher temperature rise than LEDs. Laser pumping is also practical because devices that emit in the near ultraviolet are readily available and can function effectively.
The process of moving from LED to laser does not happen overnight. In contrast, LED-based luminaires will remain the primary means of generating white light for the foreseeable future. However, laser diode-based solid-state lighting systems have found application in high-brightness lighting applications and are now available from several manufacturers.
The driving force behind the development of laser diode-pumped lamps is to prevent LED efficiency from falling. However, this is not the only benefit. With the exception of Soraa and several others, manufacturers of other LED sources use chips that do not contain wavelengths shorter than about 450 nm. Therefore, when using these inexpensive LEDs with a peak wavelength of 450-460 nm for phosphor pumping, the resulting light source will be deficient in the violet portion of the spectrum.
White light using a 405 nm UV LED laser avoids this situation. These devices are cheap, efficient and available on the market. With short wavelength phosphor pumping, the spectral output is more abundant, the spectral coverage is greater, and the color rendering index is higher than typical LED-pumped white light sources (see Figure 1).
Figure 1. The chromaticity point of light from a laser diode-pumped white light is almost neutral (x = 0.3305, y = 0.3309), demonstrating the spectral richness of the light source. The black line in the figure is Planck's trajectory.
In many cases, the quantity is more important than the quality of white light. Judging in this way, the laser diode pumps emit light so they are the ideal high-intensity light source for architectural lighting, searchlights and car headlamps. Another property of a laser source is that the emitted light beam has very little divergence and is nearly parallel.
The advantages of laser pumping light sources draw the attention of automakers, who are actively installing laser headlamps for high-end models. Just as LED lighting infiltrates the automotive industry, high-end models lead the trend, including some of BMW's models (see Figure 2).
Figure 4. An efficient pump for small laser pump lights. The pump light (red beam) enters from the left, passes through the concave beam expander lens, spreads to the right phosphor-coated glass surface, and impinges on the silicone-bonded phosphor layer. Down converted light is displayed as blue light. In the figure, the phosphor plate is shown as opaque and separated from the green mirror in order to see the path of the light inside the reflector housing. In an actual lamp, a phosphor plate (a transparent piece of glass that allows light to escape outward) is sealed to the edge of the reflector. Only a small amount of light is reflected back to the left laser module (not shown).
Although the white light source produced by laser pumping has many advantages, it must also consider its limitations. Like LED-based lamps, the emitted light contains two distinct components: the down-converted light from the phosphor and the remaining unconverted laser. However, the biggest difference when using lasers is inherent coherence, which results in spots - the visible and dark spots that appear on the bright surface.
Can not ignore the shortcomings of spots, in addition to distracting, it has a negative impact on visual perception, preventing the detection of spatial detail of lighting objects. Tests have shown that blotches reduce visual acuity by 40%, reducing the ability to perceive spatial and spatial frequencies. These problems are part of the reason why direct laser lighting is not popular.
Please note that when using a laser projector, there are a few ways to reduce the coherence of the laser and to convert the spot to an acceptable level. However, using a laser to illuminate the phosphor, spots can even be reduced to lower levels. In this case, the spots are too small to be completely ignored. This is mainly because the good lamp design ensures that most lasers are converted to longer wavelengths without speckle. An additional benefit is that the remaining laser exhibits very little speckle due to its reduced coherence as it undergoes multiple scattering as it passes through the phosphor layer. Phosphorescent pump laser system produces strong light, rich colors, no spots, better than LED lamps produce light.
Pro and opponents
The biggest drawback of laser pumping white light sources is diode-related. First of all, the price of this chip is much higher than the LED. Although widely used in data storage applications, laser diodes are much more expensive than LEDs for equivalent luminescence levels. Due to the high cost, laser-pumped phosphor systems are expensive and can only be deployed in specialized, less-expensive lighting applications at this time.
One advantage of laser-pumped lighting systems is the ability to cover a wide range of output powers. Commercial laser diodes are rated for power ratings from a few milliwatts to a few watts and can form a more powerful source by combining the output of several lasers. However, this is detrimental to the life of the system. This is because the diode life is lower than the life of the LED, especially when they are operating at high drive currents. Why diode life is low? This goes back to tiny, pre-existing crystal dislocations in the laser structure. These defects are greatly increased when the device is driven at high power levels. Running in this way will gradually form a wide network of dislocations called dark line defects. They act as a non-radiative recombination site, reducing the light output of the device. The brightness of the diode diminishes over time and the brightness is too low until the light goes off due to the pump light source output dropping.
Encouragingly, as the sophistication of laser diodes has increased, the magnitude of the problem has weakened but not eliminated. The solution to this problem is to transform into a native GaN substrate for epitaxial growth because this reduces the number of interfacial threading dislocations and increases the lifetime of the laser diode.
Another breakthrough that uses laser-based white lighting is the high cost of economies of scale. This will open up new markets for UV and near UV laser diodes. Although LED lighting undoubtedly leads today, but in the ensuing years, please pay attention to the laser system.
link by : www.likeyli.com
Currently, laser sources are based on semi-polar GaN laser diodes and incorporate advanced phosphor technology. As the laser focuses on a tiny spot on the phosphor that emits light and converts it to white light, the light source can output safe and highly collimated white light. Laser-based methods, whose properties include a much higher temperature rise than LEDs. Laser pumping is also practical because devices that emit in the near ultraviolet are readily available and can function effectively.
The process of moving from LED to laser does not happen overnight. In contrast, LED-based luminaires will remain the primary means of generating white light for the foreseeable future. However, laser diode-based solid-state lighting systems have found application in high-brightness lighting applications and are now available from several manufacturers.
The driving force behind the development of laser diode-pumped lamps is to prevent LED efficiency from falling. However, this is not the only benefit. With the exception of Soraa and several others, manufacturers of other LED sources use chips that do not contain wavelengths shorter than about 450 nm. Therefore, when using these inexpensive LEDs with a peak wavelength of 450-460 nm for phosphor pumping, the resulting light source will be deficient in the violet portion of the spectrum.
White light using a 405 nm UV LED laser avoids this situation. These devices are cheap, efficient and available on the market. With short wavelength phosphor pumping, the spectral output is more abundant, the spectral coverage is greater, and the color rendering index is higher than typical LED-pumped white light sources (see Figure 1).
In many cases, the quantity is more important than the quality of white light. Judging in this way, the laser diode pumps emit light so they are the ideal high-intensity light source for architectural lighting, searchlights and car headlamps. Another property of a laser source is that the emitted light beam has very little divergence and is nearly parallel.
The advantages of laser pumping light sources draw the attention of automakers, who are actively installing laser headlamps for high-end models. Just as LED lighting infiltrates the automotive industry, high-end models lead the trend, including some of BMW's models (see Figure 2).
Figure 2. BMW's laser headlamps have separate laser diodes, beam combiners, phosphor targets and reflector assemblies.
Advantages and Defect Analysis of Laser White Light and Vehicle Lighting
2017-09-15 11:42:43 [Edit: nicolelee]
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FAIZ RAHMAN of the University of Ohio, Ohio, believes switching from LEDs to lasers for phosphor-pumped white light sources increases efficiency, extends lifetime and produces highly collimated beams.
Currently, laser sources are based on semi-polar GaN laser diodes and incorporate advanced phosphor technology. As the laser focuses on a tiny spot on the phosphor that emits light and converts it to white light, the light source can output safe and highly collimated white light. Laser-based methods, whose properties include a much higher temperature rise than LEDs. Laser pumping is also practical because devices that emit in the near ultraviolet are readily available and can function effectively.
The process of moving from LED to laser does not happen overnight. In contrast, LED-based luminaires will remain the primary means of generating white light for the foreseeable future. However, laser diode-based solid-state lighting systems have found application in high-brightness lighting applications and are now available from several manufacturers.
The driving force behind the development of laser diode-pumped lamps is to prevent LED efficiency from falling. However, this is not the only benefit. With the exception of Soraa and several others, manufacturers of other LED sources use chips that do not contain wavelengths shorter than about 450 nm. Therefore, when using these inexpensive LEDs with a peak wavelength of 450-460 nm for phosphor pumping, the resulting light source will be deficient in the violet portion of the spectrum.
White light using a 405 nm UV LED laser avoids this situation. These devices are cheap, efficient and available on the market. With short wavelength phosphor pumping, the spectral output is more abundant, the spectral coverage is greater, and the color rendering index is higher than typical LED-pumped white light sources (see Figure 1).
Figure 1. The chromaticity point of light from a laser diode-pumped white light is almost neutral (x = 0.3305, y = 0.3309), demonstrating the spectral richness of the light source. The black line in the figure is Planck's trajectory.
In many cases, the quantity is more important than the quality of white light. Judging in this way, the laser diode pumps emit light so they are the ideal high-intensity light source for architectural lighting, searchlights and car headlamps. Another property of a laser source is that the emitted light beam has very little divergence and is nearly parallel.
The advantages of laser pumping light sources draw the attention of automakers, who are actively installing laser headlamps for high-end models. Just as LED lighting infiltrates the automotive industry, high-end models lead the trend, including some of BMW's models (see Figure 2).
Figure 2. BMW's laser headlamps have separate laser diodes, beam combiners, phosphor targets and reflector assemblies.
Laser pumping structure
Unlike LEDs, different arrangements are required when using a laser diode to pump the phosphor. Due to the directional nature of laser radiation and its high intensity, it is impossible to simply deposit the phosphor on top of the pump device. In contrast, more optical arrangements are needed, such as a combination of a phosphor plate and a reflector, or a phosphor-coated integrating sphere (see FIG. 3).
For both methods, one or more diodes may be used for pumping as long as multiple laser beams are combined using suitable techniques. This means there is no upper limit on the optical power of the laser pump optical module. An additional advantage of using a remote pumping structure is that the phosphor is not on the hot part, which prevents the phosphor from heating during operation, greatly extending its useful life.
A relatively simple method of laser pumping is to direct the laser to the phosphor plate and collimate the resulting radiation with a reflector (see Figure 3 (a)). However, the phosphor-coated integrating sphere is more effective in terms of optical power conversion (see FIG. 3 (b)).
Figure 3. Typical optical setup of a laser diode pump
Depending on whether low-power or high-power light sources are needed, other solutions can be used. For example, small luminaires can use a beam expander to pump the entire phosphor plate. This method, shown in Figure 4, depicts the ray tracing simulation of a single pass-through lamp.
Figure 4. An efficient pump for small laser pump lights. The pump light (red beam) enters from the left, passes through the concave beam expander lens, spreads to the right phosphor-coated glass surface, and impinges on the silicone-bonded phosphor layer. Down converted light is displayed as blue light. In the figure, the phosphor plate is shown as opaque and separated from the green mirror in order to see the path of the light inside the reflector housing. In an actual lamp, a phosphor plate (a transparent piece of glass that allows light to escape outward) is sealed to the edge of the reflector. Only a small amount of light is reflected back to the left laser module (not shown).
Although the white light source produced by laser pumping has many advantages, it must also consider its limitations. Like LED-based lamps, the emitted light contains two distinct components: the down-converted light from the phosphor and the remaining unconverted laser. However, the biggest difference when using lasers is inherent coherence, which results in spots - the visible and dark spots that appear on the bright surface.
Can not ignore the shortcomings of spots, in addition to distracting, it has a negative impact on visual perception, preventing the detection of spatial detail of lighting objects. Tests have shown that blotches reduce visual acuity by 40%, reducing the ability to perceive spatial and spatial frequencies. These problems are part of the reason why direct laser lighting is not popular.
Please note that when using a laser projector, there are a few ways to reduce the coherence of the laser and to convert the spot to an acceptable level. However, using a laser to illuminate the phosphor, spots can even be reduced to lower levels. In this case, the spots are too small to be completely ignored. This is mainly because the good lamp design ensures that most lasers are converted to longer wavelengths without speckle. An additional benefit is that the remaining laser exhibits very little speckle due to its reduced coherence as it undergoes multiple scattering as it passes through the phosphor layer. Phosphorescent pump laser system produces strong light, rich colors, no spots, better than LED lamps produce light.
Pro and opponents
The biggest drawback of laser pumping white light sources is diode-related. First of all, the price of this chip is much higher than the LED. Although widely used in data storage applications, laser diodes are much more expensive than LEDs for equivalent luminescence levels. Due to the high cost, laser-pumped phosphor systems are expensive and can only be deployed in specialized, less-expensive lighting applications at this time.
One advantage of laser-pumped lighting systems is the ability to cover a wide range of output powers. Commercial laser diodes are rated for power ratings from a few milliwatts to a few watts and can form a more powerful source by combining the output of several lasers. However, this is detrimental to the life of the system. This is because the diode life is lower than the life of the LED, especially when they are operating at high drive currents. Why diode life is low? This goes back to tiny, pre-existing crystal dislocations in the laser structure. These defects are greatly increased when the device is driven at high power levels. Running in this way will gradually form a wide network of dislocations called dark line defects. They act as a non-radiative recombination site, reducing the light output of the device. The brightness of the diode diminishes over time and the brightness is too low until the light goes off due to the pump light source output dropping.
Encouragingly, as the sophistication of laser diodes has increased, the magnitude of the problem has weakened but not eliminated. The solution to this problem is to transform into a native GaN substrate for epitaxial growth because this reduces the number of interfacial threading dislocations and increases the lifetime of the laser diode.
Another breakthrough that uses laser-based white lighting is the high cost of economies of scale. This will open up new markets for UV and near UV laser diodes. Although LED lighting undoubtedly leads today, but in the ensuing years, please pay attention to the laser system.
link by : www.likeyli.com
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