01-01-2013, 10:34 AM
Future prospects of high-end laser projectors
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ABSTRACT
During the last years, a trend to replace commonly used short arc lamps in projection systems with alternative light
sources is seen. Next to LEDs for low light output products, lasers try to enter the projection area and have the ambition
to infiltrate from low (picoprojection) towards high light output systems (digital cinema).
One of the benefits of lasers is their narrow spectral bandwidth. As a consequence, the display can have a very large
colour gamut, if the lasers are carefully selected. Another benefit is the very low intrinsic étendue of the source. One can
imagine using less complex, more efficient, smaller but more powerful optical systems. This not only for scanning
projectors, but also for 2D light valve based projectors (LCOS, LCD, DLP). In addition, the limited lifetime of lamps has
serious impact on a system’s cost of ownership, and puts light source reliability/lifetime high on the list of priorities for
future developments. For this reason, Barco entered the European FP6-project OSIRIS, in a subtask where a 300 lm laser
projector demonstrator has to be developed and evaluated.
So far, we found out that next to obvious challenges such as laser cost and laser power, the most critical issue regarding
image quality is speckle interference which counteracts the beneficial nature of the light source. This phenomenon is a
direct consequence of the coherent nature of a laser and cannot be solved as easily as is often claimed. We will describe
laser speckle in the context of laser projection and the theoretical limits of several reduction techniques. This leads to
guidelines which can make laser projection worth considering.
INTRODUCTION
Today’s projectors in the high-end market segment
Most of today’s projectors use short arc lamps as a light source. Although solid state light sources for projection
applications are very promising, they are not used that often, because at the moment the technology is not mature enough.
LED-based projectors are limited to low output powers, which makes them not suited for high-brightness applications.
Laser projectors are currently only available for niche markets, mainly due to the price of RGB lasers and laser speckle.
There are two main types of lamps for projection systems available: Mercury and Xenon lamps. The efficiency (lumen
per electrical Watt) and the lifetime of Mercury lamps are higher than for Xenon lamps, but Xenon lamps reach higher
power levels. The spectra of these lamps are also different; where the spectrum of Mercury lamps show several peaks,
Xenon lamps have a smoother spectrum (apart from a spike in the blue color band), and they offer more flexibility to
design the filters for the primaries.
The main light valves used in today’s projectors are based on micro mirror or liquid crystal technology. LCoS offers
superior contrast ratio and resolution. The switching speed is good for three chip architectures, but it is not always
enough for single chip projectors. DLP chips, on the other hand, can be used in single and three chip setups, but DLP
light engines do not reach the resolutions and contrast ratios of the state-of-the art LCoS imaginers.
Applications of high-end projectors
As an example two high-end applications are described. By means of these two examples, it will be shown in section 2
that short arc lamps have some important limitations, which are (partly) solved by using lasers instead of these lamps.
Multi-projector display systems
The requirements for projectors in multi-projector simulation and training display systems (shown on the left in Figure 1)
are very demanding. These setups are used as simulators, as large CAD-walls for automobile design, etc. For contrast
ratio and resolution there is no upper limit: they should be as high as possible, which limits the choice of the light
modulator to LCoS-technology. As motion of the images should be as natural as possible, special precautions have to be
taken to reduce the smearing effects typical for liquid crystal imagers, and only LCoS panels with a relatively fast
response time can be used. Projection surfaces are often curved (as the one shown in the figure), so the depth of focus
should be large enough to allow this, without the need of very complicated and expensive projection lenses. In order to
obtain a seamless image in both day and night time simulation conditions, the projectors need to have excellent warping,
blending and dimming capabilities[1]. The primary and secondary colors, and the white point of all these projectors have
to be adjustable in order to obtain good color uniformity throughout the complete screen. The lifetime of the light source
(at the moment Mercury lamps are common) is important, as the complete display has to be very reliable, even for near to
24/7 applications.
Digital cinema
Projectors for cinemas need very high brightness levels (up to 30,000 lm for large screen installations) and therefore, they
currently use Xenon lamps (up to 7 kW). Due to such high power requirements one usually uses (3-chip) DLP projector
technology. The wall-plug efficiency of the light source is an important parameter at these brightness levels: not only to
reduce the power consumption, but also to make the cooling less demanding. The projectors have to be certified
according to the requirements set by the Digital Cinema Initiatives (DCI), which imposes minimum performance
specifications regarding color gamut, contrast ratio, etc[2]. As many movies are released in 3D, the projectors need to be
compatible with one of the 3D stereo technologies (e.g. Infitec[3]). The lifetime of the projector’s light source and its
power consumption are important to reduce cost of ownership by avoiding expensive lamp replacements and saving
electricity cost.
Lifetime and reliability
A first and important advantage of lasers (and other solid state light sources) is their lifetime. While a high-power
compact Mercury projector lamp has a lifetime of 1,000 to 1,500 hours (some very high power Xenon projector lamps
have even shorter lifetimes), lasers offer lifetimes of some 10,000’s of hours. Light sources with longer lifetimes increase
the reliability of the display system, which is important for critical applications like security monitoring in control rooms,
which are operated 24/7, but they also reduce the cost of ownership as the lamp replacements are costly.
Étendue
The étendue of a light source is determined by the product of the emitting surface and the solid angle of the light
emission. For a short arc lamp the étendue is defined by the arc length. More powerful lamps have longer arcs, and thus a
larger étendue. Lasers, on the other hand, have very small étendue, which is determined by the diameter of the beam
waist and the divergence angle of the beam and both of them are very small for most lasers. The étendue of a projector is
determined by the f-number of the light incident on the light valve and the size of the light valve.
The performance of today’s lamp-based high-end projectors is limited by the law of étendue: in a perfect optical system
étendue is conserved, but it can never be reduced without light losses and only the part of the light from the lamp that fits
inside the étendue of the light valve can be used. In order to collect more light from the light source (this can be one or
more lamps), the f-number of the system can be decreased. This increases the étendue acceptance of the system and the
system becomes brighter.
Spectral bandwidth
A third group of benefits of lasers in projection is related to the bandwidth of the lasers, leading to more saturated
primaries, as illustrated in the left panel of Figure 4, where the spectrum of the green primary of a typical single chip
business projector is compared to the spectrum of a green frequency-doubled laser (532 nm). More saturated primaries
have a positive impact on the color gamut, the color matching in multi-projector setups, the efficiency and improves 3Dpossibilities.
As the primaries are more saturated, their color points will lie closer to the spectrum locus in the x-y color space. This
makes the color gamut of the projector much larger with respect to lamp projectors. In principle one can make lampbased
projectors with more saturated primaries, but this affects their brightness as it can only be achieved by cutting
away a part of the spectrum of the lamp. The right panel in Figure 4 shows a comparison between the color space of a
single chip business projector and a laser projector. By an appropriate choice of the laser wavelengths, the DCIspecification
can easily be met.
DRAWBACKS OF LASER PROJECTORS
Unfortunately, the current lasers are not the perfect solution for projectors. The main issues regarding laser projection are
speckle, cost of powerful RGB laser sources, cooling and stability, and the availability of wavelengths.
Speckle
Speckle appears as a granular pattern on the screen when an image is formed using coherent light. The principle is shown
in Figure 5. Light reflected at different parts of the screen will have covered different optical path differences, caused by
the height profile of the rough screen. Due to this, all rays arrive at the position of the observer with a random phase and
constructive or destructive interference can occur, resulting in the granular speckle pattern. For a complete description of
speckle we refer the reader to reference[6].
Laser safety
As lasers are potentially harmful for eyes and skin, laser projectors have to be designed in such a way that they are safe:
One has to guarantee that no permanent damage occurs, even if someone looks accidently in the light beam coming from
the projection lens. In scanning laser projectors the peak intensity of the laser light projected on the screen is much higher
than for DMD or LCoS projectors. A short high energy laser pulse poses a higher risk of eye damage then a continuous
laser illumination with the same average intensity level, since thermal spreading cannot mitigate the risk of damaging the
eye retina. This means that safety precautions have to be more restrictive for scanning laser projectors, where safety also
has to be guaranteed in case of a failure of the scanning mechanism.
If the intensity of the laser light leaving the projector exceeds certain thresholds, one has to make sure by the setup of the
application, that it is impossible to look directly in the beam of the projector, or one has to provide a safety interlock, that
shuts down the projector when someone enters a safety zone around the projected beam. To give an example: for a
projector using a 1.2” DLP imager with a light output of 30,000 lm, which projects an image of 30 m wide on a screen at
40 m from the projector, this safety zone (the nominal ocular hazard area in the IEC 60825 standard) would extend about
1.8 m in front of the projection lens. Outside that zone the projector can be classified as a class 2M laser device, which
means that if someone accidently looks into the beam, the eye blinking reflex (0.25 s) is fast enough to avoid permanent
eye damage. The calculation is based on the IEC 60825 standard, and local regulations and laws might deviate from this
standard.
[attachment=45915]
ABSTRACT
During the last years, a trend to replace commonly used short arc lamps in projection systems with alternative light
sources is seen. Next to LEDs for low light output products, lasers try to enter the projection area and have the ambition
to infiltrate from low (picoprojection) towards high light output systems (digital cinema).
One of the benefits of lasers is their narrow spectral bandwidth. As a consequence, the display can have a very large
colour gamut, if the lasers are carefully selected. Another benefit is the very low intrinsic étendue of the source. One can
imagine using less complex, more efficient, smaller but more powerful optical systems. This not only for scanning
projectors, but also for 2D light valve based projectors (LCOS, LCD, DLP). In addition, the limited lifetime of lamps has
serious impact on a system’s cost of ownership, and puts light source reliability/lifetime high on the list of priorities for
future developments. For this reason, Barco entered the European FP6-project OSIRIS, in a subtask where a 300 lm laser
projector demonstrator has to be developed and evaluated.
So far, we found out that next to obvious challenges such as laser cost and laser power, the most critical issue regarding
image quality is speckle interference which counteracts the beneficial nature of the light source. This phenomenon is a
direct consequence of the coherent nature of a laser and cannot be solved as easily as is often claimed. We will describe
laser speckle in the context of laser projection and the theoretical limits of several reduction techniques. This leads to
guidelines which can make laser projection worth considering.
INTRODUCTION
Today’s projectors in the high-end market segment
Most of today’s projectors use short arc lamps as a light source. Although solid state light sources for projection
applications are very promising, they are not used that often, because at the moment the technology is not mature enough.
LED-based projectors are limited to low output powers, which makes them not suited for high-brightness applications.
Laser projectors are currently only available for niche markets, mainly due to the price of RGB lasers and laser speckle.
There are two main types of lamps for projection systems available: Mercury and Xenon lamps. The efficiency (lumen
per electrical Watt) and the lifetime of Mercury lamps are higher than for Xenon lamps, but Xenon lamps reach higher
power levels. The spectra of these lamps are also different; where the spectrum of Mercury lamps show several peaks,
Xenon lamps have a smoother spectrum (apart from a spike in the blue color band), and they offer more flexibility to
design the filters for the primaries.
The main light valves used in today’s projectors are based on micro mirror or liquid crystal technology. LCoS offers
superior contrast ratio and resolution. The switching speed is good for three chip architectures, but it is not always
enough for single chip projectors. DLP chips, on the other hand, can be used in single and three chip setups, but DLP
light engines do not reach the resolutions and contrast ratios of the state-of-the art LCoS imaginers.
Applications of high-end projectors
As an example two high-end applications are described. By means of these two examples, it will be shown in section 2
that short arc lamps have some important limitations, which are (partly) solved by using lasers instead of these lamps.
Multi-projector display systems
The requirements for projectors in multi-projector simulation and training display systems (shown on the left in Figure 1)
are very demanding. These setups are used as simulators, as large CAD-walls for automobile design, etc. For contrast
ratio and resolution there is no upper limit: they should be as high as possible, which limits the choice of the light
modulator to LCoS-technology. As motion of the images should be as natural as possible, special precautions have to be
taken to reduce the smearing effects typical for liquid crystal imagers, and only LCoS panels with a relatively fast
response time can be used. Projection surfaces are often curved (as the one shown in the figure), so the depth of focus
should be large enough to allow this, without the need of very complicated and expensive projection lenses. In order to
obtain a seamless image in both day and night time simulation conditions, the projectors need to have excellent warping,
blending and dimming capabilities[1]. The primary and secondary colors, and the white point of all these projectors have
to be adjustable in order to obtain good color uniformity throughout the complete screen. The lifetime of the light source
(at the moment Mercury lamps are common) is important, as the complete display has to be very reliable, even for near to
24/7 applications.
Digital cinema
Projectors for cinemas need very high brightness levels (up to 30,000 lm for large screen installations) and therefore, they
currently use Xenon lamps (up to 7 kW). Due to such high power requirements one usually uses (3-chip) DLP projector
technology. The wall-plug efficiency of the light source is an important parameter at these brightness levels: not only to
reduce the power consumption, but also to make the cooling less demanding. The projectors have to be certified
according to the requirements set by the Digital Cinema Initiatives (DCI), which imposes minimum performance
specifications regarding color gamut, contrast ratio, etc[2]. As many movies are released in 3D, the projectors need to be
compatible with one of the 3D stereo technologies (e.g. Infitec[3]). The lifetime of the projector’s light source and its
power consumption are important to reduce cost of ownership by avoiding expensive lamp replacements and saving
electricity cost.
Lifetime and reliability
A first and important advantage of lasers (and other solid state light sources) is their lifetime. While a high-power
compact Mercury projector lamp has a lifetime of 1,000 to 1,500 hours (some very high power Xenon projector lamps
have even shorter lifetimes), lasers offer lifetimes of some 10,000’s of hours. Light sources with longer lifetimes increase
the reliability of the display system, which is important for critical applications like security monitoring in control rooms,
which are operated 24/7, but they also reduce the cost of ownership as the lamp replacements are costly.
Étendue
The étendue of a light source is determined by the product of the emitting surface and the solid angle of the light
emission. For a short arc lamp the étendue is defined by the arc length. More powerful lamps have longer arcs, and thus a
larger étendue. Lasers, on the other hand, have very small étendue, which is determined by the diameter of the beam
waist and the divergence angle of the beam and both of them are very small for most lasers. The étendue of a projector is
determined by the f-number of the light incident on the light valve and the size of the light valve.
The performance of today’s lamp-based high-end projectors is limited by the law of étendue: in a perfect optical system
étendue is conserved, but it can never be reduced without light losses and only the part of the light from the lamp that fits
inside the étendue of the light valve can be used. In order to collect more light from the light source (this can be one or
more lamps), the f-number of the system can be decreased. This increases the étendue acceptance of the system and the
system becomes brighter.
Spectral bandwidth
A third group of benefits of lasers in projection is related to the bandwidth of the lasers, leading to more saturated
primaries, as illustrated in the left panel of Figure 4, where the spectrum of the green primary of a typical single chip
business projector is compared to the spectrum of a green frequency-doubled laser (532 nm). More saturated primaries
have a positive impact on the color gamut, the color matching in multi-projector setups, the efficiency and improves 3Dpossibilities.
As the primaries are more saturated, their color points will lie closer to the spectrum locus in the x-y color space. This
makes the color gamut of the projector much larger with respect to lamp projectors. In principle one can make lampbased
projectors with more saturated primaries, but this affects their brightness as it can only be achieved by cutting
away a part of the spectrum of the lamp. The right panel in Figure 4 shows a comparison between the color space of a
single chip business projector and a laser projector. By an appropriate choice of the laser wavelengths, the DCIspecification
can easily be met.
DRAWBACKS OF LASER PROJECTORS
Unfortunately, the current lasers are not the perfect solution for projectors. The main issues regarding laser projection are
speckle, cost of powerful RGB laser sources, cooling and stability, and the availability of wavelengths.
Speckle
Speckle appears as a granular pattern on the screen when an image is formed using coherent light. The principle is shown
in Figure 5. Light reflected at different parts of the screen will have covered different optical path differences, caused by
the height profile of the rough screen. Due to this, all rays arrive at the position of the observer with a random phase and
constructive or destructive interference can occur, resulting in the granular speckle pattern. For a complete description of
speckle we refer the reader to reference[6].
Laser safety
As lasers are potentially harmful for eyes and skin, laser projectors have to be designed in such a way that they are safe:
One has to guarantee that no permanent damage occurs, even if someone looks accidently in the light beam coming from
the projection lens. In scanning laser projectors the peak intensity of the laser light projected on the screen is much higher
than for DMD or LCoS projectors. A short high energy laser pulse poses a higher risk of eye damage then a continuous
laser illumination with the same average intensity level, since thermal spreading cannot mitigate the risk of damaging the
eye retina. This means that safety precautions have to be more restrictive for scanning laser projectors, where safety also
has to be guaranteed in case of a failure of the scanning mechanism.
If the intensity of the laser light leaving the projector exceeds certain thresholds, one has to make sure by the setup of the
application, that it is impossible to look directly in the beam of the projector, or one has to provide a safety interlock, that
shuts down the projector when someone enters a safety zone around the projected beam. To give an example: for a
projector using a 1.2” DLP imager with a light output of 30,000 lm, which projects an image of 30 m wide on a screen at
40 m from the projector, this safety zone (the nominal ocular hazard area in the IEC 60825 standard) would extend about
1.8 m in front of the projection lens. Outside that zone the projector can be classified as a class 2M laser device, which
means that if someone accidently looks into the beam, the eye blinking reflex (0.25 s) is fast enough to avoid permanent
eye damage. The calculation is based on the IEC 60825 standard, and local regulations and laws might deviate from this
standard.