Trip the light fantastic with laser projectors
Projector manufacturers are beginning to use lasers, as opposed to lamps, as a light source for projectors in both home and commercial cinema applications. Jacob Harris explains.
Lasers, it would appear, are set to be the next big thing in the front projection market. And considering they can offer increased brightness and colour gamut while eliminating the need to replace lamps, it’s not hard to see why.
There are currently two main variations of the technology being used as a light source for projectors: RGB laser which, as the name implies, uses an array of red, green and blue lasers to create the exact colours needed; and laser phosphor, which uses either one or two blue lasers that are combined with phosphor to create the other wavelengths of light.
RGB, or ‘pure’, laser delivers significant improvements in terms of colour gamut and luminance. However, due to physical size and cost, RGB laser projectors are currently (by and large) relegated to the domain of digital cinema.
“The most important driver for the adoption of RGB laser has been the possibility to achieve higher brightness levels. With lamps, digital cinema projectors reached their maximum brightness around 30,000lm. RGB lasers have enabled projectors to project up to 60,000lm,” says Barco senior product manager Tom Bert.
“The underlying reasons why this can be achieved are a higher f-number and lower étendue (a characterisation of how diffuse the light is in area and angle). This also leads to some ‘free’ gains such as significantly higher contrast, higher uniformity, more efficient 3D and wider colour gamut (up to Rec. 2020). Overall, the image quality is visibly better: even if you calibrate the image to a brightness level achievable with lamps, studies have shown that audience members come out of the room reporting an enhanced experience.”
The increased brightness made possible by the use of RGB laser makes its impact felt most on bigger screens and when viewing 3D media. Brighter projectors help achieve compromise-free image quality levels where 3D is screened at 7-9foot-lambert (fL) (according to Tom there have previously been cases where movies were screened below 3fL).
“Low light levels for both 2D and 3D presentations directly impact the viewing experience of movie-goers, particularly in a 3D presentation. Low light levels cause significant discomforts such as headache, eye strain and fatigue. Furthermore, the finer details of the image are often lost as the black levels are washed out,” says Christie regional sales manager for entertainment solutions Henry Noel.
“With laser projection, it is possible to double the luminal output of Xenon lamp based illumination methods. Thus, the standard light levels for 2D and 3D presentation on Premium Large Format (PLF) screens and IMAX format size screens can be easily achieved. The current DCI recommended 3D light levels for Pure RGB Laser based projection is 14fL ± 2fL.”
RGB laser projectors also make it easier to go beyond the current standards for colour gamut (Rec.709, DCI-P3). Just how far beyond those standards you can go depends on the technological choices that have been made with the laser light source.
“Monochromatic lasers with three primaries (red, green and blue) can achieve the Rec.2020 gamut – the ultimate wide colour gamut standard. However, these lasers introduce an image artifact known as ‘speckle’. Therefore, manufacturers are introducing some wavelength-diversity, which brings the realistic wider gamut below DCI-P3 and Rec.2020. This balance between theory and practice is now also understood by the standardisation committees and a kind of tolerance will very likely be applied to the theoretical Rec.2020 specification,” says Tom.
Laser phosphor technology, or ‘hybrid laser’ as it’s often called, uses blue lasers to stimulate yellow phosphor, thus converting the narrow band blue to wide spectrum green, yellow and red. According to Tom, these laser phosphor projectors occupy the middle-ground between lamp and RGB laser projectors.
“From RGB lasers, hybrids inherit long lifetimes and improved image quality while their more attractive price points and wider spectral distribution are similar to lamp projectors. Their ‘lamp-like’ attributes do mean they can’t reach the same brightness levels as RGB laser and also don’t have the ability to go to as wide colour gamuts.”
However, the fact that they have an attractive total cost of ownership (TCO), make them the default solution for the mass market. Possibly the largest driver behind the so called ‘hybrids’ is that blue lasers can be mass manufactured at moderate cost.
“Hybrid technology offers a cost effective route to achieving high luminance levels. For example, Digital Projection has just launched a 13,000lm, 3-chip DLP projector in its Highlite chassis. The small size of the light source has enabled us to develop a projector at a lumen point previously only achievable with larger, more expensive light modulators and optics. This technology will continue to be refined to increase lumens and reduce cost,” says Digital Projection international marketing manager Mark Wadsworth.
“Although Digital Projection has directed its effort largely at the cost effective use of blue lasers to stimulate phosphor (the hybrid approach), there is no doubt that the way to achieve ultimate brightness is to use very high power RGB lasers in a direct manner without any phosphor. Some of our competitors have already demonstrated projectors at 60,000-70,000lm; however, the cost of the lasers and the bulk of the associated cooling equipment makes them suited to a niche market only.”
For many applications, pure laser illumination as it stands is not practical in terms of cost, size and power consumption. According to Mark, the key innovation in the field of laser illumination was in harnessing the lasers as both a light source and a stimulation source – a hybrid system. That approach allows the use of lower power lasers, which are more cost effective and are already in large scale production.
“The predictable behaviour of these light sources is bringing added value to systems integrators. While service and maintenance contracts for projectors will continue to exist, integrators will be able to better plan and schedule how these are fulfilled. In addition to this, new business model opportunities arise where the long lifetime light sources are offered and sold in new ways: pay-per-hour models and the like are already being deployed, bringing new opportunities for integrators,” says Tom.