We have been talking about laser-based digital cinema in this space for years, but one troubling issues has always been reduction of laser speckle to a barely detectable level. At ShoWest this week, Laser Light Engines (LLE) announced they have solved the speckle problem, opening the door for laser-based digital cinema.
Insight Media Analyst
Lasers have a lot of advantages for projectors in general and digital cinema in particular. The low étendue of a laser allows high-brightness out of small microdisplays. The lack of UV in laser light can be important too, especially for LC-based projection systems. Laser primary colors are very saturated, leading to very large color gamuts and lasers can also be energy efficient. They can certainly be more energy efficient than the xenon lamps typically used in digital cinema and in the long term they may be more efficient than UHP-type lamps or LEDs.
So why aren’t there more laser projectors out there? The fundamental reason is the cost of purchasing the lasers: they aren’t cheap. Secondary reasons are the effect of laser speckle on image quality and laser safety issues.
The laser safety issue is fairly straightforward to solve by making it impossible for anyone to access the output of the projection lens. In a digital cinema theater, this lens is normally well out of reach of the customers. Standing at the screen and looking back at the projector is not significantly more hazardous with laser illumination than it is with lamp illumination.
There are a number of solutions to speckle, and for IP protection reasons, most companies will not discuss in detail exactly what their solution is. In a phone interview with Bill Beck, Co-founder and EVP of Business Development at LLE, he was surprisingly open about how LLE tackled the speckle problem. He said that speckle for the red and blue channels was not an issue in the LLE laser design, which is similar to the Optically Pumped Oscillator (OPO) laser shown in the image. He said the OPO process produced a bandwidth in these channels sufficient to eliminate visible speckle, although he declined to say exactly what that bandwidth was.
Green, he added, was the key speckle issue. This occurs for two reasons. First, their laser design normally produces a narrow bandwidth, coherent green output. Second, the human visual system is more sensitive in green than other colors. This higher sensitivity and better ability to perceive small features allows people to see undesired small features like speckle.
Typically, multiple techniques are used simultaneously since no single technique is normally sufficient to eliminate visual speckle completely. Techniques typically involve the reduction of the spatial and temporal coherence of the laser beam. Beck said one of the techniques LLE has used for some time is increasing the angular divergence of the laser beam, thereby reducing its spatial coherence. Spectral broadening of the laser beam also decreases the coherence and speckle, as in the red and blue channels of the LLE system. The breakthrough at LLE, according to Beck, was broadening the spectral bandwidth of the green channel as well, although he declined to say either exactly how that was done or how broad a bandwidth was achieved. He did say that the despeckle system was entirely contained within the laser system, which is currently fiber coupled to the LLE demonstration projector. The light coming out of the fiber is fully despeckled and no modification to the normal digital cinema projector is required.
LLE did not demonstrate this system at ShoWest, but Beck says potential customers who have seen the results in the LLE labs are satisfied with the result. Now, the only remaining barrier to laser cinema is the cost issue.
Like any start-up, Beck was reluctant to discuss costs, prices or manufacturing plans. He did drop some interesting tidbits which will be in the expanded version of this article to be published in the upcoming issue of Large Display Report, along with other digital cinema news from ShoWest. And, Beck will be a presenter at the up-coming Projection Summit conference in June.