High Dynamic Range (HDR) display is coming to the home. The question in many people’s minds is how will it eventually come to the cinema. Last month the impressive developments taking place in direct view LED displays were reviewed. This report looks more closely at the challenges faced by projector technologies in HDR.
The HDR achieves an impressive visual effect through the display of brighter highlights. Standard dynamic range cinema can display peak luminance of 48 nits (14 ft-L). Human eyes perceive light in a logarithmic manner, which indicates that it would require approximately 500 nits to achieve a doubling of brightness in highlights. Without further justification, let’s simply assume that a peak luminance of 500 nits makes for effective HDR in cinema. Further, let’s say we want to achieve HDR on a 24m (~80ft) premium large format (PLF) screen. Doing the math, with a unity gain screen and assuming operation of the light source at 80% of full power, one would need approximately 600,000 lumens of light. This is the stunning equivalent of 10 laser-illuminated projector heads. Of course, a screen gain of 2.0 would reduce this to a mere 5 laser-illuminated projector heads. If you’re shaking your head, then you understand the reason people ponder how HDR can truly come to cinema.
Cinema requires both light and sound. Projectors alone offer the ability to project light onto perforated screens, allowing traditional sound systems to be situated behind the screen. But the downside of projectors is their low degree of efficiency. Projectors are naturally inefficient. They accept as input a steady stream of light, from which light is subtracted to achieve the image shown on screen. The subtracted light is simply lost.
One company, MIT Innovation, a Canadian company founded by former employees of Brightside (acquired by Dolby), has been studying the degree to which projectors are inefficient, determining that, on average, less than 20% of the available peak luminance of standard dynamic range projectors is presented on screen. In other words, more than 80% of light in standard dynamic range projectors is thrown away. The company’s focus has been to find ways to better utilize the available steady stream of light that is input to the projector.
A fully efficient projector could accept substantially less light at its input. For example, an ordinary projector having 60,000 lumens of light is needed to light a 24m PLF screen with the peak luminance required for standard dynamic range. If only 20% of the available peak luminance was actually needed, an efficient projector having 12,000 lumens of light could be used to light the same screen. The benefit would be a substantial savings in both purchase price and operational cost.
For HDR, the inefficiency factor would grow substantially. The amount of light needed at the input to an HDR projector would be around 10x that of a standard dynamic range type, with a much smaller percentage of available peak luminance presented with HDR images on screen. If an efficient HDR projector can be realized, then such projectors could become practical in cinemas.
MIT Innovation says they’re confident of producing light management technology that will achieve high efficiency in projectors. Whether or not it is MIT that succeeds, or an unknown company yet to be announced, it is efforts such as theirs that underscores the recognition of the HDR problem in cinema, and the attempts underway to solve it. They serve as another pointer to the fact that the future of cinema technology will be quite different from what it is today.
