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You may have heard of the term ‘HDR’, or High Dynamic Range, over the last few months and have been wondering what it means and how it may be relevant to your cinema business. It is the next big thing that improves image quality by enabling the reproduction of images with the brightest highlights whilst maintaining details in the shadows creating a more realistic viewing experience as seen in nature. There are HDR capable devices everywhere from the latest TV sets to even some premium smartphones but how does it relate to your cinema and how can I tell if my digital cinema projection system will be future proofed to be able to support it in the future?
Firstly let’s start with a couple of definitions:
Dynamic Range – this is the ratio of the largest to the smallest intensity of brightness that can be reliably transmitted or reproduced by a particular imaging system.
Contrast Ratio – this is a property of a display system, defined as the ratio of the luminance of the brightest colour (white) to that of the darkest colour (black) that the system is capable of producing.
You should notice something about the two definitions above… they are the same thing!
Contrast and dynamic range relate to the same image parameter. Contrast is typically used to describe the capability of display device, such as contrast for cinema projection systems or a professional monitor, and dynamic range is used more regularly during the capture and post production phases and more recently also for display devices. The higher the achievable contrast by your projection system the greater the amount of dynamic range that can be displayed on screen. HDR capability is directly related to the contrast ratio of the projection system.
The other important aspect of High Dynamic Range is brightness. HDR enables images to be captured, post produced and displayed with very bright highlights, such as the sun in a landscape or light glinting of a car’s chrome bumper, these are known as specular highlights and these areas of particular scenes need to be displayed at higher brightness levels than Standard Dynamic Range (SDR) content. Therefore your projection system will need more lumens than usual for a given screen size to show HDR content.
Let’s have a look at a couple of diagrams to explain how brightness and contrast levels relate in a cinema:
Fig 1 – Benefits of contrast in Digital Cinema Projection (SDR)
For cinema the brightness level is set for 2D at 14fL (48nits) with a contrast ratio of 2000:1 as recommended by the Digital Cinema Initiative (DCI). This specification was limited by the display technology available at the time. Each projection system manufacturer publishes their contrast ratio achievable by their systems, with many different products being available on the market. With a simple calculation of dividing the 14fL by the achievable contrast ratio you can get a rough figure for the achievable black level in Foot Lamberts (fL) for the projection system. In the diagram above you can see that the higher contrast ratio the better the black level and the greater the dynamic range that can be represented on screen, allowing for more accurate and realistic images.
Cinema auditoriums are also not completely dark when displaying content. There are emergency exit lights, reflections off surfaces such as walls, chairs and people in the audience and all of this affects the image onscreen. This has the effect of reducing contrast onscreen. In fact any contrast much beyond 10,000:1 will more than likely be absorbed by the stray light as the black point will be below the auditorium ambient light level.
As I mentioned earlier though you need extra brightness for those specular highlights in HDR content. There is no cinema industry specification for HDR but there have been initiatives such as Éclair Color (which makes use of the Sony SRX-R500 series projection systems) or Dolby Vision that specify approximately 30fL for the peak brightness value with great results.
Let’s have a look at the diagram again if the brightness increased to 30fL to allow for specular highlights:
Fig 2 – Benefits of contrast in Digital Cinema Projection (HDR)
Firstly if the projection system does not have sufficient contrast it doesn’t make sense to increase the brightness to 30fL as the black level will be raised too high and the image will appear washed out. Dolby Vision has a theoretical contrast ratio of 1M:1 however in a cinema environment there is ambient light and whilst it can be reduced it is unavoidable to some degree meaning a portion of the contrast will be lost. By increasing the peak brightness of the Sony SRX-R500 series to 30fL the black level is maintained at a better level than the DCI recommended level for standard projection systems set at 14fL. This enables the shadows to be maintained whilst allowing the additional brightness for the highlights, creating a more realistic and natural viewing experience.
We can see from the discussion above that there are two key projection system qualities that enable HDR projection and they are brightness and contrast. Note that light source is independent of these specifications, laser is not a requirement for HDR. HDR is already in theatres now with content being mastered for both Éclair Color and Dolby Vision and whilst at this time there is not a standard in place for cinema I know what I would look out for my new or replacement digital cinema projection system.
Sony SRX-R500 series projection systems are available for all size screens from 9,000lm – 60,000lm all with 8000:1 average contrast ratio with a proven industry leading cost of ownership.
If you would like to know more I will be giving an in depth presentation about ‘HDR in Cinema’ at the 3rd ICTA Technical Seminar event in Munich on the 24th January 2017, please go here for more information about the ICTA. Alternatively, if you are unable to attend the event, contact us here with any further questions you may have.
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