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Optical Efficiency in Digital Cinema Projectors

What isn’t compared is the energy and materials that go into making a remarkably simple film projector against a remarkably complex digital projector, plus the energy and materials that are needed to create the media server and the local central storage, plus the data farms that create and store the movies in the distribution chain, plus the network operation centers that oversee their constant quality and security. Like projectors, data centers use an astounding amount of energy to keep the air cool and what it takes to constantly keep the disks moving and processors humming.

With just a little thought, one can make a long list of data centers in the digital cinema chain: at the animation studios, at each post-production house, at each specialist render farm, at the sites which create the master Digital Cinema Distribution Master (DCDM), and those facilities that create each separately keyed Digital Cinema Package (DCP).

The efficiency of a digital cinema projector was the topic of a side-discussion at a recent technical meeting, involving a number of engineers who have been intimately involved in the digital cinema evolution. There was only conjectured and estimations based upon hearsay. It was surprising. It came up again on an online chat, where engineers who are not centimeters deep/kilometers wide (as is your author), also didn’t have an answer. It was also part of the discussion in our article:
3Questions – Laser Light Engines

So it is a pleasant and interesting relief to be able to present this document from Barco on the subject. Created by Barco Product Marketing Manager Tom Bert, it lends detail into some of the nuance of optical efficiency inside a digital cinema projector.

Optical efficiency in Digital Cinema projectors

How to get as much light as possible from your lamp onto your screen?

1 Introduction

In this article, we want to discuss everything that has an impact on how much light falls onto the screen… except for the lamp. We will have a close look at all the components that influence parameters such as image quality, total cost of ownership and serviceability.

It is our goal to clarify the impact of product design, craftsmanship and manufacturing on projector performance. We want to provide you an inside view on what’s under the hood of the projector and how it really impacts performance.

We hope that, after reading this article, you better understand how a digital cinema projector works and what contributes to the stunning performance of these state-of-the-art devices.

2. The life of Ray

Digital Cinema projection is all about getting the light from your lamp onto your screen. In this paragraph, we will discuss the different impacting parameters on a ray of light leaving the lamp. We will try to do this in a sequential approach, following the build-up of the light path of your digital cinema projector.

The image below gives an overview of the components involved:

Components of a Digital Cinema Projector2.1 Reflector

The reflector is an elliptical or parabolic structure that is placed around the lamp, in order to capture as much light as possible from the lamp and send it into the optical path of the projector. An important parameter is the surface roughness of the reflector: high end devices use perfectly smooth reflector materials, while others use processing techniques that introduce small imperfections. This can have an impact of as much as 15% on the reflector efficiency.

Important to remember is that a badly designed and manufactured reflector can destroy all of the light concentration that comes from using short arc lamps. A well designed reflector also corresponds with the heat generation and dissipation capacity of the projector.

2.2 Filters

After being captured and sent into the optical path by the reflector, the light is filtered. First it goes through a UV-filter and later on through an IR-filter. These filters get rid of that part of the light that can damage the interior of the projector and have in this way a huge impact on the projector lifetime. Filtering out too much of the spectrum of your light, means you will have to compensate later on, leading to a lower overall optical efficiency. However, it is important to know that these filters work far from the sweet spot of the visible light in the spectrum. This means that their influence on image quality is low. Their impact on lifetime is much more important.

2.3 Optical engine

After being filtered from the dangerous components in the spectrum, the light enters the actual optical engine of the projector. This engine contains passive (light rod, lenses and prism) and active (chip) components. At the interfaces of these components coating materials are applied to improve performance.

2.3.1 Passive optical components

The optical engine contains relay lenses and a light rod to transport and spread out the light. The materials that these components are made off, have an impact on the performance. Badly designed engines use materials that absorb too much of the short wavelength (blue) light. This causes them to turn yellow (cfr. you get a sunburn when you forget to put on your sunblock) or even melt! It is clear that this has a major impact on image quality.

It also defines the energetic capacity of your projector, this is the amount of light you can send through. When not designed to transmit the high load that bright projectors bring, the material can break. The brightest digital cinema projectors, like Barco’s DP2K-32B, were specifically designed to carry the load that comes with projecting more than 32,000 lumens.

2.3.2 Active optical components

In digital cinema, two technologies are used for the active chip: DLP™ (from Texas Instruments, which has ~90% market share) and LCoS (from Sony, which has ~10% market share). A digital cinema projector contains three of these chips, one for each color channel (red, green and blue). This is by far the most important component of your digital cinema projector, also from an optical efficiency viewpoint. Its compact size and accurate angular performance makes it the central component that defines the design of all other optics.

For DLP™ technology, the overall optical efficiency of the chip is defined by a combination of:

  • The chip size: it is easier to “aim” light on a bigger chip than on a smaller chip. This is one of the reasons why projectors using the 1.2” DLP Cinema® chip have a 10% higher efficiency compared to those with the 0.98” DLP Cinema® chip. For that same reason, 4k projectors with a 1.38” DLP cinema® chip will have a higher optical efficiency than those using the 1.2” DLP Cinema® chip.
  • The fill factor: the fraction of the surface that is active and reflects the light.
  • The surface reflectivity: the amount of light that bounces back from each individual mirror. This is defined by the maturity of the manufacturing process and is close to the maximum achievable value for DLP™.
  • Diffraction: when hitting structures with a small features size, such as DLP™-mirrors, a small portion of the light is always diffracted (sent off in non-perpendicular angles). As technology miniaturizes, it will become more important to manage this aspect.

With LCoS projection technology, other parameters come into play, like the inefficiencies related to using polarized light. All this leads to DLP™-based digital cinema projection yielding higher efficiencies than LCoS-based projection.

To maximize the chips’ lifetime and maintain a high image quality over time, it is crucial to keep them away from dust. Sealed engines, as patented by Barco, keep the chip surface clean and help avoid scattered light and spots on the screen.

2.3.3 Coating

Two types of surface coating are typically used on the optical materials:

  • Anti-reflection (AR) coatings: these minimize the light being bounced off the optical materials and maximize the light being injected in and transmitted through the engine. A well designed engine uses these AR-coatings wisely and can achieve a transmittance of more than 99% through the passive optical components. The avoidance of reflection also reduces the amount of the scattered light in the engine, which has a positive impact on contrast ratio.

Be aware: any small dust particles on the interface surfaces undo the benefits of using AR-coatings. They also lead to dark spots or zones on screen. That is one of the reasons why the sealed engine design, as patented by Barco, is so important.

  • Color separation coatings: these are used in the heart of the light engine (the prism) to separate the incoming light into the separate red, green and blue channels that are sent to the 3 different chips. As the name indicates, the coatings have to provide a clear separation between the different parts of the spectrum. Just like you need a sharp knife to have a clean cut, you need a sharp filter to have a good spectral separation. This so-called cut-off of the filter has improved significantly of the last years by improved manufacturing techniques. High-end devices use filters with a sharp cut-off, leading to better color separation. This has impact on optical efficiency and image quality.

2.4 Color calibration

The DCI (Digital Cinema Initiative) standard emphasizes (amongst other things) the white point and color gamut of the image on the screen. These parameters do not 100% match the output of an uncalibrated projector (one where you build in the components, not tune or tweak them and power it up). Achieving the color points set by the DCI specification, means you have to slightly compromise on light output. This has an impact on optical efficiency of 5-10%.

2.5 Projection lens

The final component the light ray goes through before leaving the projector is the projection lens. In order to achieve the high level of focus that we know in digital cinema (and from the distances typical in theatre environments), this has to be a piece of optical top design. While we call it the projection lens, it is actually built up of multiple small lenses, each contributing to the overall performance. Like the other passive optical components described above, lens design also depends largely on choice of materials and careful coating of interfaces. State-of-the-art lenses yield an overall efficiency of about 85%.

2.6 Beyond the projector

Most people neglect the impact of parameters that influence the light after it has left the projector. This can go from dirty porthole windows to dirty screens (with low or high gain). Now that you understand what design and work goes into getting the most out of your projector, you should appreciate what a waste it is to loose your light on dirty material.

3 Putting it into numbers

In order to objectively quantify this, we typically use the unit lumen per watt (lm/W): the light output (in lumens) divided by the electrical power input (in watt). This unit helps taking a holistic approach, making it possible to compare across technologies and across brands.

A typical digital cinema lamp (only the lamp!) achieves 40-50lm/W. A typical digital cinema projector (everything described above + lamp) achieves 4-5 lm/W. This means that all the contributions we discussed before yield on average an optical efficiency of 10%!

4 Conclusion

Many factors influence the optical performance of your projector. Even more, we hope you understand that designing a projector is an art as well as a science. It takes know-how and experience in optics, electronics, mechanics and, cooling techniques to design a top class projector.

When selecting your projector, be sure to inquire on how the supplier took into account these aspects. Choosing a brand that cares for all of them will make your projector a high-quality, high-performance and safe choice for the future!

Dr. Ir. Tom Bert
Product Marketing ManagerBarco logo

Based on the inputs from: Rik Defever, Peter Janssens, Nico Coulier

Barco Digital Cinema

Noordlaan 5
8500 Kortrijk

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