Digital Cinema Projection Technologies

         Digital Cinema projectors are the last and most important component in the Digital Cinema system. In the 100 year history of cinema lot of advances came through in cameras, lenses, widescreen movies, audio and so on. But projectors remained unchanged for very long time. “Star Wars” movie was a trend setter. It was shot in digital camera and it was projected using digital projectors [1]. This was a real break through.  In the world there are around one lakh (100 thousand) cinema screens are there and around 6000 screens are converted into digital every year [2].  Projection technologies can be broadly classified into three categories; Digital Light Processing (DLP), Liquid Crystal on Silicon (LCoS) and Grating Light Valve (GLV). DLP technology from Texas Instruments (TI) is the only successful commercial product until up to 2007. It will be dealt in detail in this post.

Projection system is a combination of projector and cinema screen which is made up of silver halide. That is why movie theatres are referred as silver screens [citation reqd]. Light from the projector falls on the screen and incident light is reflected and it is viewed by patrons (a respectful term for movie goers). It is expected the entire projection system is in dark enclosure. Any stray light inside theatre will have a detrimental effect on projection system performance. But in TV light is emitted and room with minimal illumination is a requirement.

All projection system will contain following components; light source, light splitter, light modulator, light combiner and projection lens. For block diagrams of projection systems refer [2]. Normally light source will be Xenon arc lamp that can emit red to long wavelength blue with equal intensity. This lamps will consume 1200 W to 6500 W, operating voltage will be around 20 V to 33 V and current rating range from 60 A to 120 A. The power consumption of projection unit will be much higher than arc lamp. The life span of lamp will be around 2000 hours.  Laser is very power efficient but laser based GLV has to become commercially successful. Dichroic mirrors (wavelength selective) or similar devices split the light into red, green, and blue and send them to three light modulators. The light modulator is the real heart of the projector. It changes the incoming light amplitude depending upon pixel value. This operation is called modulation. As we know colour pixel is made up red, green and blue channels and each channel when viewed separately will look like a gray scale image. Thus if the incoming light is blue then output from light modulator will be from darkest blue to lightest blue. Modulated red, green and blue lights are overlaid by the light combiner (prism arrangement) to produce the desired image.  The projection lens will project the image into the screen. Everyone would have noticed that projection booth is above from the center of the screen. This means the projector sends the light downward to project it on the screen. This introduces  trapezoidal distortion to the image and it is rectified by using trapezoidal mask in the aperture of the projector [3].

Figure 1.  A typical theatre arrangement

1.  DLP Light modulator

It was developed by TI who is a leading player in semiconductor industry.  Jack Kilby who got Noble prize in 2000, for inventing Integrated Circuits (IC) has served Texas Instruments. Digital Micromirror Device (DMD) is the heart of DLP. It was developed in 1977 and in 1990 only it came out of R&D cocoon [1]. Size of the DMD will be around one square inch and it contains 13 lakh (1.3 million) aluminum micro mirrors. They are hinged to the CMOS surface of the IC and mirrors can be tilted 5000 times within a second. Three DMDs are used and each DMD is allotted to one primary colour (red, green, blue). One DMD with colour wheel can be used instead of 3-DMD arrangement. With this arrangement projected image quality and reliability of the system take a beating.

Figure 2.  Leg of an Antenna in the backdrop of DMD mirrors

Each DMD mirror occupies an area of 16 micro square metre.  The underlying CMOS circuits acts as an actuator and tilt the mirror by either +10 degrees or -10 degrees [4]. Thus when the mirror is tilted +10 degrees illumination light is turned away from projection lens. At -10 degrees illumination light is sent into the projection lens. Thus there is only ‘on’ and ‘off’ states only. With this kind of binary systems how colour images are projected onto the screen is the question that arises in our mind. Our human eye is fooled by flipping mirrors in three light modulators very quickly so that our integrating eye sees the colour. This process uses pulse width modulation. Manufacturers Barco and Christie  are licenced to produce TI DLP projection systems. NEC is also building digital cinema projectors using DLP.

2.  LCoS Light modulator

It sounds like LCD and it share lot of similarity between LCD. Earlier to LCoS transmissive LCD were experimented. Problems like colour shift, low contrast, large area of LCD chips hampered its candidacy for digital cinema [1].  In the LCoS a very thin layer of liquid crystal impressed with a voltage that is proportional to the pixel value. Depending upon the voltage the incoming light is polarized. Thus LCoS acts as a ‘window blinds'. Depending upon the polarization either light will be reflected fully or absorbed. If light is reflected fully then white colour is seen by the patrons. Backside of the LCoS contains the necessary circuits and it is opaque. Thus LCoS can only reflect light. LCoS is a perfect analog system as its polarization capability is directly to proportional to applied voltage. Electro-optical characteristics of liquid crystal used to vary across devices. Most importantly it is susceptible to temperature variations [5]. 

        JVC has developed Direct Drive Image Light Amplifier (D-ILA) and Sony has developed Silicon Xtal Reflective Device (SXRD). Both these are primarily LCoS based systems only. In [5] it is reported that SXRD based projector uses 4.2 kW lamp but DLP based Christie uses only 3.3 kW lamp to produce same amount of lumens. 

3.  GLV light modulator

It is developed by Silicon Light Machines/Cypress Semiconductors. Stanford University has played significant role in its development. It is made up of ribbons that are placed adjacent to each other and their ends are attached to device. Below them there is a substrate of transistors. With the use of transistors, ribbons can be flexed by fraction of wavelength. Thus a diffraction pattern is created. A 2K GLV will have only 1080 ribbons. Entire length of each ribbon has to be divided (virtually) into 2048 locations and each location should be scanned by laser. This will produce 2048 x 1080 pixels.

Note

• lakh is a Indian unit to represent 100 thousand
• Reference material’s bias is not eliminated.
• Presented information may not be up to date.

 Source

1. “Digital Cinema: The Revolution in Cinematography, Postproduction and Distribution”, by Brian McKernan, McGraw Hill, 2005.
2. Digital Cinema, Xilinx Corporation (PDF, 1537 KB, https://www.student.cs.uwaterloo.ca/~cs781/digital_cinema.pdf). 
3. “Understanding Digital Cinema: A professional Handbook”, Edited by Charles S. Swartz, Focal Press, 2005, ISBN: 0-240-80617-4.
4. G.P. Pinho, Optics of Digital Cinema, Christie Digital Systems. (PDF, 195 KB, https://www.student.cs.uwaterloo.ca/~cs781/PinhoDigitalCinemaTalk.pdf ).
5. Alen Koebel, A white paper on Digital Cinema projection, choosing the right technology, Christie Corporation. (PDF, 2641 KB).