The display is that which will allow the audience to see the image. When speaking about projection, this will include both the projector and the display surface. There are a variety of methods and display devices ranging from reflective (light reflecting off the surface) and transmissive (light transmitting through the surface) to emissive (light being emitted from the surface) technologies. Even though some of the types of equipment may seem archaic, all are important beyond their historical relevance as they may have continuing use in modern productions.
At first, we will look at the surface itself. The portion of the system that is often neglected, or at least not often considered, is the surface to be projected upon. This could have some impact on emissive displays, but primarily this section will be dedicated to the discussion of the surface for projectors. Generally, a production will use whatever surface is available. Understanding how this affects the image will greatly improve the design. It should be noted that light emanating from the projection surface will always be less than the luminous flux being transmitted from the light engine of a projector. Knowing how much light is lost is critical to the design. As can be seen in Figure 4.1, in a demonstration by Dataton (maker of Watchout software), images can be effectively combined with different surfaces to create a single display (mixture of LED panels and projection on various geometric surfaces).
Reflective and emissive surfaces.
Source: Dataton Watchout
SCREEN PROPERTIES
In order to best choose your projection surface, or the best use one that has already been acquired, the designer should understand how to read the design of the material. The importance of each variable will hold different weight depending on the content to be displayed and where the audience will be situated. When you are purchasing a new screen, these variables will be readily available from the manufacturer. Unfortunately, if you have previously acquired material or are using a non-traditional surface, many of these variables will be unknown. With the right measurement equipment, some information can be identified.
Gain
It may seem obvious, but it should be stated that the projection screen is a passive element of the system. It does not add light. Instead, a manufactured screen will have controlled optical properties in order to prevent light from becoming fully diffused, which dramatically lessens the amount of light given to the viewer. The measurement of light being transmitted perpendicular from the center of the screen, divided by the luminance of the projector, is known as gain. As has been stated, the human eye is most sensitive to changes in light to dark; having an image that is the correct brightness will be an essential part of the design. If the surface does not transmit that light to the audience, then other parts of the design will need to compensate, potentially to the detriment of the production.
Gain is always used to describe front projection. For rear projection surfaces, gain is known as transmittance, though the calibrated standard is based on the front projection luminance factor equal to one. When a manufacturer is to properly measure a surface, there is a bit more than what a designer will do, but it is good to understand where these measurements come from. The British standard for gain measurement (BS 5382; 1976) states that “a freshly cut surface of magnesium carbonate should be placed centrally and parallel to the surface of the test sample, with the room pitch black and the projector with its luminance at the center of the block measured from a horizontal plane at the center, but at 5 degree horizontal angle from perpendicular. After this measurement is taken, then the block is removed and the screen surface is measured. The resultant measurement is the gain” (L2/L1*100%). As humans perceive a difference in brightness when it is reduced by more than 50%, calculating where the ½ angle of a screen surface is recommended. This is the point from center where the gain is halved.
Contrast Ratio
Display devices will have a contrast ratio associated with them, determined by the amount of light produced versus the amount of light prevented from being shown. A contrast ratio is that comparison between the brightest white and the darkest black of the image. In order to calculate this according to American National Standards Institute (ANSI) standards, a measurement of a checkerboard of 16 alternating black and white rectangles is measured. The ratio is then determined through the average of the white measurements to the average of the black measurements taken from the center of each rectangle. Gain is adversely affected by ambient light as this raises the average of the black levels; thus, a strict control of light spill on the surface is imperative. The lower the eventual contrast, the more difficult it will be for the audience to perceive the image, especially in imagery that is more dependent on that contrast, such as photography. It allows for a separation of tones within the image. A traditional projection screen will be designed with methods of enhancing contrast.
ANSI contrast checkerboard.
When projecting an image onto a screen, the contrast ratio will be the combination of the projector contrast, the screen contrast, and ambient light on the screen. The greater the contrast ratio in the screen, the less overall light will be returned to the audience, but also it reduces the amount of ambient light interference. It cannot be stressed enough that brightness is the most important part of the human vision system. Improper control of contrast will result in a muted image which will lose the detail necessary for content to be accurately viewed. There is no way to project black; it is simply the absence of light. Any light present, whether from ambient light or poor contrast ratio within the system, will degrade the image.
Contrast is all about information (details of image) and emotion (high contrast can have a different feel and mood than low contrast). It should be noted that there is no target contrast ratio for all content. The system can have a much lower ultimate ratio for general viewership than for watching a motion picture in a cinematic environment. For instance, viewing a presentation in a classroom or at a conference requires a much lower contrast ratio (often targeted at around 15:1) than in the theater, where we often look to see fine detail (50:1 is optimum). These are a far cry from what is often required in cinema with an extreme control over light on the screen, setting the contrast ratio to 80:1. While there will be no standards police coming after the designer of a theatrical performance for not meeting the standards set forth, audiences and critics can be very unforgiving. If the contrast is too low, it is known as flat or soft, which can appear to be less sharp of an image even when in perfect focus. Meanwhile, too high of a contrast is known as hard, which can be jarring and disruptive to some images.
Viewing Angle
Knowing where the audience will be viewing the image from is a crucial part of the design. A viewing angle is determined from the center point of the screen in either the horizontal or vertical axis. This is specified in order to guide the optimum brightness from the screen center to either side. In a cinema situation, where the audience is mostly sitting directly in front of the screen, below the level of the projector, the viewing angle does not need to be very great. Conversely, if the audience is situated in a more traditional, radiating pattern, the viewing angle will need to be greater. The consequence of the wrong viewing angle is that the perceived brightness of the image will not be equal to all parts of the audience or that brightness is wasted, potentially contributing to a decrease in contrast due to adding to ambient light.
With a formal projection screen, the viewing angle will be calculated by the manufacturer, giving the designer known variables to work with. This is based on the uniformity of the surface. It will be described as the angle from center in a horizontal plane. As most screens are designed with the audience being in a plane generally no greater than +/–10% vertical, there is no intent for a vertical viewing angle. This could have an impact on a designer in a theater with one or more balconies.
Texture
Screens will be textured to enhance certain characteristics of light. Textures will include lenticular and Fresnel type surfaces, which help to direct the light in particular ways, and at times assist in the rejection of ambient light. The texture will be directionally specific, requiring the screen to be put in a specific orientation to benefit from that texture. Artistically choosing to orient the screen in a different fashion could result in undesirable effects, as noted in the viewing angle designed into particular surfaces.
In addition, textures such as Fresnel were more common on rigid rear projection surfaces than temporary flexible screens. They are most effective for older rear screen display technologies, which needed greater increase in light transmission. Lenticular and polarized screens are finding favor in designs intended for 3D display. In some office settings, screens are textured to have an increase in ambient light rejection. This is primarily for light coming from above, which is usual in that type of setting. The texture of older, high gain screens can have a negative effect when using high resolution projectors. This can cause a specular aberration, where spots appear throughout the image through the interference of light, as discussed in Chapter 1.
REAR OR FRONT PROJECTION
The majority of projection will be front projection, or having the projector on the same side of the projection surface as the audience. This is often required when the surface is something other than a traditional projection screen. There are benefits and challenges to both methods of displaying the image. Considerations need to include how the surface presents the image to the audience. When looking at a traditional screen, there are a variety of known variables to allow the projection designer to understand how the image should look based on strict calculations. As soon as other theatrical fabrics and set pieces are introduced, the variables become unknown and the designer must build additional flexibility into the system to compensate. As can be seen in Figure 4.3 below, the image is drastically different depending on which material it is on and how much ambient light it is competing with.
Rear Projection
When you are using a rear projection screen, the projector will be situated on the opposite side of the surface with respect to the audience. The light from the projector will transmit through the surface to the audience, resulting in a certain amount of light loss as a portion is reflected back towards the projector. The contrast ratio with a rear screen is often better than that of front projection, as it allows light to transmit in both directions. This allows much of the ambient light to pass through the screen, which interferes less with the black levels of the projected image. Another advantage is that the projectionist can at times be a little less precise with the edges of the image, which can be blocked by the frame or soft goods, making setup time faster.
Examples of different screen materials with a projected image.
On the other hand, there can be some distinct disadvantages. The biggest issue that plagues this form of display is the amount of unobstructed space required behind the surface. There are a few ways of handling this using front surface mirrors, but small performance areas will always find this challenging (more on this later). Depending on screen material, a rear projected image may suffer from a “hot spot”, where the image in the direct path of the projector is bright, and where the remainder of the field diminishes greatly. This can also greatly reduce the viewing angle. Manufactured screens suffer much less from this than alternate materials do. Additionally, positioning of the projector can be more challenging, as short throw lenses have difficulty in being off axis and maintaining an image in focus across the surface.
Front Projection
As previously mentioned, the use of a front projection screen is more common. This method requires the consideration of many factors. Ambient light is the greatest concern for the front projected image. As the front projection surface is designed to reflect light, it cannot differentiate between “good” light and “bad” light. The texture of the surface may help in environments such as the classroom, where the majority of the light is coming from an overhead source. However, ambient light rejecting screens have great difficulty compensating for the multitude of sources in the theater.
A front projection screen can have a much greater range of viewing angles than a rear screen and has the benefit of uniformity in the extent of the image. There are also very specialized screens, including polarized and wavelength rejecting, which aid in enhancing the image and adding to the contrast or even promoting 3D projection. It must be understood that the entirety of the image will be seen by the audience, so great care must be taken by the designer to deal with an image that may not entirely fit the surface, and the accuracy of the projectionist, to accomplish the design.
NON-TRADITIONAL MATERIALS
As opposed to designs for corporate events and other standardized designs, theatrical productions often utilize non-screen materials as the projection surface. This is one of the benefits to using projectors in a media design, as almost anything can be used for the surface. When using non-traditional materials, the designer must be very aware of the physical properties of light in order to realize the design and prepare for the needs of optimizing the image on site. As can be seen in Figure 4.3, the difference in ambient light can change how the image looks.
Some theatrical fabrics will have some known properties in how they handle light. Generally, these properties will often be descriptive of how much light they absorb. These fabrics are not likely to be the projection surface, but may surround the projection area. An example is commando cloth.
The most common material to be projected on will be the muslin drop, often referred to as a cyc (though distinctly different from a traditional cyclorama). It will often be an off-white material, though it has been known to have a light blue hue as well, so color correction may be necessary in the image. Similar to some large screens, the muslin drop can often be made without seams for smaller venues. The material has a regular diffusion across its surface, which has the benefit of a wide viewing angle both vertically and horizontally, but also means that it requires a fairly bright projector in order to have a quality image. As a large surface, it will rarely conform to the aspect ratio of a video projector, so the designer will need to make the decision on whether to attempt to fill the entire surface, which may require some advance techniques, alter the image to allow it to blend into the non-projected areas, or request that carpentry mask it to the size and aspect of the projected image.
Another common fabric will be the theatrical scrim. This material does not have a solid surface, but is a type of mesh. While this offers some challenges when using it, there are some unique advantages. It allows for a front projected image to be displayed, while having the potential for an object to be seen behind it, as seen in Figure 4.4. This effect will require working closely with the lighting designer to pull it off correctly. As this is often used as a lighting effect material, the lighting designer should be able to easily accommodate this request.
Projection on a scrim.
Source: Maureen Selwood
Pretty much any material will be able to be used as a front projection surface, as long as the designer understands how well it reflects and diffuses the light and can compensate with bright enough projectors. Some materials will require more trial and error than others, as there will be a discovery of the material properties.
VIDEO AND LED WALLS
In addition to projection surfaces, a variety of emissive displays are a part of media design. These are displays that are not dependent on a surface to display an image, as the surface is what emits the image. There are several different types of displays used for this purpose and there are variations between them.
The monitor is what most people are most familiar with, as it is a part of computer systems used in daily life. In addition to monitors, the next most recognizable emissive display is the LED wall. These are a staple of many music concerts because of their brightness, but can also be seen in a variety of theatrical productions. More detail will be given later about each of these technologies and how to use them.
Not all display sources are equal. Without side by side comparison, the layperson may not be able to detect a difference. In fact, depending on some of the other design factors, it could be difficult for the expert to distinguish between them even when they are side by side. Regardless, as with the projection surface, there are distinct differences that can greatly affect the outcome. Understanding how the image is created can help the designer create an amazing visual experience.
Example of video wall as digital signage.
Monitor
While the monitor is seen by most of us on a daily basis, there are distinctions among them. A television is a specialized monitor that includes either an analog or a digital tuner to allow for selecting channels. It may or may not be of use to the media designer. In an analog system, this was a variance in the radio frequency (RF) spectrum. This has been altered under the Advanced Television Systems Committee (ATSC), which oversees the transmission of digital television. This will likely be of little importance to the theatrical environment outside of CCTV (closed circuit television is used for monitoring activity in the performance area). What is good to know is that outside of the tuner, the television is just like other monitors that accept some form of video signal and display that on the front surface.
There are differences in how these are created, such as LED, OLED, LCD, plasma (mostly phased out), and the much older CRT. Some will be back lit while others are sidelit. Sidelit monitors will almost never be desired due to the color shift if the surface is not viewed from directly in front. Surfaces will most often be flat, but they could be curved: convex for older CRTs or concave for some newer LEDs. Each will have different benefits and drawbacks which should be considered, but we can only touch the surface (no pun intended), as there are too many variables to discuss in detail here. Common to almost all monitors is glare, which can possibly be avoided when working with the lighting designer.
The CRT was the standard monitor used under NTSC. It was the traditional TV all the way back to the beginning of broadcast television. As these are no longer manufactured, if a designer needs one for a production, he or she will likely be looking at second hand electronics stores or similar situations. Until the past few years, many production studios continued to use them, as they had the best color reference. It is safe to say that this is no longer the case. The CRT should probably not be used unless necessary as they are bulky and heavy, and have a very limited resolution. As all units will be used at this point, it will likely be harder to find units of matching quality.
Another technology that is out of production is the plasma monitor. The surface of these monitors was individual RGB pixels which contained a rare gas that, when excited with electricity, produced light (similar to a fluorescent light). While these had a superior image and rich blacks and wide viewing angles, they had a rough time competing for a few reasons. These monitors were heavy, similar to CRTs, especially in comparison to LED monitors. They were also very fragile in that the slightest crack could allow the gasses to escape, making the monitor useless. Even when they were operational, they often suffered screen burn, where an image that was represented for a long time would leave a ghostly image when changed. To compensate for this, some monitors would shift the pixels of the image on a regular basis, which could be a distraction.
Liquid Crystal Display (LCD) monitors are the most common in use today. These either are sidelit, using a fluorescent light source, or are LED backlit, having the light pass through a liquid crystal medium with RGB filters. The LCDs are very thin and are voltage regulated as to how much light passes through each individual pixel. They can have good color representation, but they can suffer with blacks. Similar to a projected image, you have to contend with video black as they will always emanate some light. Manufacturers are constantly improving this technology, and there are some reference quality LCD monitors available. These tend to have a limited viewing angle, especially those which are sidelit.
Video black with a sidelit monitor.
A technology that is also being improved upon and which will likely become more popular is the Organic Light Emitting Diode (OLED). This is a thinner film display which has the potential of replacing other LCDs in the future. This is similar to the plasma monitor in that when electricity is applied, the OLEDs phosphoresce (glow) and provide their own illumination. This means that they can provide truer blacks and offer much better contrast. Since they do not require an extra source of illumination, these are lighter and thinner monitors. As they continue to develop, they are also becoming much more flexible, which could mean that in the future they will be able to be applied to a surface like wallpaper.
Currently, monitors are set in size as individual units. They can be combined when creating a larger display, as seen in Figure 4.5; this is commonly known as a video wall. When creating a video wall, the choice of monitor will generally include the size of bezel, or the “picture frame” surrounding the display area. Monitors which are designed with the purpose of being part of a video wall will have extremely narrow bezels, minimizing the mullion (gap between active display areas), while consumer televisions are likely to have much thicker bezels. In addition, professional models intended to be used as part of a video wall will often have built into their firmware the ability to deconstruct an image to display only a portion, allowing for the total wall to show the complete image. Monitors not designed with this purpose will require additional hardware or software to complete this task. Of course, even if the monitor has the ability to work as part of a video wall on its own, the designer could choose an external hardware or software in order to better meet the needs of the design. Many of these technologies are found in the digital signage market for retail display operations.
Rear Projection
In the era where the CRT reigned supreme, large display monitors were impractical. To increase the size of the display, rear projection televisions were created. These continued in popularity for a while even when much larger flat panel displays were available, as they were generally considerably less expensive. While these have mostly fallen out of favor as other monitor technologies have surpassed the older technology in quality and cost, there are a couple of modern examples which are still being used.
Rear Projection Cube
While rear projection televisions lost favor in the consumer market for their bulk and quality of image, newer versions made their way into the display market. These displays are intended to work together as part of a complete system as opposed to individually, as their predecessors were. Each cabinet will be uniformly designed in order to stack together, similar to other video walls. They all contain their own short throw projector, with all the advantages and disadvantages of such. In addition, their circuitry can allow them to communicate with their neighbors and help match brightness through internal luminance sensors. Generally, they will run on solid state lighting technology to grant them the many hours necessary to run continuously. Also, as they are using projection technology, they can have a very high pixel density, benefitting designs where the audience could be near the display. Also, the mullion is nearly nonexistent, as little as 0.7 mm for some models, giving them a much greater advantage over other video walls.
Christie’s MicroTiles is a popular rear projection cube.
Source: Christie Digital Systems
Laser Phosphor Display
Another re-envisioned technology is the laser excitation phosphor display. Older CRT technology used an electron gun to activate phosphors embedded on a screen to create images. A newer technology, Laser Phosphor Display (LPD) replaces the cathode tube with laser diodes in a similar fashion for displaying images on a phosphor embedded screen. These units are much slimmer and lighter than their CRT ancestors, and also use considerably less power per given area of the display. A significant advantage of the LPD is their size scalability. The modular nature of the laser light engine allows for establishing an end use case determined display of any screen size, by merely racking and stacking an array of light engines. These have mainly been used to create video walls for digital signage or use in control rooms, but could be of unique interest for a designer who may be looking for a long-lasting, large-format solution within an entertainment venue.
Prysm LPD tiles.
Source: Prysm, Inc.
Panel
When the designer wishes to create a larger, more seamless display, the choice is generally to use an array of LED panels, creating what is known as a LED wall. These panels are either solid or open spaced and will have the individual LEDs spaced at regular intervals. The benefit is that they can be configured in many different orientations to give the designer exactly the size of display desired. This is generally a very expensive option when looking to create a large image, as compared to projection, but has the ability to compete with high amounts of ambient light. This will often even allow for daytime use.
The spacing between the LEDs is known as pixel pitch. The lower the number, the more tightly they are spaced, granting higher resolution. Tighter pixel pitch will be required the closer the audience is to the display. The greater distance that the viewer is from the display, the more space can be allowed between pixels, and the perception will be the same. In addition, the type of image that is being created may determine the pixel pitch. The more detail required, such as photographic representation, the tighter the pitch required. Each pixel of a panel is made up of multiple LED sources covered with a lens. As seen in Figure 4.9, sometimes individual colors are doubled in order to better represent color as a whole.
Video walls differ from LED walls in that the LED panels are only designed to work together to create a display. While the panels will have a set number of pixels, these do not correspond to specific computer resolutions. When combined together, they can make common resolutions or can greatly depart from them to fill the image space as envisioned in the design. The display in Figure 4.10 shows a 1920x1080 output with a white display (note that some were still being color matched) and the top portion with the color bars is just as wide, but is not 1080 high. This required multiple media servers to create one image.
Pixel pitch.
Outside of the performance space, LED panels are often seen as digital billboards. This is where a designer can see the challenges of using them. In a long-running production or with rentals from companies who use them in many different situations, additional challenges can arise. In a projected display or video wall, when a portion of the image fails to display, it is usually single pixels. With the LED wall, entire sections of the panels will fail, leaving a cluster of pixels missing from the image (note the black area in the white image). These can be replaced. However, the replacement will have some differences from the rest of the wall (note the variance in white). In some cases, especially when used outdoors, the physical panel will be a different color, which is noticeable in areas of contrast (see the front section of the wall in Figure 4.11). In addition, the color of the diodes can be different, which require calibration to match colors to those of nearby panels. Depending on the manufacturer of the panels, there will be various levels of ability to color match and it may require considerable time to do so.
Example of LED wall.
The back and front of an LED wall.
As LED panels do not require any additional screen for the display to be seen, the viewer is looking directly at the light source. This allows additional flexibility in how the panels are designed. Often for indoor displays, and outdoor displays with a sturdy structure, the surface will be solid and not allow the audience to view what is behind the panels (as in the wall above). However, when designing for an outdoor production where a solid structure may have difficulty contending with air movement, many panels will have openings, which allow airflow. Obviously, this will cause some issue with what is seen behind, potentially requiring an additional light blocking surface behind the panels. On the other hand, some panels will have virtually no structure between pixels, creating an almost transparent image. Figure 4.12 is an excellent example of this type of panel, where you can clearly see the image but also see behind the panels. With the proper lighting, this could emulate the use of theatrical scrim.
LED wall with open space between pixels.
Another benefit that the LED wall will have over other video walls is that some panels will have flexibility in their surface. While a video wall can have separation in individual displays, creating a more dynamic and interesting video surface to view, they cannot truly fit a curve. With some LED panels, the surface will actually allow the panels to curve to create a more visually striking surface.
Transparent Display
A number of manufacturers have created displays which look like a window pane, in that they are visually transparent but also act as a video monitor. Although these were developed with retail situations in mind, they could have expanded use for special effect video in live performance. The designer will need to look at the benefit as compared to cost, as this could add considerably to the budget. However, effects similar to Pepper’s Ghost may be created without requiring a projector and screen, plus it will require considerably less space to accomplish the effect. On the other hand, display panel sizes are limited (note the division in panels in Figure 4.13).
Transparent display used in retail situation.
Electrowetting and Electrophoretic Technology
Once known only for handheld display devices for reading digital copies of books and magazines, these technologies are getting bigger and may find their way into the live performance market once the price and availability are right. While these are mainly passive reflective devices, not actively producing light, they may be able to be sidelit or backlit. As they are not natively producing light, this technology would require working with the lighting department to make the image seen. They have a good viewing angle, similar to paper. These technologies could truly replace painted backdrops.
How electrophoretic technology works.
Source: E Ink Corporation
Electrophoretic technology is commonly known as E ink or electronic ink. It has its roots back in the 1970s when researchers from manufacturers such as Xerox were looking at an electronic alternative to ink on paper. Individual cells have particles that are positively or negatively charged, which are moved around by a charge beneath them. This can even provide split cells if different charges are placed on different halves of each cell. The technology is also known as bistable, meaning that the image on the surface remains when power is removed; the only use of power is during the change of the image. For this reason, electronic book readers that use E ink displays will have a much better battery life than the ones that use LED or LCD displays.
E ink is most often seen as a two pigment system, creating a black and white (not monochrome) image. This technology cannot provide video, but still images could be mixed with appropriate lighting to create a landscape. It can be applied to very thin flexible display film which may provide other opportunities to add texture. This is limited to paper-size surfaces at the moment, but that would not preclude its use, as it could likely work its way into props for magical effects. There are offerings for color E ink, but at this time this is only suited for static images, as it has a very slow refresh rate and the entire surface must change at once.
A similar type of reflective display is through the use of colored oils in a process called electrowetting. As opposed to using positive and negative charges, it uses varying voltage between a liquid and an electrode beneath a hydrophobic insulator. As this is a reflective surface, this technology uses subtractive color mixing using CMY (cyan, magenta, and yellow), starting with a white surface and working towards black as the colors are applied. This is the process of color printing. Each cell will contain the three different layers of colored oils. It uses the physical principal of surface tension of each of the liquids. By applying various voltages to the layers, the oils contract or spread out across the cell, allowing for color mixing. Similar in design to LED panels, at least one European manufacturer is making this technology with the ability to combine multiple panels into large displays that can be used as billboards, even covering the sides of buildings, at about 1/100th the power consumption. It provides rich colors and has refresh speeds fast enough for use for video. As this is a new technology, it may be cost prohibitive to develop into theatrical use at this time, but will certainly find its place in the near future.
How electrowetting technology works.
Source: Etulipa