No matter the application—be it a mobile small business presentation, a multimedia conference room, a classroom, a sports venue, a church, a commercial space, or a medium to large-scale auditorium—the projection systems range from compact to cinematic in size, featuring either front projection or rear projection. There seems to be no third method of projection. Front and rear projection essentially encompass the entirety of projection technology.
So, "front projection" simply means that the projector is installed on the same side as the audience relative to the screen. The projector emits light that projects onto the screen, forming an image, which is then reflected back to the viewer's eyes.
Similarly, what is referred to as "rear projection" is when the installation position of the projector and the audience are on opposite sides of the screen. The light emitted by the projector shines straight from one side of the screen and passes through it to enter the audience's eyes. Front projection screens can be made in any size, but controlling ambient light is necessary for optimal viewing. They are particularly suitable for dark rooms with a large audience. Rear projection screens are smaller in size but do not require ambient light control. They are more appropriate for environments with fewer audience members and good lighting.
In terms of historical development, front projection has a long history, dating back to the era of movies and slides. Rear projection, however, has a relatively short history, spanning only the past 20 years. Nevertheless, rear projection has introduced a completely new concept to the world. It can project images with the same quality as television screens, and the size can be expanded hundreds of times larger than that of a TV.
Front and rear projection each have different applicable scenarios.
The ideal projection environment is one with minimal lighting and the use of very bright projectors. If the environment is too bright, front projection may appear dimmer, in which case, rear projection is the preferred choice. However, to optimize the quality of rear projection, the space required for installing a rear projection screen is larger than that for front projection, and the space for the projector must be sufficiently dark.
Front projection is suitable for situations where there is no space for rear projection. An impromptu, unprepared room is a great example of when to use front projection, such as during business presentations or regular meetings where a large group gathers suddenly in a smaller room. If the number of people is between 50 and 100, a smaller screen is sufficient, and there is no need for a larger one. "Front projection screens can be made in any size, and the advantage of front projection is that you can make it very, very large with just one screen, much like a movie theater screen." However, front projection requires turning off the lights in the projection room to achieve a comfortable image, similar to movie theaters, which need to control ambient light, making it highly suitable for dark rooms with many spectators.
Front projection is a preferred and effective projection method, but due to limitations in projection space, it's not suitable for many environments. Additionally, the size of rear-projection screens is limited, yet they do not require control over ambient light. They are particularly well-suited for environments with fewer viewers and good lighting. "The application conditions for rear projection are: there are more than 100 viewers, the presenter has enough space behind the screen to install the projector, and a full-size rear-projection screen can be installed."
Spaces like classrooms and multimedia conference rooms may not have the room required for rear-projection systems. However, many larger presentations, especially in large auditoriums or outdoors, utilize rear-projection screens due to the need for multiple suitable projectors. "For instance, in sports arenas, it's impossible to turn off the lights; bright lighting is necessary to see the game clearly. Most people in sports venues opt for rear-projection because it's the only way to produce clear, vibrant images." "For movie theaters, front projection is the obvious choice. It's a no-brainer since the screen sizes are so vast and unwieldy. It would be a waste of space and costly to provide a rear-projection room behind the movie screen." "In schools, since all that's needed during church services is to see the images, students have lower demands on the projection environment and image quality, and there's also the need to save space and equipment costs, so rear-projection is generally not chosen." If the budget is limited and the available space is small and narrow, front projection is a good option. Rear-projection requires more space, so if the client insists on a very wide and grand effect, like that of a movie theater, front projection is the only viable choice, as it can meet their needs with a single screen.
Technical Classification of Projector Screens
Screens are divided into two main categories: front projection screens (reflective) and rear projection screens (transmissive). Front projection screens are not limited by size (can be customized) but are affected by ambient light, causing a significant drop in image contrast. Rear projection screens have a stronger overall sense of the image, are not affected by ambient light, and can accurately reflect the quality of information, making the colors vibrant and images lifelike. Currently, large single optical rear projection screens can reach 200 inches, with an effective image area of 4034mm wide by 3018mm high.
Front-projection screens and rear-projection screens are divided into the following types based on optical structure:
Curved Screen: Metal Arc Screen, Flat Screen:珠光幕/Metal Soft Screen/White Screen
Front Projection Screens: Rigid Rear Projection Screen (Transmitting Screen), Soft Rear Projection Screen (Transmitting Screen)
Front projection screens have lower gain, wider viewing angles, and require lower ambient light. Curved screens have higher gain, narrower viewing angles, and can handle stronger ambient light, although the screen reflects incident light unevenly in various directions, and the high temperature generated by light focusing can easily cause color distortion in the image. They are categorized by material into glass screens, metal screens, embossed plastic screens, and so on.
Projector screens are categorized into optical rear-projection screens, diffused screens, and soft rear-projection screens, each with different imaging principles. These rear-projection screens overcome many drawbacks of traditional front-projection methods: the projector is installed in a dark room behind the screen, which does not disrupt interior decoration, shields the noise generated by the projector's operation, and is less susceptible to interference from ambient light.
Scatter-type screens distribute projection light by means of scatter particles, thus, scatter-type screens have the following obvious drawbacks:
Low light utilization efficiency; wasteful of projector brightness output
Color drift is severe, with very low contrast.
Limited viewing angle; not suitable for large-scale display.
Scatter-type screen has limited market applications.
Optical projection screens rely on a fine optical mechanism for the distribution of projection light energy and are currently recognized as an effective display method. The technology of Fresnel lenses is widely applied in the manufacturing process of optical screens, with the Fresnel lens structure capable of converging incident light into parallel beams, thereby enhancing the screen brightness within a certain viewing angle. The technology of cylindrical lenses is also extensively used in the production of optical screens, allowing for the control of light distribution in both horizontal and vertical directions through the screen's front cylindrical lens structure. This feature expands the viewing angle range. Customers can design the projection light path and screen focal point with a large angle, based on the actual application environment, to achieve a favorable light path coverage angle range.
The drawbacks of optical rear-projection screens include the necessity to choose a lens with a matching focal range to achieve image focus (which has requirements for projection distance). Most optical rear-projection screens offer a range of focal lengths, covering the majority of lens focal ranges for projectors.
The rear-projection soft screen, made by spraying a high-transparency film, offers advantages such as affordability, seamless (customizable in size), easy transportation and installation (can be folded), and no specific requirements for the projector's lens focal length. It is commonly used in the rental industry. However, due to the use of the light scattering imaging principle, the rear-projection soft screen inevitably has the drawbacks of a diffused screen: poor brightness uniformity, severe "sun effect," low light energy utilization, severe color drift, and a narrow viewing angle. The low resolution and low contrast make the rear-projection soft screen unsuitable for data display applications. Additionally, in terms of the physical properties of the manufacturing material, the soft rear-projection screen has the following disadvantages:
Stability is easily affected by temperature, humidity, and air currents, which can cause image jitters.
Easy to wrinkle, fade, deform, mildew, and age, with a brief quality guarantee period.
To ensure the field of view (typically around 160°), the gain is set relatively low (usually around 1.5).
The installation of rear-projection systems is relatively complex, while front-projection is simpler.
The installation of rear-projection systems is more complex, as not all projection environments have sufficient projection distance to achieve ideal results. "The installation process for rear-projection systems is a bit more intricate, involving wall design for the installation screen as well as the house structure suitable for transporting the rear-projection screen, such as door widths, elevators, etc. The installation also requires the operation of professionals, making it more complex."
Front projection is less limited by projection distance. As the concept of front projection suggests, the installation position of the projector in a front projection system is on the same side as the audience relative to the screen, so the installation space for the projector in a front projection system is already within the audio-visual space area, and no additional space is needed for placement. The installation and placement of projectors in a front projection system are also relatively simple; placing a projector in front of a screen is sufficient. Most people understand how to design such a system. The design is easy and intuitive.
The rear-projection distance can be less than that of front-projection.
If a projector directly projects an image onto the screen, the space required for both front and rear projection is the same. If a rear projection system employs a reflective system, the required space can be reduced. A typical example is that the thickness of a rear projection TV is only 50 to 60 inches.
The rear projection screen is transparent, while the front projection screen is not.
Front projection screens are white or gray and are opaque as they need to reflect light back into the audience's eyes. Rear projection screens are transparent, and there are two options: a slow-reflection screen and an optical screen, with different display effects. The optical screen absorbs light from the projector and evenly distributes it to the audience, with light evenly spread across the center and corners of the screen. A slow-reflection screen is a standard transparent plate or glass layer with added diffusing materials. The images displayed on a slow-reflection screen do not send light evenly to the audience's eyes but irregularly in all directions, resulting in a strong sun effect and darker corners.
Front projection is highly affected by environmental factors, while rear projection is less impacted.
The overall image effect of front projection is greatly influenced by ambient light and lighting; rear projection is not as affected. "The image seen in front projection is not just the light from the projector reflecting off the screen and into the viewer's eyes; it's a combination of one or more types of light. One is the light projected by the projector, reflected off the screen, and into the eyes; the other is the ambient light hitting the screen and reflecting into the eyes. Both are multiple-reflected lights. In front projection mode, if there's a light around the screen, its light will also hit the screen, reflecting off it into the eyes, which can affect the viewing quality of the image. Even if all the lights in the front projection environment are turned off, making the room pitch black, the light from the projector hitting the screen will still reflect off it onto the ceiling, which is white. This light will then reflect back onto the screen, affecting the image. The reflection principle of projector light is similar to the echo in sound, where the sound from a speaker hits a wall and reflects back to the listener's ears; the wall affects the sound quality. The reflection principle of the projector is similar, as it projects light onto the screen, which then reflects off the screen onto the wall and finally into the eyes, affecting the image quality. However, rear projection systems are not affected by this to the same extent, and even if there is some effect, it won't be as significant as in front projection."
Previous investments are not suitable for speakers standing in front of a screen.
When using front projection, there are some inconveniences during the demonstration and explanation process: the speaker cannot stand in front of the screen, as they would block the light and cast a shadow on it. Additionally, the front projection light can enter the speaker's eyes, which is very detrimental to their eyesight. Rear projection is not affected by these issues. Rear projection is often used in situations where a good visual effect is needed, or when there is someone speaking or conducting research in front. This includes conference rooms, control centers, audio-visual centers, and similar settings.
