How Does Rear Projection Work? Exploring the Technology Behind Rear Projection

Projectors are designed to use one of two types of projector screens. In this case, the placement of these projectors or beam sources can either be in front or behind the screen. This configuration gives us front and rear projection screens respectively. In this article, we cover everything that you need to know about rear projection.

What is Rear Projection Technology?

To understand rear projection technology, you need to have an idea of how projectors work. A projection setup contains two major components. 

  • Projector or beam source – A projector is a device that projects an image onto a screen. This image is constructed using specialized optical equipment that is housed by the projector. These devices can range from simple to complex depending on the requirements.
  • Screen – This is the display screen where the light containing the image information is displayed. Projector screens also vary in design and specifications based on the modalities and use cases.

Apart from these two components, other peripheral devices support the primary components in the projection.

During projection, the projector needs to be strategically positioned concerning the position and direction of the screen. In other words, it is the basics of projection and it applies regardless of the projection technology used.

When the projector is placed or positioned behind the projector screen, this is referred to as rear projection. This can be done for various reasons, but the main reason is to ensure that the projector is not blocking the viewer’s line of sight.

In this approach, the projector is mounted to the ceiling, and the screen is placed such that the light beam from the projector hits it from behind. This might require some firmware to make sure the image comes out correctly when looking at the screen. Rear projection provides large and high-quality images.

There are two types of rear projection setups — passive or active.

Passive rear projection setup

In this setup, the screen is coated with a reflective material. For the viewer to see the image created in this setup, the projector shines light at the screen, which is then bounced back by the reflective material to the viewer. In other words, the reflective material acts as a light amplifier. 

One downside of this system is that the image might appear dimmed if there are no enhancements made. On the flip side, it is a less expensive method when compared to active projection setups.

Active rear projection setups

This setup has an integrated light source on the screen. This is the light used to amplify the image coming from the projector. Objectively, active projection systems are more efficient at projection than passive systems.

Active rear projection setups end up producing better images because of the integrated light source. Therefore, the display looks brighter and more detailed compared to passive setups. This convenience is also reflected in the cost, meaning active setups are more expensive. Besides, they are more difficult to install and set up as well.

How Does Rear Projection Work?

The mechanism of this technology is quite straightforward — a projector placed behind a screen to display images. While this is self-explanatory, there are a lot of intricate processes that happen inside the projector for an image to be produced.

You need to understand the core concept of projection for you to understand rear projection. A projector is a device that converts image information into light beams and displays it on a flat surface that constructs the image in two or three dimensions.

The optical components of the projector are responsible for creating the beam of light that is produced by the projector. This process happens through the reflection of light. Then, a special type of lens focuses the light on the screen in a beam which now creates the image. Many components have to work together for the setup to produce an image.

Components of a Rear Projection Setup

As we said, a rear projection setup has several components that work together to produce an image on the screen. Let us discuss some of the major components.

Rear projection screen

Without a screen, the beam of light from the projector would not be visible. Therefore, this is a critical component of the setup. 

While some projectors can project an image on any surface, the quality of the output would look different than if there was a screen. For instance, the lack of a light reflector means that the images will appear even darker.

Rear projection screens come in different sizes and aspect ratios. This diversity is important since it ensures that you can project content that has different aspect ratios. They can also be formulated from various materials so you might have to consider this when choosing a projector screen.

Data projector

This is the main projector that is positioned behind the screen. A projector acts as a light signal emitter. It has a processor that is programmed to function like how you would expect a projector to function.

Projectors vary in terms of size and resolution. This is seen in the cost of the device, with larger and higher resolution projectors costing more than their low-tier counterparts. Also, you will find them from different brands, each having a variety of features, both basic and advanced.

Video Source

For a projector to work, it needs to be fed information that it converts into light information. In most cases, this information is stored in conventional memory equipment such as discs and other multimedia players. These video sources have to encode the video information in a format that the projector can decode. This is important to avoid incompatibility issues. 

Computer

Modern rear projection setups have a computer. The function of this camera could range from controlling the output to everything in between. Also, this computer could be used to house the video or image projected from the projector.

Common Terminologies Related to Rear Projection

Still, on the subject of understanding the ins and outs of rear projection, there are several terms you need to learn. You will encounter these terms a lot when dealing with anything related to rear projection. Let us expound on them.

Keystone correction 

Keystone correction is the process of adjusting the shape of a projected image so that it is rectangular. This is necessary when the projector is not directly in front of the screen, or when the screen is not flat. Keystone correction can be done manually, or automatically using the projector’s built-in controls.

Throw distance

In layman’s terms, you can think of the distance at which the image is thrown. Of course, projectors do not throw images, but thinking of it like this will make things easier. The technical description is the distance between the screen and the projector.

Throw distance can be short or long depending on the installation of the components. Also, this depends on the type of projector. This means that projectors can either have a long or short throw distance. Therefore, the installation takes this aspect into account. 

To calculate the throw distance, you will need to know the following information:

  • Screen size – The width and height of the screen
  • Projector lens: Projector focal length in millimeters.

Using screen size and projector lens, we can find the throw distance quite easily. The following formula is usually used.

Throw distance = (screen height x screen height) / projector lens

Screen gain

This is another term you should keep in mind about projectors. It is defined as the measure of lumen that hits back at the viewer. A screen can have a low or high screen gain. 

Screen gain is an important aspect because it is used to control the visibility of the screen in different ambient lighting conditions. Low-gain screens are used in darker rooms while high-gain screens can be used in brightly-lit rooms.

Use this table to compare the level of screen gain that would be appropriate based on the lighting conditions of a room.

Ambient lighting level Recommended screen gain
Darkroom 1.0 – 1.5
Moderately-lit room 1.5 – 2.0
Bright room 2.0 – 2.5

 

Hotspotting

One thing you need to understand is that hotspoting is a problem. Also, this problem is mostly associated with rear projection screens. 

In simple terms, hot-spotting is a problem that occurs when light from a screen is concentrated on one spot. This leads to a bright spot. The reason why this is a problem is that the content on the spot is usually hard to see clearly.

Many factors can lead to this phenomenon, one of them being the type of projector being used. Also, the position and type of screen used can contribute to hot spotting.

Hotspotting is corrected by using projectors that have a high screen gain and also making sure the projector does not face brightly lit spots like windows or other openings.

Ambient light

Ambient light is a common phenomenon that is used when display technologies are being discussed. It means the amount of light or lack of it in the room where the projector is hoisted. This light can interfere with the quality of the display. 

Too much ambient light will affect how bright the projector appears while low ambient light makes the content look good. This is why you find that most rooms where the projector is installed are pitch dark when the projector is turned off.

Common Projector Technologies 

Rear projection is highly influenced by projector technologies. These are the schemes that different types of projectors use to create an image. Let us discuss three of the most common types of projector technologies.

DLP, LCoS, and LCD are the three common types of projector technologies. 

DLP (Digital Light Processing)

As the name implies, digital lighting processing is a type of projector technology that uses digital components to produce light. More specifically, DLP uses a tiny micromirror to create an image. The main aspect that controls this is the arrangement of the mirror.

In modern DLP projectors, these mirrors are arranged in a special grid to enable individual control of them. As a result, there is more control of the image produced, a factor that resultantly contributes to better color accuracy among other things. 

LCD (Liquid Crystal Display)

Everything you know about LCDs is what this type of projector technology implies. Like the mirrors in DLP projectors, the liquid crystals in this projector are arranged in a grid matrix. While the control of these crystals is similar to that of DLP, you cannot control the efficiency of mirrors to that of crystals. This means that DLP is more effective than LCD.

LCoS (Liquid Crystal on Silicon)

Have you ever heard of hybrid systems? LCoS is a hybrid system that combines both DLP and LCD projector technologies in one. In LCoS projectors, the mirrors are usually made of crystal elements. Therefore, the projector ends up being as color-accurate as a DLP while also being as bright as an LCD projector.

Rear Projection Screens

The type of screen used in rear projection is quite important. As such, you need to be deliberate when choosing these screens. It is important to understand the features of these screens and the different types available on the market. 

Features of rear projection screens

Learning about the features of rear projection screens will help you understand this technology even better. These screens are specifically made to have particular features that you would typically not find in a regular type of screen.

  • They are translucent: You know what translucent means. It cannot be compared to transparent since only limited light passes through. Translucent screens enable the viewer to see the content without seeing behind the screen.
  • Diffusion: The addition of a diffusion coating on rear projection screens is a great touch. One of their main functions is to distribute light evenly produced by the projector. This results in a better viewable image.
  • They are either rigid or flexible: These screens are usually either rigid or flexible. This refers to the form factor. Rigidity and flexibility are influenced by the substrate material of the screen. In this case, the material used to make rigid screens is typically glass. On the other hand, flexible screens are made of vinyl or other flexible materials.
  • Variety of sizes and shapes: The size of the screen matters a lot. This is why there are different sizes and shapes of the screens. This also means that you can choose a screen based on the specific use case to ensure there is a match.
  • Home and commercial applications: Projectors can be used for both commercial and recreational purposes. To account for this, manufacturers make sure that there are grades that are meant for home and commercial. To achieve this, they have to factor in longevity and design during the manufacturing process.

If you are on the lookout for projector screens, consider the following aspects and features that will help you find the most suitable screen.

  • Size: Size matters a lot. This will depend on the size of the room where you intend to mount the screen. 
  • Shape: For the shape, you will have to consider practicality and also the aesthetics. The two have to be accounted for to achieve good results.
  • Material: The material of the screen is quite an essential consideration. This will also help you decide whether you are going for a flexible or rigid screen.
  • Features: The added features of the screen are supposed to help you decide. For the most part, you should consider screens with features such as anti-glare and diffusion coatings. These features improve the viewing experience of the screen.

Types of Rear Projection Screens

The type of screen is also an important consideration to make. Let us discuss the different types of screens that you will come across.

Fixed screens

As the name implies, these screens are usually fixed on a wall or ceiling. This is the installation procedure used. Fixed screens are usually made of flexible but also durable enough to sustain installation. They also have enhanced visibility.

Wall-mounted screens

Just like fixed screens, wall-mounted screens are like the name implies — they are mounted on a surface. This is done with the help of a bracket or a mounting bracket. This type of screen is usually good in case you want a temporary installation.

Pull-down screens

The good thing about these names is that they are quite descriptive. For instance, when you hear pull-down screens, you immediately imagine the action of pulling down. These screens are usually made of vinyl to reduce their weight. They are there for cases where space constraints are an issue.

Roll-up screens

The way to access this type of rear projection screen is to roll it up. This is different from other types of screens such as pull-downs. They can be made from any material as long as it is durable enough to sustain long-life usage. The good thing about these screens is that you do not have to worry about space since they can fit in any place.

Ceiling-mounted screens

The screen is also a great place to install a rear projection screen. This is of course if it is practicable based on where it is installed. The installation of this type of screen will depend on your installer. In some cases, you might be forced to angle it on the ceiling so that it can be visible. Either way, this is also a great way to maximize the use of space.

DIY Rear Projector Screen

Did you know that you can also make your type of projection screen? This is the DIY or do-it-yourself approach. For this, you can use available materials such as fabric and a frame to make a screen from scratch. Alternatively, you can buy screen equipment and assemble them in the form factor of a screen.

Factors to Consider When Investing in Rear Projection Setups

It is important to have a set of factors to consider when you are in the market for rear projection setups. Some of these factors include;

  • Intended use case: What have you planned to use the screen for? Is it for home or commercial use? These are some of the questions you need to ask yourself before embarking on this journey.
  • Layout of the space: The layout means the interior design with respect to the space and size of your interior. In most cases, the space is usually a major factor when contemplating the type of screen you want.
  • Screen specifications: Different screens have different specifications. This includes the features and other aspects of the screen. The best screens have top-end features, but they are also expensive. You can opt for this type of screen if you are looking for the best viewing experience.
  • Projector technology: The projector technologies we discussed above will also be a factor that influences the type of screen you should get. This is due to compatibility issues.
  • Ambient light conditions: In case there is a lot of light to be expected in a certain condition, you need to consider its impact on the screen. This will help you choose a screen with the right screen gain to avoid poor images.
  • Budget: The budget is also an important factor. As you already know, the cost of a screen will depend on many factors. Therefore, you need to choose a screen based on these factors. 

Additional considerations:

  • Throw distance: The best screen is the one that has a good throw distance. This means that the screen can show images from the right distance without distorting them. 
  • Mounting options: Whether you are going for a fixed or retractable mount, you need to make sure that the screen supports it. To elaborate, some screen materials cannot handle fixed mounting since they might break. The same applies to retractable screen mounts.
  • Screen maintenance: From time to time, you will be required to service your projector screen. In this case, you will need to have adequate skills or have someone do it for you. The reason you need to consider this is to ensure that you can handle basic maintenance of any screen.

Why Rear Projection instead of Front Projection?

There is a lot of debate between front projection and rear projection. Here are some reasons why you should consider the rear instead of the front.

  • Brighter images. Rear projection screens are usually enhanced by filters and lighting systems to improve the image quality. This results in a bright display overall.
  • Better contrast. Contrast is quite important when viewing content on a large screen. It is what helps you differentiate between colors and shapes. Rear projection screens generally have better contrast than front projection screens.
  • Wider viewing angle. The viewing angle is another essential factor that needs to be considered when choosing a rear projector screen. It helps the viewer be able to clearly see what is on the screen from any position in the room.
  • Less expensive. In most cases, rear projection screens are less expensive than front projection screens. This is important since it means that you can get a lot of value for less.

Some of the drawbacks of rear projection screens include.

  • More space is required. You will find that you might need more space to set up a rear projection system than you would need for front systems. This could be an issue when trying to adapt the system.
  • More difficult to install and maintain. The installation and maintenance of rear projection screens require skilled personnel to do it right. This means that you will have to have these skills or hire someone who can. This could be a setback that can influence your decision-making.

Conclusion

Projectors are here to stay. They provide concrete solutions to people looking for a more efficient way to display content either for large or small audiences. Particularly, systems like rear projection improve the projection and make it a desirable setup. Therefore, if you are looking for a reliable projection setup, consider going with rear projection.

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