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The Differences and the Connections of Orthographic Projection and Rear Projection

The Differences and the Connections of Orthographic Projection and Rear Projection
At present, whether it is a medium and large projection system for mobile small business presentations, multimedia conference rooms, classrooms, stadiums, churches, commercial venues, large auditoriums, virtual displays or even large screen projection systems in movie theaters, they either use orthographic projection method is either the rear projection method, otherwise there seems to be no third projection method. Front projection and rear projection can be said to be all of the projection technology.

Front projection, simply put, is that the projector is installed on the same side of the screen as the audience. The light from the projector is projected onto the screen, forming an image on the screen, and then the light is reflected back to the human eye.

Rear projection means that the installation position of the projector and the audience are located on the two sides of the screen. The light emitted by the projector is directed from the side of the screen to the screen, and the light enters the eyes of the audience through the screen.

The front projection screen can be made in any size, but you need to control the ambient light to get a good viewing effect. Very suitable for darkroom environments with many audiences. 

The rear projection screen is not as good in size as the front projection, but there is no need to control the ambient light. It is more suitable for environments with few audiences and good ambient light and lighting.

The rear projection solution brings a whole new concept to the world. Rear projection can project a picture with the same image quality as the TV, and the size can be expanded to hundreds of times that of the TV.

There Are Different Applications for Front and Rear Projection

The ideal projection environment is basically no lighting and use a very bright projector. If the ambient light is very bright, then the front projection will appear darker, then the rear projection is the best choice. However, in order to optimize the quality of the rear projection, when installing and using the rear projection screen, the required space is larger than that of the front projection, and the space for installing the projector must be dark enough.

Rear projection is a preferred choice and a better projection method. However, due to the limitation of projection space, many environments cannot use it. In addition, the rear projection screen has a limited size, but it does not need to control the ambient light. It is more suitable for the environment where the audience does not have much light. David Hull said: "The best application conditions for rear projection are: more than 100 people, the presenter has the space to install the projector behind the screen, and can install a complete rear projection screen.

Technical Classification of Projection Screen

The screen is divided into two categories from the front projection screen (reflective) and the rear projection screen (transmissive). 

The front projection screen is not limited by the size (customizable), but it will be seriously affected by the ambient light and cause a serious decrease in the contrast of the picture. 

The rear projection screen has a strong overall feeling, is not affected by ambient light, and can accurately reflect the information quality, so the picture is colorful and vivid, and the largest single optical rear projection screen currently has 200 inches, and the effective image range is 4034mm * high 3018mm.

Front projection screen and rear projection screen are divided into the following types from the optical structure:

Front projection screen: curved screen (metal arc screen), flat screen (bead screen / metal soft screen / pure white screen)
Rear projection screen: hard rear projection screen (transmission screen), soft rear projection screen (transmission screen)

The front projection flat screen has a small gain, a large viewing angle, and the ambient light must be weak; the arc screen has a large gain, a small viewing angle, and the ambient light can be strong, but the incident light reflected by the screen varies in all directions, and the light is focused the high temperature is easy to cause picture color distortion. It is divided into glass curtain, metal curtain, embossed plastic curtain and so on.

Rear projection screens are classified into optical rear projection screens, scattering screens and soft rear projection screens. 
These rear projection screens with different imaging principles have overcome the shortcomings of many front projection methods: the projector is installed in the darkroom behind the screen , It will not cause the adjustment of the decoration, and also shield the noise generated by the operation of the projector, and it is not easily interfered by the ambient light.

Rear Projection Installation is More Complicated, Front Projection is Relatively Simple

Rear projection installation is more complicated, because not any projection environment has enough projection distance to achieve the ideal projection effect. Stevie Ng said: "The installation process of the rear projection system is more complicated. This includes the design of the wall where the screen is installed, as well as a house structure suitable for handling the rear projection screen; such as the width of the door, the elevator, etc. The installation project also requires professionals It ’s more complicated to operate. "

Front projection has fewer restrictions on the projection distance. As the concept of front projection is said, the installation position of the projector used in the front projection system is on the same side of the audience as the screen, so the installation space of the projector of the front projection system is already within the area of the audio-visual space, and no additional space is required. The installation and placement of the projector in the front projection system is also relatively simple, just place a projector in front of a screen. Most people know how to design such a system. The design is easy and intuitive.

The Projection Distance of the Rear Projection Can Be Smaller Than the Front Projection

If the projector projects an image directly on the screen, the space required for front projection and rear projection is the same. If a reflective system is used in the rear projection system, the required space can be smaller. The most typical example is the thickness of the rear projection TV is only 50 to 60 inches.

Rear Projection Screen is Transparent, Front Projection Screen is Opaque

The front projection screen is white or gray, and because it reflects light back into the viewer's eyes, it is opaque. Rear projection screens are transparent. There are two options for rear projection screens. One is a slow reflection screen and the other is an optical screen. Their display effects are different. The optical screen absorbs the light from the projector and sends it to the audience evenly. The light is evenly distributed in the center and corners of the screen. The slow reflection screen is an ordinary transparent plastic sheet or glass layer, but some diffuse reflection material is added inside. The light displayed on the slow reflection screen is not evenly distributed to the eyes of the audience, but is irregularly divergent in all directions. The solar effect is very strong and the corners are dark.

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