Why use Fresnel for Rear-projection
A VisionScreen is not a traditional screen, but an advanced optical lens-structure, which enables you to control the projected image and project the light where it’s seen and create response.
Conventional front and rear projections screens spread the light in all directions – also into ceiling and down to the floor. VisionScreens enhance the image for optimum viewing by combining the focusing abilities of a Fresnel lens with the distribution of diffused surface or a lenticular lense.
The result is a brighter and more uniform image without “hot-spots” and dark corners than using a conventional diffusionscreen.
Contrast is important
Power of light is not all. Contrast is important to the perfect image - especially when using single lens projectors. For use in bright rooms and outdoor environments, VisionScreen.eu manufacture ultra contrast screens to enhancement the contrast and black levels of your image.
Depending on your application requirements, you have freedom to choose between different type of rear-projection screens to optimize features, such as:
· Enhanced contrast
· Enhanced brightness
· Enhanced image uniformity
· Anti-reflective or non-glare coatings
· Ultra-short built-in depths
Fresnel screens from DAF Screens support you installation, especially when short projection-distance is an issue.
Choosing the correct screen focal for the installation is essential for achieving a perfect rear projected image.
In our product range you will find our screens are available in different focal lengths.
This allows you to obtain a perfect match between the projector and the screen, where the Fresnel lens focuses the light perfectly and distributes it straight towards the viewer’s eyes.
To obtain the ideal configuration, the screen’s focal length has to match the projection distance, which is determined by the projector lens.
In most situations, the light needs to exit the screen at right-angles, perpendicular, from the screen to the audience. The same principal applies to installations where two or more screens are positioned next to each other with a limited seam. This makes the brightness uniformity
even more important. And the best brightness uniformity is achieved by having a good match between screen focal and projection distance.
In general there are three different possibilities:
Projection Distance EQUALS Screen Focal Length
A projection distance equal to the screen focal is recommended in most cases as all light is directed perpendicularly towards the audience. It also provides the best brightness uniformity.
Drawing here
Projection Distance are LESS THAN Screen Focal Length
Avoid situations where the projection distance is much shorter than the screen focal, as the screen might exhibit a hot spot – like a diffusion screen.
Drawing here
Projection Distance LONGER THAN Screen Focal Length
A projection distance longer than the screen focal can be helpful in situations where you want to install a large screen in a relatively small room. By using this technique you can design a meeting room and place the table closer to the large screen than you would normally do.
However, please note that the viewer should not be closer to the screen than two times the screen height. If seated closer, the viewer can’t see the entire screen, and the steep angles from
top to bottom of the image will result in a difference in brightness uniformity.
To get the best possible result, you can use the equation below, which only applies in situations where the projection distance is longer or equal to the screens focal length.
The second conjugate distance is the distance from the screen where the light path coming from the top of the screen intersects with the light coming from the bottom of the screen.
If we take an example with a 100" VisionScreen in a meeting room, you will notice the table
should be no closer than 2 x 1,524 mm = 3,048 mm to the screen. If the screen focal length is 1,850 mm the screen should normally be used together with a projector lens of 1,850/2,032:1 = 0.91:1 in order to achieve projection-distance equal to focal length and perpendicular light distribution.
If you use an 1.2:1 lens instead, providing a projectiondistance of 1.2 x 2,032 mm = 2.438 mm you will find that the distance from the screen to the second conjugate point is longer away from the screen.
If the seat height of the chairs is 420 mm and the screen position is 500 mm from the floor, you will have the two situations you can see on the drawings above.
As you can see in the drawings, the brightness uniformity is increased due to the longer projection distance resulting in a second conjugate. The vertical 12°/14° viewing angle is helped by the 6° angle of the light.
As a general guideline the projection distance should not be more than 40% longer than the screen focal (in the example the projection distance is 32% longer than the screen focal). If the projection distance gets longer, it will have a negative effect on the brightness uniformity.
As the above principle indicates, the match between screen focal and projection distance is essential for the quality of the final image. Furthermore, it is clear we have a good possibility to optimize the installation in the design phase by using the screens ability to focus the light in the direction required.
