Projector screens and the moiré effect

Moire effect  
Flashing lines across the screen display the moiré effect.
Photo courtesy of ednl, used under the CC BY 2.0 license.

As the resolution in projectors grows, the difference between projector and projector screen is also growing, and increasing the risk of what is called the 'moiré effect'.


What is moiré?

The moiré effect refers to the lines you may see on your projector screen when you are trying to watch TV or a film. They might also appear on a regular TV when people on the screen are wearing striped clothing, creating an almost 'strobe-like' image. On projectors, it often has to do with the pixellated image from the projector being interfered with by the weave pattern of the projector screen.

The problem is related to resolution; if a pixel cannot be seen a very short distance from the screen, then it is the projector screen that is causing the problem. However, if a pixel can be seen in detail it is the projector which is causing the problem. This is often the case with screens made from fine fabrics.

Particularly bad cases of moiré will give you feeling that you are viewing your screen through a mesh door.


How does the moiré effect occur?


Moiré occurs when the natural patterns on your projector screen align with the pixel patterns of what is being projected, merging to form lines that run across your screen. This can occur when the projection screen has been weaved too loose, in the case of a woven screen, or has an uneven patterning.

The pattern of the screen overlaps with the field of the pixels, creating the moiré effect. Moiré is becoming a larger issue now with HD projection monitors, as the higher resolution makes the visual more susceptible to imperfections in a coarse screen.

Regardless of what some advertisements claim, there is currently no such thing as a full-HD or HD-ready projection screen. Having said that, there are specific ways to ensure a smooth experience on your projector screen.


How can I prevent moiré?

The moiré effect can be prevented by the use of a smoother projection screen to view HD images on. Additionally, woven screens have more natural variations in their thickness and pattern, and are therefore effective in reducing alignment with the projector and reducing the effect of moiré.

Before buying any projector or screen, speak to someone qualified in projector hardware to ensure that your setup will be compatible and free from the moiré effect.