The rainbow effect explained
It is considered one of the main drawbacks of single-chip projection.
While some viewers don't notice the rainbow effect at all, others complain of fatigue or even headaches.
Current projectors sometimes differ considerably with regard to the rainbow effect. Unfortunately, the only technical specification that indicates the "strength" of this effect has been chosen by manufacturers in a very abstract and hard-to-understand way: colour wheel speeds and colour wheel segments are advertised without explaining any direct connection.
How the effect arises
1. Sequential colour display
Single-chip projectors don't produce the primary colours on the screen simultaneously, but one after the other – "sequentially".
The white light of the projector lamp is split into its primary colours by three filters on a colour wheel and projected onto the screen one after another.
With this method, the actual colour mixing only takes place in the viewer's brain, thanks to the persistence of vision.
The colour display is, so to speak, too fast for us to perceive it as separate R, G and B images.
2. Colour speed
As already explained, all the colours of the video image are created by mixing the individual additive primary colours red, green and blue.
For each individual video frame, the RGB sequence must therefore run through once.
To reduce the rainbow effect, the speed has to be increased. But the faster the colour wheel spins, the greater the strain on the motor, axle and glass wheel. Users also want a quiet projector, and a colour wheel spinning at 200 revolutions per second is difficult, if not impossible, to "get quiet".
As a solution, engineers came up with an ingenious trick:
3. Colour wheels with more segments
Instead of increasing the rotation speed of the colour wheel, the wheel's red, green and blue filters are simply doubled. The colour wheel now has not "just" three segments, but six. This is currently the most commonly used variant and is called an "RGBRGB" colour wheel.
Six-times speed delivers consistently good results with very few rainbow artefacts. Despite the high frequency, our eyes can still occasionally detect rainbow effects in certain scenes with fast movement and strong contrasts.
4. Special colour wheel variants
The RGBRGB colour wheel described above is currently the most widespread way of achieving a high RGB frequency at a moderate speed.
But for some manufacturers that isn't enough, and they have developed the colour wheel further to achieve other image benefits.
Here we present the two most interesting variants.
4.1. Double red segments in a 6-segment colour wheel
The area of the red segments is doubled. Thanks to this trick, the primary colour red is projected onto the screen for longer relative to green and blue, which our brain interprets as a boost in red.
4.2. Eight-segment colour wheel with additional dark green segments
Green is the primary colour that carries the most brightness for our eyes. When a dark area is to be projected, the additional dark segments come into play.
Matching product categories in the shop:
Further guides:
- Projector buying guide using the BEAM method – overview of all guides
