Chromatic Aberration
From Lumenlab
Chromatic aberration is caused by a lens not refracting all wavelengths of light equally. Blue light is refracted more than green and green more than red. This is the reason a prism can separate the different wavelengths and show the colors of a rainbow.
This causes images to appear to have a colored fringe because the different wavelengths can’t focus at the same point. By combining lens elements made from materials that have a different refractive index, the projected image can be improved. The triplet is one such lens that has been optimized for chromatic aberration.
The fresnel lens suffers from chromatic aberration just like any other single lens. If there is dust or smoke in the air you can see the separation of colors if you look at the converging light cone after it has passed through the two fresnels in a projector. There should be visible a red band of light surrounding the main white cone. After the cone has converged to a point the rays cross over and diverge where now the band appears blue.
Generally the fresnels won’t be too much of a problem. As long as they are kept behind the LCD or close to the LCD when used as a split fresnel set up. If the front fresnel is moved too far forward, in a split set up, then the chromatic aberration can cause the individual pixels in the corners of the projection to appear as ghosted or fringed with color. The chromatic aberration from the fresnels can also appear in the projection as colored corners, either red/yellow or blue. This is due to misalignment and can help in final tweaking if you know what to look for.
Here’s a ray trace of a properly aligned projector showing the arc image forming inside a simplified triplet. Instead of a sharply focused arc image forming, chromatic aberration from the fresnels causes the arc image to be blurred. As explained above this won’t effect the final image, the triplet is focusing on the LCD, as long as it is passing unobstructed through the triplet.
Problems occur when not all of the entire arc passes through. Here the arc is too far back from the rear fresnel, causing the arc image to form short of the triplet.
The converging light has enough room to cross over and the outer color band to change from red to blue. Notice how the blue band is being blocked by the triplet. In this situation the corners of the screen image will be receiving less blue light and appear red/yellow.
The arc in this next trace is too close to the rear fresnel causing the arc image to form too far forward of the triplet
This time the red band is blocked by the triplet so the corners of the screen image will appear blue from lack of red light.
Tip for tweaking
This is useful to get close to the optimum position for the arc. Move the arc back until you start to see the corners change to red/yellow and mark this position. Then move the arc forward until the corners start to change to blue. There will be a position in between these two marks that is optimal.
Generally I find that that the best position is just prior to the corners turning blue gives the greatest lumens.
