Lens Shifting
From Lumenlab
Lens shift is a way of projecting a square screen image with the projector positioned above or below the centre of screen. It’s comparable to ["Keystone Correction"] by tilting the collector fresnel. There are a few slightly different configurations to achieve lens shifting. Fig1 shows the basic approach. It simply shifts the arc in one direction and the triplet in the other. The fresnels stay in the same position. The disadvantage in this method is that the fresnels don’t like to have the arc so far off center so less light will pass.
Fig2 shows a slightly better approach. This time the arc is positioned at the centre of the condenser fresnel as usual. But the collector fresnel is shifted along with the triplet. So now the two fresnels are still able to work efficiently. The disadvantage to this approach is that the collector fresnel will need to be physically bigger in the direction of shift. The dashed lines indicates the center of the fresnels
Fig3 shows a slightly better approach. Because they is less light at the edges of the fresnels compared to then center when shifting this dimness can sometimes be visible on the screen. It can be dimmer at the top or bottom of the screen depending on which way the shift is. So to try to alleviate this, the collimator fresnel along with the arc can be shifted in the opposite direction. Again a larger fresnel will be needed.
Here’s a variation that tilts the fresnels and lamp.
All these approaches so far have been with unsplit optics so the image quality of the LCD can be preserved. But that tends to limit how much shift can be achieved is the field of view of the triplet. The screen image can become out of focus or the corners of the screen can start to curve outwards ("Pincushion Distortion"). So by using the fresnels in split configuration some of these problems can be adjusted out. Fig4 shows that by tilting the collector fresnel as well as lens shifting the projector can be moved slightly more.
All these methods will push the triplet and fresnels to their limits so some experimenting will be needed to achieve satisfactory results.
Continue to LCD Panels or go back to Keystone Correction.
