Identifying the Object and The Image

From Lumenlab

To ray trace a projector we first have to separate it into smaller more understandable light paths. The object is anything that a lens collects light from. The image is just that, an image of the object created by the lens refracting light. So these three things, the object, the lens and the image, can be grouped together as a ray path.

In a projector with both fresnel lenses behind the LCD two ray paths can be identified. One path is from the LCD through the triplet to the screen. The other is from the lamp arc through the two fresnel lenses to the triplet. Fig 2a shows the two ray paths in an un-split fresnel set-up.

When the projector is set up with one of the fresnel lenses in front of the LCD, a third ray path now has to be considered. In the un-split fresnel set-up the LCD was the object for the triplet but because the field fresnel is now used in front of the LCD the field fresnel creates what is called a virtual image. So now the triplet uses the virtual LCD image as its object instead of the LCD. Fig 2b shows the three ray paths.