Light Flow

From Lumenlab

In our projector we need to control the flow of light through the projector. Anyone who has ever started a fire with a magnifying glass will know that the lens concentrates the sunlight to a point, thus concentrating the heat in the light. If you were to measure the distance from the lens to the point of light your magnifier focused, you would have the focal length* of your lens, measured in millimeters. Imagine reversing the situation. We want to take a point of light (our bulb) and distribute it evenly over a large surface (our LCD panel). We want to use a point light source because we can accurately control the light with a fresnel lens to be distributed very evenly over the entire LCD module This is where we will use our first fresnel lens. When using the fresnel lens this way we are using it as a collimating or condensing lens (fig 1a)

Focal length is defined as the distance from the lens to a point where parallel rays are focused to a point (converge), traditionally measured in millimeters (mm).

By placing the collimating fresnel in front of our point light source we get a nice parallel (collimated) distribution of the light. This evens the light across the entire surface of the fresnel which ideally is the same size or slightly larger than our lcd panel.

If we took our collimating fresnel and turned it around, we would have a collector or field lens (fig 1b). A collector lens bends diffuse light towards a point. This is useful for our projector as well because we want all of the available light focused on our projection lens. Otherwise much of the light would be wasted since it didn’t pass through our projection lens. Figure 1c illustrates this point.

We can see that much of the light falls outside of the projection lens’ (A) area. We could solve this problem if we could use a projection lens that was as big as our object (LCD module). Since the projection lens needs to be a high quality optic (a ‘real’ lens) it would be prohibitively expensive to use, and nearly impossible to find. This is where our collector fresnel lens comes in. Figure 1d shows the best arrangement of a lamp, 2 fresnel lenses and a projection lens.