Sunday, August 7, 2011

Telecentric Lenses with Collimated Light: Part II, Alignment

[Here’s another post from Guest blogger Spencer Luster. Don’t forget to check out the range of telecentric lenses on his website!]

A practical challenge when using collimated light with telecentric lenses is alignment and adjustment. The collimator is simply a small light source (S) at the back focus of a collimating lens (C). Rays from the source hit the lens and come out mostly parallel. (How parallel depends on the size of the source, the lens focal length, and other factors not discussed here.)

The parallel light then enters the first receiving element (R) of the telecentric lens and converges to form an image (I) at the entrance pupil (E) of the telecentric lens. (C has a focal length F1 and R has focal length F2). See Figure 1.

If the lenses are properly aligned and the image I is smaller than E, all the light enters the lens and is useable.

But what if the alignment is off? Figure 2 shows C being shifted up, but still with the correct angle relative to R.

Here we see that the image of the light source still completely "falls into" the entrance pupil, and will be useable, and most of the image will be uniformly bright. The problem is that the lower portion of the field viewed of R won't be illuminated at all.

This condition by itself tends to be obvious when viewing a live image on your monitor—uniform illumination except for a fairly sharp-edged dark region. You can also check for it with no monitor. Take plain white paper and hold it at the front of the telecentric lens. You can then see if the disc of collimated light is centered on it.

Unfortunately, the paper trick doesn't help discriminate between light source shift and angular misalignment. The off-angle condition is shown in Figure 3.

For this case, I is not completely contained by E. The overall light level as seen at the monitor will be reduced. Furthermore, the system will have asymmetric sensitivity. That is, defects that redirect light in one direction won't produce the same signature as otherwise identical defects that redirect light in another direction. This is bad!

The way to check for angular misalignment is to look down the barrel of the telecentric lens and observe I on the surface of E. (A small dental mirror can be of great use.) It's best to do this with the room lights off. Adjust the collimator angle until I is well-centered. If I is smaller than E, it will be "lost" in the hole. Temporarily increase the telecentric lens f-number to make E smaller. You can iterate between checking for shift and checking for angle until everything is perfect.

In part 3, we'll discuss more alignment/adjustment issues.

Spencer Luster

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