Telecentric Lenses with Collimated Light: Part IV, Collimated Dark.

[Note to readers: Here’s the final installment in a series of four posts by Guest Blogger Spencer Luster. Spencer runs a business that specializes in telecentric imaging, and if you don’t know what that is, I suggest you read his articles and visit his web site, www.lw4u.com.]

In Part I we explored using telecentric lenses and collimated lighting for subtle defect detection. We then moved on to alignment and adjustment issues. Now we're ready to take a look at a powerful technique for detecting very subtle defects—Collimated Dark. Figure 6 shows such an arrangement.

This is the same as for the collimated light set-up, but the source (S) is much larger, and a blocker has been added on-axis. The blocker size is chosen so that its image just fills the entrance pupil (E) of the telecentric lens. It's as if a "dark source" is emitting "darkons" or "dark rays" toward the telecentric lens. Of course what's really happening is that light from the source is being projected by C in many, many directions except parallel to the optical axis. Drawing all those light rays, however, would just make a mess, so it's handy to think of dark rays. But remember, lack of dark rays equals light.

When no object is between C and R, or it's just a flat, smooth, transparent object, the dark rays are unchanged and enter the telecentric lens. When a defect deviates a dark ray, however, light enters the lens and a bright signal is produced.

Like most dark field techniques, the otherwise unused source can be made extremely bright and thus produce a highly sensitive system. Why? Suppose a defect in a light field system has a "natural" contrast of 1%. A background of 200 grey levels will only be changed by 2 grey levels for such a defect. Tough to detect. A dark field system, however, could have a source with a theoretical light level of 2,000 grey levels, or 20,000 grey levels. With no defect there is no signal, but if the 1% defect appears, the signal spikes to 20 or 200 grey levels!

Remember the images of the safety glasses from Part I? Let's take a look at exactly the same object, but this time using collimated dark and a very bright source.

Figure 7: Collimated Dark

Now we see the curved "viscous thread" defect showing up as a bright object because it refracts the dark rays away from the telecentric lens. Furthermore, every little dust speck, and even the long hair that was deliberately placed appears bright with high contrast. Why should such a dark, blocking object show up as bright? The answer is diffraction. We won't go into details here, but the edges of any object deviate rays slightly and produce a weak signal. When the source is very bright, even a weak signal can easily be detected.

Spencer Luster

LIGHT WORKS, LLC