Thursday, April 19, 2012

Optical Crosstalk Cure or Light Pollution Solution

Much as I enjoy sharing clever solutions to problems, I recognize that others in our industry have far greater expertise than do I. One such is Spencer Luster of LightWorks. On occasion Spencer is kind enough to contribute a guest post to these pages, and after much cajoling (he keeps saying how he has to earn a living,) he has sent me another article.

If you’re interested in learning more about what LightWorks could do for, I suggest check out the lw4u website where you’ll find information on telecentric lenses, hypercentric lenses (sometime known as pericentric lenses,) and the subject of this blog post, optical view splitters.

Here's an old application trick that recently raised its head again, so I thought it would be worth describing. Opposed views of an object can sometimes present challenges. One of them is optical crosstalk or light pollution.

Figure 1

The basic inspection is to measure the height of excess flash above the rim or "finish" of a plastic bottle. In concept, two views on opposite sides of the bottle let you see all that's needed. To save money on cameras and lenses, you might want to do this with an optical view splitter such as those available from a swell company in Toledo.

Figure 2

Figure 2 shows a possible arrangement, with either a fiber optic or LED ring light to illuminate the bottle finish.

But the flash from one side can appear in the image of the opposite view, especially when each view must have a long depth of field to accommodate the bottle mouth curvature.

Here's an exaggerated cartoon showing the two views from an optical splitter combined onto one camera detector. A high feature (excess flash) can be seen from both views.

Figure 3

Figure 4 shows the real captured view.

Figure 4

The edge on the right of the image is of the near flash only, but the left side shows two edges. The green arrow points to the near edge, while the red arrow points to the far edge on the opposite side of the bottle. When the far edge shows up, this is cross talk. Unless a very fast and obsessive person paints arrows on all your image captures, how can you tell the difference?

Try using two different color light sources and two non-overlapping color filters. Figures 5 and 6 illustrate the concept.


Figure 5: Spectral Curves of 2 Non-overlapping Filters

Figure 6

In this way the far side of the bottle is illuminated by a colored light that can't pass through the near filter. Take a look at Figure 7. The cross talk effect is greatly reduced.

Figure 7: Far Edge Brightness is Reduced

Of course you don't have to use an optical splitter. Maybe you enjoy spending extra money on more cameras and lenses. (Depending on the size and nature of a two-way splitter, typical prices can range from $1,500 to $3,500 USD.)

The main point, however, is to illustrate the concept of light pollution reduction. Furthermore the technique can be used for other inspections—let your mind wander. Mine does all the time!

Spencer Luster

No comments: