How Laser Sorters Work

So.... if laser sorters are not so accurate in terms of reflected color, what good are they? Why do people commonly use them for controlling product quality? The short answer: they look at things fundamentally differently than cameras. So see how this works, grab your laser pointer, and shine it at piece of metal. What you see is a bright spot. That is the direct laser light reflecting off of the metal. Now shine it against your skin. What you see is the same bright spot (perhaps a bit less bright), PLUS a softly glowing area around the spot, several times the diameter of the bright spot.

Look at the image below, of a laser shining against a piece of steel and a carrot slice. Now look at the same image above. This is how the laser views things: it blocks the bright spot (where you see the black spot) and looks at light reflected from the circle.

A laser system looks at the glowing area around the spot, not at the spot itself. Very translucent objects (including most vegetables) have a relatively bright glow. Many foreign materials, such as metal and most stones, do not glow at all. So, even if they are the same color as the vegetable, the laser sees them as VERY different in translucency. The glow is often called "backscatter".

Take a peek at the photo below: it shows two versions of the same information. On the right is a laser-generated image of various objects: two whole potatoes (one with a defect, both look generally red), a white golf ball (looks blue/gray), white plastic (looks pink), red ceramic (looks medium blue), natural wood splinter (light in color, looks mottled pink/light blue), a white stone (looks dark gray), a piece of clear glass (looks very dark blue).

The left image shows how the same information/image is interpreted by the sorting computer according to class: gray indicates background, white is good potato, blue is defect in potato, red is foreign material. Click on the image to see a larger view. Impressive, no?

In this mode, a laser system is effective in identifying most foreign materials that look the same as potatoes to cameras.

So.... what about clear glass?

The laser system also uses a background surface, usually a round bar that the laser shines against when nothing is being sorted. The laser sensors continually look at the translucence of the bar, and compare it with the translucence of each object going by. For most applications, the background bar material is of translucence similar to the good product, so that when a change is sensed, foreign materials are likely present.

Now, when a piece of clear glass (or any other color, for that matter) passes between the laser and the background bar, the laser light is bent as it passes through the glass. This displaces the spot away from the background bar, far enough away from the laser sensors that they see no backscatter at all- the clear glass piece looks black to the laser.

So the laser can see and identify many foreign materials that a camera cannot.

Does this mean that cameras are not valuable for foreign material control?

That will be our next topic.

Tim

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