Rose-tinted glasses

Aug. 1, 2011
In older systems, better lenses may deliver a clearer image, but sometimes defects are best left unseen.

In older systems, better lenses may deliver a clearer image, but sometimes defects are best left unseen

by Andy Wilson, editor
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Every year, I visit my local optician to check to see how badly I have damaged my eyesight by sitting at a computer all day. After a barrage of tests, my doctor decided that it was perhaps time for me to invest in a pair of bifocal lenses. I was horrified. Being a narcissist, I pictured myself with a large pair of English-style National Health Service frames whose very thick lenses would be evenly split down the middle. Such a product, I presumed, would make me look very old and less attractive to the fairer sex.

To allay my fears, the doctor explained that these lenses were no longer fabricated in such a way and that, in fact, no one would know that I was a bifocal user! And so it was that, with some trepidation, I took my doctor's advice and invested $900 in the finest frames and lenses money could buy.

After about a week, I was informed by the optician that my spectacles were ready and, after donning them, I saw the world in a new perspective. The doctor had been correct. I could now focus on short, medium, and long distances without switching frames.

At first, I was very pleased. That is, until I visited a restaurant for lunch one bright, sunny day with a friend of mine. While the young people around me looked younger and prettier than ever, folks of my age appeared a lot older and uglier. In fact, I could discern every blemish, pit, crack, blotch, spot, and wrinkle on their faces.

I immediately panicked. If they looked like this to me, what did I look like to them? If you have ever met me, please do not send me your opinion!

A sight for sore eyes

Those developing machine-vision systems must also pay careful attention to the types of lenses and optics they use in their systems. Often, manufacturers supply high-quality lenses designed to maximize the image resolution that is captured by the image sensor. With such high-resolution lenses coupled to megapixel cameras, imaging systems can resolve the finest details of objects being inspected.

However, sometimes, as in life, the choice of a lens must depend on what you want to see—or rather what the machine you are building is designed to inspect. In a simple color inspection application, for example, where the system is checking for the presence of a specific color on a product, such high-resolution lenses may not be required. In other applications, where detailed pits or cracks in glass must be inspected, higher-resolution image sensors and lenses may be needed.

In building such systems, developers must carefully match the lens and camera combinations with the software used for image analysis. Testing these systems to ensure that the system operates in the correct manner then guarantees customer satisfaction.

However, since all lenses are not created equally, replacing a lens in a well-tested system with its higher-resolution counterpart can have horrific consequences. Defects such as blemishes, pits, cracks, blotches, spots, and wrinkles once undetected by the system may suddenly be flagged by the deployed software.

Needless to say, this may result in erroneous pass and failure rates for the product being inspected. While viewing everything through rose-tinted glasses may place the world in a much better social perspective, choosing or replacing low-resolution lenses with their higher-resolution, more expensive counterparts may not necessarily be a good idea for developers of machine-vision systems.

If you enjoy reading Andy Wilson's monthly column, watch his video blog.

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