Sunday, June 16, 2013

Color: Why you should stay away, and advice for those who can’t

While writing my previous post, “Color linescan camera options” it struck that more information on color might be useful. In this post I’m going to address why you should steer clear, color camera configurations, and how three chip color cameras work. Now that’s a book-worthy agenda, so mostly I’ll just offer links to those who’ve done the real leg work.

Why you should steer clear

It’s probably natural that the machine vision newbie would think color is superior to monochrome or grayscale imaging. After all, color TV beats black and white. But here’s the difference: you’re not in this for the artistry or to provide entertainment, you’re trying to extract data from a scene. If grayscale is sufficient, that’s all you need.

And to that point, don’t overlook that colored light and filters can let you discriminate between pairs of colors much better than can color imaging. But if you really need to know what colors you’re seeing, or actually want to measure color, then a color camera is the way to go.

If you don’t believe me, read the Dalsa white paper titled “Evaluating Color Inspection: Can Color Machine Vision Improve Results?

Color camera configurations

Most color area cameras use a Bayer filter over the sensor. Adimec’s blog post “Bayer Area Scan Color Cameras compared to 3-CCD Color Cameras, part 1” (May 14th, 2013,) explains what this is, and compares it with the three chip camera design.

The Bayer filter design has several disadvantages, but for most of us these are outweighed by its advantage: it’s less expensive than the three-chip design.

How three-chips work

Adimec provide some details in the blog posting, but I found “Separation Prism Technology” on the pages of Alternative Vision had more depth. This shows the various prism configurations, which explains why these cameras are so much more expensive.

So let’s sum up what we’ve learnt:
  • Color machine vision isn’t better, just harder
  • Bayer filter cameras have their weaknesses but are the right tool for most color machine vision tasks
  • Three-chip color cameras use prisms, which makes them good but expensive

Surprisingly enough then, it seems you get what you pay for.

1 comment: said...

Thanks for pointing to the Teledyne DALSA white paper as a resource. Agreed that not all inspection systems need colour. However, colour imaging adds a new dimension and provides better detectability than mono imaging. We suggest asking the following 3 questions to determine if color inspection is right for you:

1. Is the object’s color and consistency of color a key factor in the overall quality of your product?
2. Can the object’s color help you to ascertain the relative quality of your product?
3. Will color help facilitate detection of the object?

Compared with prism based colour cameras, tri-linear cameras significantly reduce costs at system level. Sub-pixel spatial correction also enables precise synchronization with web speed to achieve high image quality. We see a trend in MV industry moving to colour imaging. We’re here to help if you have any further questions.