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:
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.
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