Choosing a Camera for Inspection: Color or Monochrome?

Choosing a camera for inspection: Color or monochrome?

When purchasing a new suveillance system you must decide between color and monochrome cameras. Many times businesses decide on a system without understanding how the choice affects the performance. Most surveillance systems include color cameras because users feel more comfortable with color images. They are more into the aesthetics rather than performance of the system. On the other hand, some owners may want monochrome cameras because they assume they would cost less, which is not always the case. The best decision an owner can make is they must consider the strengths and weaknesses of each system so they can get the best value for their money.

High Resolution

A single pixel can yield only intensity information. Most color cameras determine pixel color with a combination of filters and interpolation from the intensity values of adjacent pixels. This interpolation reduces resolution. For example, the arrangement of blue and red pixel sites in a Bayer filter makes a single-imager color camera more prone to horizontal and vertical interference, particularly on objects with straight edges that follow a row or column. Therefore, designers often favor monochrome cameras when resolution is most important such as in banks and stores.

Owners who want their systems in both color and the highest possible resolution need another alternative, a color-imaging system that does not interpolate. In this case, three separate sensors gather three color channels of information. A separating prism placed in front of the three sensors directs the red, green, and blue light to the proper detector. While the three chip color camera present resolution that matches that of a monochrome camera, the prism and additional sensors make this system more expensive. Also, these cameras use more power than their counterpart. Finally, although the sensors in these color cameras may equal the resolution of the monochrome sensors, they are not always easily available. Many times, you can get the required resolution on a large-format single-chip color camera by choosing the lenses with an appropriately adjusted magnification.

Inspection Example

An inspection system that must decide whether fuses are correctly placed in an electrical harness often needs a computer to make color differentiation evaluations. Because fuses are color coded, you might be inclined to use a color camera. If the purpose requires human visual inspection on an analog monitor, a color camera would be a good choice. If the components were in color, inspectors would find it easier for them to do their job.

But if a computer performs the inspection where they must distinguish few colors, a monochrome camera may provide a more cost effective solution. Most image-processing algorithms process pictures to a pixel depth of 8 bits. A color camera would take the color planes from each image and analyze it in three separate color channels. A final pass-fail decision would have to compare each analysis against the others. The ensuing processing time of the images could slow down the inspection step, limiting system throughput. In this case, image-processing speed and higher resolution requirements favor a monochrome camera.

However, a monochrome camera cannot distinguish colors well. Significant color variations within batches of similar filters can manufacture incorrect results. To improve contrast, adding a color filter usually works. So, a filtered monochrome camera works fast and can provide sufficient contrast for two-color applications.

If the application involves additional colors, a monochrome camera might not be enough. Instead, inspection parameters would define each fuse as a ratio between mean pixel values in the three different color planes. Consequently, applications that do not allow cooperation on either resolution or color require a three-chip color camera; the greater processing time becomes unavoidable.

The obvious choice of monochrome vs color cameras for machine-vision applications is not always clear. System designers must look at the different tradeoffs such as processing time, resolution, pass/fail-decision accuracy, power consumption, and, of course, cost—to make the best decision for each situation.