Color Camera
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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.
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Is it possible to use a digital camera to calibrate the color balance of a TV?
If I color balance the digital camera at daylight (6500K) will that allow me to properly calibrate my TV’s color balance by taking pictures of a shade of gray on the TV screen using the digital camera?
Using a digital camera in place of a proper color measurement instrument will not work when it comes to any aspect of instrument-based display calibration. Instrument-based display calibration is intended to precisely set the colorimetric white-point at multiple—typically predetermined—gray-scale levels. The standard white-point reference target for a video display device is CIE D65 (which correlates to a color temperature of 6504 K,) within a margin of error—or CIELUV delta E (dE or ?E)—of 6 or less across a range of gray-scale levels spanning from 20 IRE to 90 IRE and a ?E of no more than 10 at the gray-scale extremes. (Note: some techs prefer to report ?u´v´ color difference values in addition to or as an alternative to ?E. Also, some calibration techs prefer to use much tighter margins of error.) A digital camera simply cannot accomplish this fundamental measurement necessary to calibrate a display device.
Proper colorimetric measurement devices can be obtained at a reasonable price. A very effective tristimulus colorimeter, which performs as well as some professional devices that cost several orders of magnitude more, is the i1Display 2 from X-Rite (formerly GretagMacbeth.)
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CIE Standard Illuminant D65 is the accepted worldwide industry standard white point reference for video displays including HDTVs. (Note: a CCT of 5400 K is the accepted industry standard for displays used in graphic arts applications.) For motion picture and video applications D65 should ALWAYS be the target calibration reference point to ensure accurate color reproduction not the derived CCT value of 6500 K.
In the field of colorimetry and color science 6500 K is referred to as a “Correlated Color Temperature” and is represented in two-dimensional color space, aka a CIE chromaticity diagram, by a straight LINE—specifically an “isotemperature line”—that transversally passes through a point on the Planckian (aka blackbody) locus. Any pair of chromaticity coordinates lying along the 6500 K isotemperature line, no matter their visual difference, will produce a corresponding CCT of approximately 6500 K (though only one pair of coordinates will be closest to the ideal D65 source point.)
D65, being a specific reference point and not a line, is a substantially more precise target—and is therefore preferred—for precision display calibration as opposed to a potentially infinite number coordinates along a line. Do-it-yourselfers and knowledgeable, well-trained display calibration technicians should be well aware of this very important fact.
######################## RESOURCES ########################
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http://www.sencore.com/uploads/files/WhatIsWrong.pdf
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Is D6487K too low of a color temperature for an LCD TV?
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AV Science (AVS) Forum
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http://www.avsforum.com/avs-vb/forumdisplay.php?f=139
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http://archive2.avsforum.com/avs-vb/forumdisplay.php?f=139
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http://www.avsforum.com/avs-vb/showthread.php?t=852536&pp=60
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http://docs.google.com/Doc?id=dhpmqnbb_4g8mtjzdf
TweakTV – Tutorials
http://www.tweaktv.com/tutorials/index.php
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Spears & Munsil
– High Definition Benchmark Blu-ray Edition
http://www.spearsandmunsil.com
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http://www.spearsandmunsil.com/pdf/UG_SM_HD_Benchmark.pdf
Joe Kane Productions
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AV Science (AVS) Forum
– New Calibration Disc (GetGray)
http://www.avsforum.com/avs-vb/showthread.php?t=586139&pp=60
– Calibration Meter Shootout
http://www.avsforum.com/avs-vb/showthread.php?t=958099&pp=60
X-Rite
– X-Rite Products
http://www.xrite.com/top_products.aspx
– [PDF] X-Rite i1 Monitor Solutions
http://www.xrite.com/documents/literature/en/985629_i1_Monitor_Solutions_en.pdf
