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Lecture
18: Video Systems
Nomenclature:
CCD Charged Coupled Device
Contrast Gray Scale
Depth of Field (DOF) the difference between the closest and farthest distances and object may be shifted before an unacceptable blur is observed. Also “Depth of Focus”.
Distortion Distortion is an optical error caused by the lens, that results in a difference in magnification at different points within the image.
Resolution Separation of individual geometry (line – space – line = 3 pixels)
Resolution - Pixel count example: 768 horizontal pixels, one pixel per line, space, next line: 768 pixels = 384 lines resolution.
Resolution - TV Lines number of equi-distant lines covering field of view; NTSC resolution limited to » 800 lines (system limitation). In actual systems, color cameras achieve typ. 330 to 480 lines, monochrome cameras 570 lines
Resolution – Line Pairs Line pairs per millimeter (line pair = line + space).
NTSC National Television Standard – Color RS170A
RS170 = EIA, monochrome
PAL
CCD
Sensor Size typ. ¼” (3.2x2.4 mm / 4 mm diagonal,
1/3” (4.8mm x 3.6 mm / 6 mm diagonal,
½” (6.4 mm x 4.8 mm / 8 mm diagonal
2/3” (8.8 x 6.6 mm / 11 mm diagonal,
1 inch (12.8mm x 9.6 mm / 16 mm diagonal.
Pixels (HxV) 640x480 (307,200), 768x494 (379,392), 1024 x 768 (786,432), 1280x960 (1,228,800),
Aspect Ratio typically 4:3 (H:V)
Min. Focal Length closest distance between object and lens, working distance
Field of View (FOV) horizontal x vertical angle
Bit resolution 8-bit (28 =256), 24-bit (28 =16,77,216),
Gray-scale: 0=white, highest number = black
Color CCD sensors cannot measure color, only intensity. To measure color, pixels are fitted with filters to measure intensity in 3 different wavelengths, to get color information.
Selecting a Video Camera
Color vs. Monochrome color less resolution than monochrome due to required color filtering of available pixels to get color information.
example - 768x494 pixels:
470 line resolution color / 570 line resolution monochrome.
Monochrome: higher resolution, better signal to noise ratio, low light sensitivity, greater contrast, IR sensitivity.
Visible vs. IR / UV Silicone CCD: 300 nm to 1100 nm w/o IR filter; most commercial cameras are with IR-filter (<700 nm).
Signal
format Common:
Composite (PAL, NTSC - BNC),
Superior Resolution: Y-C (S-video), RGB (3 color CCD)
Lens pin-hole, C-mount,
Accessories
Filters color filter, polarizer
Illumination Directional Illumination, Diffuse, Darkfield, Backlight. Illumination (spectral quality) may affect color rendition (example: blue/red LED do not provide color rendition of plants – add minimal white light for color videography.
Coatings Anti-reflection , hot mirror (reflects IR, transmits visible), cold mirror (reflects visible, transmits IR)
Other Consideration:
Color calibration may require calibration with known color reference, as digital images and color information can be different depending on system – leave target in Field of View ?
“User Psychology” End-user may prefer true-color image over higher resolution mono-chrome image.
Resolution: example: spider web – difficult to visualize with color camera and stationary video system; IR-sensitive mono-chrome camera + IR illumination not only increased resolution (color vs. B&W), but also enhanced spider web.