Underwater imaging
• Exposure• Aperture and shutter speed• Illumination
• Digital images• Formats• Resolution
• Video• Devices• Telemetry / storage
Exposure
• You need to get photons to the sensor to get a photo• Shutter speed: how fast the shutter
moves across the image• Aperture: the size of the opening• Sensitivity:• ISO / ASA• Digital equivalent
Illumination• Natural
(ambient) light
• Absorbed by water, dissolved substances, particles
• Red disappears first
Illumination options• Flood lights
• Continuous illumination• Lots of power/heat• Necessary for video*
• Strobe lights• Large capacitors
energize a gas-filled tube
• Short duration• Freezes motion
• LEDs!• Can do it all• Relatively low
power/heat
Lighting• Particles scatter best in either forward
or “back” directions• Minimize interference by lighting from
the side
Calibration/scaling
• Laser pair at known separation
http://www.savante.co.uk/
http://scini1.mlml.calstate.edu/wp-content/uploads/2008/11/2008_10_31_3_27_23_421_test-picture_asn000120-300x212.jpg
Film Camera
• Advantages*:• Disadvantages• Processing• Cost• Resolution• Dirt• Inflexibility• Mechanical failures• Size• Data density
*I couldn’t think of any
• The gray levels indicate how much light of that color was detected
• In the red quadrant, the red pixels are lightest
• To get color, the image is “demosaiced”
• Colors for each pixel are interpolated from adjacent images
• Color balancing algorithm is up to the user
Specifics• Storage• 1 megapixel = 1,000,000 pixels• 1,000 x 1,000 pixels• This includes all 3 colors
• Formats:• Raw• Jpeg: compressed = Joint Professional
Experts Group
• Sensitivity: can trade resolution for sensitivity
• Speed: takes awhile to save an image
Notes
• Lens: can be flat or domed• Flat is cheap and easy• Domes give less distortion
• Condensation: avoid it using dessiccant or dry gas
Video
• “normal” video is analogue• PAL and NTSC• “Phase Alternating Line” or “National
Television System Committee
• Transmitted as an analogue signal = varying voltage
• Based on CRT (Cathode Ray Tube)
• The video tube and CRT are synchronized
• “raster” back and forth
• Lines of resolution• Normally 480• (x 640)
• Refresh at 30 Hz
Digital video
• Use normal digital sensor• Acquire rapidly• Compress• Transmit/store
• But, each line is recorded separately
• Played back simultaneously
• “Rolling shutter”
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