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Underwater imaging

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

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Underwater imaging

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  1. Underwater imaging • Exposure • Aperture and shutter speed • Illumination • Digital images • Formats • Resolution • Video • Devices • Telemetry / storage

  2. 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

  3. Illumination • Natural (ambient) light • Absorbed by water, dissolved substances, particles • Red disappears first

  4. BYOP: Bring Your Own Photons! • Even in shallow water (10m) color is gone

  5. 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

  6. Lighting • Particles scatter best in either forward or “back” directions • Minimize interference by lighting from the side

  7. 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

  8. Film Camera *I couldn’t think of any • Advantages*: • Disadvantages • Processing • Cost • Resolution • Dirt • Inflexibility • Mechanical failures • Size • Data density

  9. Digital!! • Same lenses, optics, exposure considerations • Instead of film, digital sensor

  10. An example from our AUV • Target in a pool • 3m range • 1024x1360 pixels • 2/3” sensor • Flat port

  11. The same view in “PGM” • Portable graymap format

  12. Green Red Blue

  13. The gray levels indicate how much light of that color was detected • In the red quadrant, the red pixels are lightest

  14. To get color, the image is “demosaiced” • Colors for each pixel are interpolated from adjacent images • Color balancing algorithm is up to the user

  15. Raw AUV image, 3m altitude, 850m depth

  16. 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

  17. Notes • Lens: can be flat or domed • Flat is cheap and easy • Domes give less distortion • Condensation: avoid it using dessiccantor dry gas

  18. 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)

  19. The video tube and CRT are synchronized • “raster” back and forth • Lines of resolution • Normally 480 • (x 640) • Refresh at 30 Hz

  20. Digital video • Use normal digital sensor • Acquire rapidly • Compress • Transmit/store • But, each line is recorded separately • Played back simultaneously • “Rolling shutter”

  21. Time lapse images • Require intervalometer

  22. Summary • Film bad, digital good • Issues include: • Power • Storage • Control • Lighting • Distortion • Interpretation • scaling

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