Underwater imaging Exposure Exposure Aperture and shutter speed Aperture and shutter speed Illumination Illumination Digital images Digital images Formats.

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

• BYOP: Bring Your Own Photons!

• Even in shallow water (10m) color is gone

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

Digital!!

• Same lenses, optics, exposure considerations

• Instead of film, digital sensor

An example from our AUV

• Target in a pool

• 3m range• 1024x1360

pixels• 2/3” sensor• Flat port

• The same view in “PGM”

• Portable graymap format

Green

Red

Blue

• 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

• Raw AUV image, 3m altitude, 850m depth

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”

Time lapse images• Require intervalometer

Summary

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