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

Exposure You need to get photons to the sensor to get a photo You need to get photons to the sensor to get a photo Shutter speed: how fast the shutter moves across the image Shutter speed: how fast the shutter moves across the image Aperture: the size of the opening Aperture: the size of the opening Sensitivity: Sensitivity: ISO / ASA ISO / ASA Digital equivalent Digital equivalent

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

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

Illumination options Flood lights Flood lights Continuous illumination Continuous illumination Lots of power/heat Lots of power/heat Necessary for video* Necessary for video* Strobe lights Strobe lights Large capacitors energize a gas-filled tube Large capacitors energize a gas-filled tube Short duration Short duration Freezes motion Freezes motion LEDs! LEDs! Can do it all Can do it all Relatively low power/heat Relatively low power/heat

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

Calibration/scaling Laser pair at known separation Laser pair at known separation content/uploads/2008/11/2008_10_31_3 _27_23_421_test-picture_asn x212.jpg

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

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

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

The same view in “PGM” The same view in “PGM” Portable graymap format Portable graymap format

Green Red Blue

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

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

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

Specifics Storage Storage 1 megapixel = 1,000,000 pixels 1 megapixel = 1,000,000 pixels 1,000 x 1,000 pixels 1,000 x 1,000 pixels This includes all 3 colors This includes all 3 colors Formats: Formats: Raw Raw Jpeg: compressed = Joint Professional Experts Group Jpeg: compressed = Joint Professional Experts Group Sensitivity: can trade resolution for sensitivity Sensitivity: can trade resolution for sensitivity Speed: takes awhile to save an image Speed: takes awhile to save an image

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

Video “normal” video is analogue “normal” video is analogue PAL and NTSC PAL and NTSC “Phase Alternating Line” or “National Television System Committee “Phase Alternating Line” or “National Television System Committee Transmitted as an analogue signal = varying voltage Transmitted as an analogue signal = varying voltage Based on CRT (Cathode Ray Tube) Based on CRT (Cathode Ray Tube)

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

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

Time lapse images Require intervalometer Require intervalometer

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