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

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Underwater imagingExposureAperture and shutter speedIlluminationDigital imagesFormatsResolutionVideoDevicesTelemetry / storage

ExposureYou need to get photons to the sensor to get a photoShutter speed: how fast the shutter moves across the imageAperture: the size of the openingSensitivity:ISO / ASADigital equivalentIlluminationNatural (ambient) lightAbsorbed by water, dissolved substances, particlesRed disappears first

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

Illumination optionsFlood lightsContinuous illuminationLots of power/heatNecessary for video*Strobe lightsLarge capacitors energize a gas-filled tubeShort durationFreezes motionLEDs!Can do it allRelatively low power/heat

LightingParticles scatter best in either forward or back directionsMinimize interference by lighting from the side

Calibration/scalingLaser pair at known separation CameraAdvantages*:DisadvantagesProcessingCostResolutionDirtInflexibilityMechanical failuresSizeData density

*I couldnt think of any

Digital!!Same lenses, optics, exposure considerationsInstead of film, digital sensor

An example from our AUVTarget in a pool3m range1024x1360 pixels2/3 sensorFlat portImage acquired of a test pattern at the bottom of a pool, range 3m.Flat port housing, 1024x1360 2/3 sensor (Prosilica GC1380C)Lens focal length 9mm, f/2.0, 10ms exposureSensor gain 10dB, automatic white balance (unknown value)Processed from raw image and color corrected11

The same view in PGMPortable graymap format

Raw version of same image viewed as grayscale. Appears as a normal grayscale due to averaging by our eyes and the display software. Each pixel contains a light level from each cell of the CCD sensor. A filter is placed over the sensor such that each cell only measures a red, green, or blue value.12

GreenRedBlueA Bayer filter is common (and is used by this sensor).[A] mosaic of selectively transmissive filters is superimposed in registration with a solid state imaging array having a broad range of light sensitivity.13

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

In the left color block, red pixels have a medium level while green and blue have low values. A sample of a white stripe is also visible. All values are high in this section, although the blue levels are much lower than they would appear for a pure white. In the right color block, the red level is very low and the blue levels are highest. The green pixels are lower, but they have a higher level than they would in a pure blue. The test pattern is a spare tire cover painted with cheap spray paint, so the color values are far from perfect.14

To get color, the image is demosaicedColors for each pixel are interpolated from adjacent imagesColor balancing algorithm is up to the userThrough a process called demosaicing, colors at each pixel are interpolated. This image shows areas of distortion and lower sensitivity in the corners, partly due to the flat-port housing. The vertical band at the left side of the image is a defect of the sensor itself.15

Raw AUV image, 3m altitude, 850m depthRaw image obtained at 3m range by the Mola Mola AUV operating at ~850m depth. Gas hydrate outcrop near a crater within the Gulf of Mexico MC118 lease block.16

Demosaicked image that has been color balanced17

Image combined with 5 neighboring images to create a photo mosaic. This mosaic was produced graphically by matching control points in adjacent images. Currently in progress is a method to place images geographically within a scene using vehicle navigation logs. Graphical smoothing can then be applied to the rough edges.18SpecificsStorage1 megapixel = 1,000,000 pixels1,000 x 1,000 pixelsThis includes all 3 colorsFormats:RawJpeg: compressed = Joint Professional Experts GroupSensitivity: can trade resolution for sensitivitySpeed: takes awhile to save an imageNotesLens: can be flat or domedFlat is cheap and easyDomes give less distortionCondensation: avoid it using dessiccant or dry gas

Videonormal video is analoguePAL and NTSCPhase Alternating Line or National Television System CommitteeTransmitted as an analogue signal = varying voltageBased on CRT (Cathode Ray Tube)The video tube and CRT are synchronizedraster back and forthLines of resolutionNormally 480(x 640)Refresh at 30 Hz

Digital videoUse normal digital sensorAcquire rapidlyCompressTransmit/storeBut, each line is recorded separatelyPlayed back simultaneouslyRolling shutter

Time lapse imagesRequire intervalometer

SummaryFilm bad, digital goodIssues include:PowerStorageControlLightingDistortionInterpretationscaling