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O P T I C A L I M A G I N G S Y S T E M S D E S I G N F O R U N D E R W AT E R V E H I C L E S

A R N A U B E C A .

O P D E R B E C K E J .

A L L A I S A .

Overview

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Lights

Optical sensors Local Storage /Processing

and/or To boat through fiber

(0-6000m) fiber link

Boat: - storage - (Pre)-processing

Overview: - Lighting systems - Optical imaging sensors - Placement on the vehicles - User interfaces - Data formatting - Processing - Conclusion

Lighting systems - For Video (continuous lighting):

- HID, 400W, (90lm/W, 36000lm)

(ex: 2kW on Victor6000, custom design)

- Deepsea P&L LED 90W (90lm/W, 9000lm)

- Bowtech LED 230W -> 20 000lm

- Photo, pulsed xenon flashes (custom design)

- 1200J Flash on Victor 6000 (period 4s) ~ 21.6 millions lumens

- 200J Flash on our HROV (period 4s) ~ 3.6 millions lumens

- 150J Flash on our upcoming 6000m AUV (period 1s) ~ 2.7 millions lumens

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Optical imaging sensors - Constraints :

- Good sensitivity

- Optical lenses accounting for water

- Small aberrations (chromatic, distortion, blur)

- Pressure resistance (up to 6000m)

- Example of sensors we designed :

- Upcoming 4K from DEEPSEA power & light

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Still imaging (DSLR 24Mpx) Cameras HD Stereo RIG

Optical imaging sensors - Easiest solution for camera integration:

- Put a camera in pressure tolerant housing

- Pros : Cheap

- Cons : Aberrations due to water refraction

- Two possible port: - Dome port : Wide angle but low sharpness

- Flat port : Angle reduction, chromatic aberration, distortion

- Solution : - Dome port with corrective lenses accounting

for water/glass/air refraction

- Pros : Minimize all aberrations and good sharpness

- Cons : cost & space in the housing

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Optical imaging sensors

- Most of our camera have corrective lenses

- Our last still imaging design as an example - 24 Mpx Nikon APS-C (23.6mmx15.7mm) DSLR in dome port housing

- Our own corrective lenses design :

- Simulation in Zemax software, finding lenses for optimal sharpness

- We use MTF for sharpness evaluation

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Dome port (Glass)

2 corrective lenses

Water

Still camera (Nikon dslr)

Air

Sensor plane

HROV Design final f/1.4 3 metres

MTF represent the loss in sharpness (contrast)as a function of spatial frequencies

Optical system is a low pass filter

Optical imaging sensors - Does it worth the investment ?

- Visual difference (at corner) without and with correction at f/2.8

- This is of course confirmed by MTF comparison - MTF is analysed on real system with

slanted edge MTF algorithm

- We now use MTF analysis even for off the shelve

products

- The same way we also analyse - Distortion (calibration)

- Focal length (calibration)

- Chromatic aberration (empirically)

- Noise (empirically)

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MT

F M

od

ulu

s

Spatial Frequency (lp/mm)

Edges/ Center without correction

Edges/ Center with correction

Light/Sensor relative placement

- It could be tempting to do this : - Very powerful wide angle light

- On the same Pan & Tilt as the camera

- But because of water backscattering - Lots of light will come from water (and particles)

and not from the scene.

- Pros : - Light following camera

- Cons : - Very poor image quality

- Nothing can be seen after 2m

even in clear water

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Camera Light

Light/Sensor relative placement

- Solution : - Put light as far as possible from the camera

- More lights with smaller angles

- Pros : - Good image quality compared to previous solution

- Increase distance of visibility

- Cons : - Real illumination study must optimize

uniformity

05/06/2018 9 VideoCor Cruise

Physical limitation - Due to backscattering, wathever the power you have, you can’t

change the water/scene return ratio - The empirically observed consequence is that we recommend a maximum altitude of 8m

for good imaging

- Example of loss of contrast and color at 10m (thanks to high resolution some details are still presents)

- The light attenuation in water is dependent on the wavelength - Ex: red is more attenuated than blue, hence it reduces color identification accuracy

- Color correction is possible but only if red information remains

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User interface for acquisition

- At the surface we’ve built user friendly interfaces for easy data acquisition

- For video :

- liveview on video monitors with a video matrix

to select on which screen the user wants

the video

- simple on/off button for recording

- For still camera:

- Web interface to access camera (multiple connections possibles)

- Liveview return

- Preset to load default settings for a given configuration

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Formating and synchronizing data

- To be usable as scientific data, acquisition must be synchronous with other vehicle data

- For still imaging :

- A time stamping is generated at photo triggering time (based on vehicle ntp clock)

- Navigation information are synchronized and put in photo metadata and also in separated file

- JPEG Files or RAW data can be generated

- For video:

- Video is timestamped and its name include date and time for future synchronization when processing data

- Videos are compressed in H.264 (30mb/s)

- Upcoming 4K will be compressed in H.265

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Optical mapping

- Optical data (video or photo) can be processed to build a global representation of the scene

- 2D mosaicking

- 3D reconstruction

- On-board of after the cruise

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Conclusions - Sharp underwater images are not possible without

corrective lenses

- Lights and sensors relative position must be optimized

- Data synchronization is crucial for scientific exploitation

- Perspectives : - High resolution stereo imaging

- Panoramic imaging

- Full 4K video chain

- High repetition rate pulsed xenon flash

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