Volcano Blog by Tom Pering
  • Blog
  • About the Blog
  • About Me
  • Publications
  • Slug Calculator
  • Codes
    • Corrplot
    • Image Acquisition and Signal Processing
    • Image Processing
    • Other Matlab Codes
  • Photos
    • Etna 2011
    • Etna 2012
    • Etna 2013
    • Glencoe
    • Iceland 2011
    • Rainier
    • Sakurajima 2013
    • Stromboli 2013
    • Vulcano 2014
    • Yellowstone 2013
  • Videos
    • Dynamics of mild strombolian activity on Mt Etna Elsevier AudioSlides presentation
    • High time resolution fluctuations in volcanic carbon dioxide degassing from Mount Etna AudioSlides Presentation

Low cost ultraviolet camera measurements using the Raspberry Pi

14/10/2016

 
PictureLeft - Example image of sulphur dioxide emissions from a power station. The image was created using two different wavelengths of UV light. Right - Tom Wilkes on the flanks of Mt Etna with the Raspberry Pi UV camera system.
Last week our article "Ultraviolet Imaging with Low Cost Smartphone Sensors: Development and Application of a Raspberry Pi-Based UV Camera" was published in the Journal Sensors. The article is fully open access so can be downloaded in full! This work is the result of a continuing collaboration between the Department of Geography and the Department of Electronic and Electrical Engineering at the University of Sheffield, with the majority of the leg work done by current PhD student Tom Wilkes (Geography). In my last blog post, I talked about ultraviolet (UV) cameras, how they work, the improvements on previous techniques, and why we need them. However, the majority of current UV cameras are quite expensive! For a completely functioning system the costs may range anywhere between £10k to £50k. In our latest paper we detail a new method which uses the low cost Raspberry Pi camera and the Raspberry Pi computer to significantly reduced the costs of a complete UV camera system to perhaps <£500. There are also several other benefits of using the Raspberry Pi based set-up, including: a reduction in power needs,  a reduction in the weight, and the ease of finding necessary parts. 

The majority of digital cameras and sensors such as the Raspberry Pi camera, when they are used unaltered, are sensitive only to visible light. This makes sense, as most of the time we want to take pictures which resemble what we see with our eyes! However, by removing certain layers from the surface of the camera sensor we can significantly increase the sensitivity to UV light at the wavelengths needed to measure the absorption of UV light by sulphur dioxide for example (see Figure above and video below for example application at a power station). There are also several other UV imaging applications which could benefit. ​The development of this new method also necessitated the purchase of new lenses (to focus the UV light onto the sensor) and the design of a new lens holder which was 3D printed. For full details see the published paper. 

Being a part of the development of this new method from the outset has been fantastic fun, and it is always amazing when everything turns out better than expected!

What is an Ultraviolet (UV) camera and how does it work?

12/10/2016

 
PictureUltraviolet cameras set up on the flanks of Mt. Etna.
I have realised that I often talk about my work using UV (Ultraviolet) cameras on this blog but I have never properly explained how they work and why we specifically use them! So here I go.

Monitoring volcanic gas release is one of the major things volcanologists can do to aid with eruption forecasting (see this recent post by James Hickey to understand why we call it forecasting). A number of techniques have therefore been developed and used to measure the release of volcanic gases from volcanoes. One of such techniques is the UV camera (Bluth et al. 2007; Mori and Burton, 2006), pictured on the right. These were developed to solve the time and spatial issues with previous techniques (e.g., DOAS and COSPEC). These previous techniques required building up a plume profile using a stepper motor to scan across a plume or traversing under it. Traversing could be conducted on foot, by moped, by car, or aircraft! The UV camera immediately solves this problem as it essentially works like a conventional camera by taking still images using the UV part of the electromagnetic spectrum, instead of visible light like your everyday camera. 

The UV camera and previous techniques all work on the principle of absorption of UV light by the volcanic gas sulphur dioxide (SO2). We specifically use SO2 due to the very low atmospheric background concentration of this gas. This makes it very easy to resolve the volcanic component. We can then, using two different cameras taking images at exactly the same time, compare two images (captured at the same time from each camera) where SO2 does and doesn't absorb (310 and 330 nm respectively). The graphic below illustrates the basic concept.

Picture
Graphic demonstrating the principle of ultraviolet spectroscopy. Incoming solar radiation (sunlight) is absorbed by sulphur dioxide within the volcanic plume. Light which isn't absorbed continues to the ultraviolet camera where the light intensity is recorded.
PictureThe plume from the NEC at Mt. Etna. Lines (a) and (b) indicate locations used to calculate plume transport speed.
We can then use the Lambert-Beer law to combine the images captured at each wavelength to work out the strength of absorption in each pixel. At this stage the calculated value will preserve real fluctuations in concentration but we won't know exact SO2 values without calibration! There are two distinct ways of doing this.

The first method is to image quartz gas cells, with known concentrations of SO2, with both cameras. We can then use the Lambert-Beer law again to combine the images at the separate wavelengths (310 and 330 nm - where SO2 does and doesn't absorb), which will create a series of new images with the absorption strength in each pixel - but this time we know exactly how much gas should be in front of the image - we can therefore use this to create what we call a calibration curve (or line) to map the values in the image of the volcanic plume to real SO2 concentrations.

The second method is to combine the DOAS method (Differential Optical Absorption Spectroscopy) with the UV camera images. DOAS works in a different way, essentially measuring a single pixel in the plume, and it has the ability to determine actual SO2 concentration (based on the absorption structure of SO2 at different wavelengths). During acquisition the concentration of SO2 will change at the location the DOAS is pointing, again enabling us to calibrate our images through comparison of the absorption pixel values in the UV camera images for the same pixel with the known value. Both of these processes are illustrated in the photo slideshow below. We can then calculate the rate at which SO2 is released from a specific crater, vent or fumarole by multiplying the plume speed by the total concentration of SO2 contained along a single line (e.g., line [a] in the image above), what we call the integrated column amount. Plume speed can be determined in a couple ways, including: optical flow algorithms which map the movement of pixels from image to image (e.g., Peters et al. 2015) or by using a cross-correlation technique (e.g., McGonigle et al. 2005) which uses the structure of the gas plume and how long it takes for parts of the plume to move from one point to another (illustrated in picture of plume from Etna above as lines [a] and [b]).

So this how we make UV camera measurements! The frequency that images can be taken is generally every 1 second but this can potentially be increased to 15 or even 30. This has the obvious benefit of being able to measure rapidly changing volcanic gas emissions associated with explosive activity such as strombolian volcanism or during passive activity (the constant quiescent release of gas). We can also begin to compare SO2 flux measurements with other datasets collected at similar time frequencies (e.g., Burton et al. 2015 offer an overview of UV camera studies). The benefits offered with an increase in spatial resolution can allow us to image more than one source at the same time - perfect where more than one crater (e.g., Etna) may be emitting gases or we may be observing a number of fumaroles (e.g., Vulcano). 

I hope this has been interesting aside into one of the major pieces of equipment which allows me to do my research! My next post will be all about a low cost UV camera system developed at the University of Sheffield and recently published in the Journal - Sensors.

References
  • Bluth, et al., 2007. Development of an ultra-violet digital camera for volcanic SO2 imaging. J. Volcano. Geoth. Res. 161, 47-56.
  • Burton, M.R., Prata, F., Platt, U., 2015. Volcanological applications of SO2 cameras. Journal of Volcanology and Geothermal Research 300, 2-6.
  • McGonigle, A. J. S., Hilton, D. R., Fischer, T. P., Oppenheimer, C., 2005. Plume velocity determination for volcanic SO2 flux measurements. Geophysical Research Letters 32 (L11302).
  • Mori, T., Burton, M., 2006. The SO2 camera: A simple, fast and cheap method for ground-based imaging of SO2 in volcanic plumes. Geophys. Res. Lett. 33.
  • Peters, N., Hoffmann, A., Barnie, T., Herzog, M., Oppenheimer, C., 2015. Use of motion estimation algorithms for improved flux measurements using SO2 cameras. Journal of Volcanology and Geothermal Research 300, 58-69.





    RSS Feed

    Volcano Blog Logo
    Picture
    Eruptions Blog
    Picture
    Magma Cum Laude
    Donate to the IVM-Fund! Click on the picture to go to the website!
    Share this page

    Archives

    February 2020
    December 2019
    November 2019
    April 2019
    January 2019
    May 2018
    April 2018
    March 2018
    February 2018
    January 2018
    October 2017
    August 2017
    June 2017
    January 2017
    October 2016
    July 2016
    June 2016
    May 2016
    April 2016
    January 2016
    December 2015
    July 2015
    June 2015
    May 2015
    April 2015
    March 2015
    January 2015
    December 2014
    August 2014
    July 2014
    June 2014
    May 2014
    April 2014
    January 2014
    November 2013
    October 2013
    September 2013
    August 2013
    July 2013
    June 2013
    May 2013
    April 2013
    March 2013
    February 2013
    January 2013
    December 2012
    November 2012
    October 2012
    September 2012
    August 2012
    July 2012
    May 2012
    April 2012
    March 2012
    February 2012
    December 2011
    November 2011
    October 2011
    September 2011
    July 2011
    June 2011
    May 2011
    April 2011
    March 2011

    Author

    I am currently studying volcanology in the UK and want to share this interest with others. 

    Categories

    All
    Adams
    Arthurs Seat
    Australia
    Avachinsky-Koryaksky
    Basalt
    Bbc
    Blog Update
    Cascades
    Castle Rock
    Chile
    Colima
    Column
    Costa Rica
    Decade Volcano
    Decade Volcanoes
    Dolerite
    Dubbi
    Edinburgh
    El Chichón
    El Hierro
    Eritrea
    Etna
    Eyjafjallajökull
    Fumarole
    Further Study
    Galeras
    General
    Geoinformatics
    Glencoe
    Grimsvotn
    Harmonic Tremor
    Hawaii
    Haystacks
    Hidden Journeys
    Hotspot
    Iceland
    Ignimbrite
    Importance Of Gas
    Indonesia
    Information
    Insar
    Jokulhlaup
    Kagoshima
    Katla
    Kilauea
    Lake District
    Langdale
    Lava Flows
    Lava Fountain
    Lava Lake
    Lava Stalactites
    Lava Tube
    Lightning
    Maar
    Mantle
    Mauna Loa
    Media
    Merapi
    Mercury
    Mexico
    Modelling
    Mount Hood
    Myrdalsjokull
    Mythology
    Nabro
    Nyamuragira
    Nyiragongo
    Passive Degassing
    Planetary Volcanism
    Plume
    Popocatepetl
    Popocatépetl
    Puyehue Cordon
    Puyehue-Cordon
    Puyehue-Cordon Caulle
    Puyehue-Cordón Caulle
    Pyroclastic Deposits
    Rainier
    Rift Valley
    Ruapehu
    Sakurajima
    Santa Maria
    Santiaguito
    Skaftafell
    St Helens
    Strokkur
    Stromboli
    Strombolian
    Summary
    Super Eruption
    Svartifoss
    Tag Cloud
    Teaching
    Terminology
    Thrihnukagigur
    Tuff Ring
    Turrialba
    Uk
    United Kindom
    Vei 8
    Vidgelmir
    Volatiles
    Volcanology
    Volcano Puzzle
    Vulcanian
    Xenolith
    Xenoliths
    Yellowstone

An informative blog for anyone interested about all things volcano related!