Orb Photography of Dust Particles

(supervisor: Tony Cook)

Nature of project: experimental, instrumental

Available to full-time physicists or joint students.

Project description and methodology

With the popular use of digital cameras, many users have noticed "orb" or "disk" like artefacts that appear in flash photography taken at night. To see some example images see: http://en.wikipedia.org/wiki/Orb_(optics). The effect is caused by backscatter from dust, ice particles, or water droplets, close to the camera lens which are illuminated at a near zero phase angle from the flash bulb. A previous project showed how the orb size was linearly related to distance from either side of where the camera was focussed, and how the colour of the particle could be determined.

The main project aim is to use orb imaging as a simple particle analysis system with possible potential future applications for monitoring particle environments on planetary surfaces, in polluted atmospheres, and for use down mines, or in factories where lung diseases could be an issue.

A successful project will develop beyond the above in one/some of the following directions:
The purpose of this project is to:

1) Use the brightness cross-section through the edge of each orb to quantify what size the particles are

2) To determine the smallest sized particle that can be detected in this way

3) To demonstrate the technique in daylight using difference image photography

4) To explore orbs produced by forward scattering, to see if colour, size, or other physical properties can be determined.

When considering where to take your project, please bear in mind the time available. It is preferable to do fewer things well than to try many and not get conclusive results on any of them. However, sometimes it is useful to have a couple of strands of investigation in parallel to work on in case delays occur.

Additional scope or challenge if taken as a Year-4 project: To automate the detection and measurement of the orbs, perhaps using two cameras or two separated flash lights (different colours for distinguishing purposes) using stereo photogrammetry. Automatic image analysis would be done using lab view. Also to use a strobe light and a single long exposure in order to be able to track the particles in 3D.

Initial literature for students:

  1. Lock, J. A. and Fen g Xu. Optical Caustics Observed in Light S cattered by an Oblate Spheroid. Applied Optics 49 (2010): 1288 - 1304.
  2. Padley, P. Diffraction from a Circular Aperture Connexions. Rice University, Houston, TX, USA, 8 Nov. 2005 - see: https://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=3&ved=0ahUKEwif8LWl5ovWAhUCD8AKHSheCtcQFggyMAI&url=https%3A%2F%2Fcnx.org%2Fexports%2F75308055-281a-4989-b660-8dca9da92b2f%4033.6.pdf%2Fwaves-and-optics-33.6.pdf&usg=AFQjCNEFvIz5xKOKXFL7waerUHds9TmPDQ
  3. http://en.wikipedia.org/wiki/Orb_(optics)
  4. AC Cook et al. (2014) Close Range Remote Sensing of Levitated Dust Particles. EPSC abstr. 817 see... http://meetingorganizer.copernicus.org/EPSC2014/EPSC2014-817.pdf

Novelty, degree of difficulty and amount of assistance required

This is an interesting new application

It is of an average difficulty level - but of course more advanced students can take this further.

Some equipment used from a past orb project will be required and a dark room to do your work or a card board box in the lab to keep stray lght out.

Project milestones and deliverables (including timescale)

milestoneto be completed by
Decide upon materials and equipment neededend of October
Produce some orb images and study sharpness of edges of orbs and compare brightness gradient to particle sizesend of February
Compare back scatter with forward scatter light orbsmid-March
Investigate what other properties can be detremined from orb images other than distance and sizeEaster

Students taking this project will have to submit a full risk assessment form