Full text: XVIIIth Congress (Part B5)

  
A 3D-SENSOR FOR THE MEASUREMENT OF PARTICLE CONCENTRATION FROM IMAGE SEQUENCES 
Peter Geißler * 
* Interdisciplinary Center for Scientific Computing 
University of Heidelberg 
INF 368, D-69120 Heidelberg, FRG 
Peter.Geissler@iwr.uni-heidelberg.de 
Bernd Jähne *? 
? Scripps Institution of Oceanography 
Physical Oceanography Research Division 
La Jolla, CA 92093-0230, USA 
bjaehne@giotto.iwr.uni-heidelberg.de 
Commision V, Working Group V/III 
KEY WORDS: Camera,Depth-from-Focus, Experiment, Optical,Sensor, Three-dimensional, Underwater 
ABSTRACT 
A sensor for the measurement of the concentration and size distribution of small particles is described. The particles are 
visualized by a light blocking technique and imaged by a CCD camera. The concentration measurement is based on a depth- 
from-focus approach to determine the positions of the particles in three-dimensional world coordinates. From these coordinate 
sets of all observed particles both the measuring volume as well as the concentration can be determined. The sensor has been 
developed for the underwater measurement of size distributions of air bubbles submerged by breaking waves to investigate the 
influence of air entrainment to air-sea gas exchange. 
1 INTRODUCTION 
The investigation of the exchange of clima-relevate trace 
gases such as CO» and oxygen is a current research topic 
in modern oceanography. The underlying physical processes 
are still poorly understood. At low windspeeds, gas exchange 
is controlled by molecular diffusion through the air/sea phase 
boundary. With increasing windspeed the situation changes 
dramatically. Breaking waves entrain air in form of gas bub- 
bles directly to the water. During their lifetime underneath 
the water surface, these bubbles form an additional reservoir 
which exchanges its gas contents with the water. Indeed, a 
strong increase of the gas exchange rates is found when waves 
start to break [Merlivat,83]. The influence of the air bubbles 
strongly depends on parameters such as life time, penetration 
depth and the amount of gas confined. All these parameters 
are determined by the size of the bubbles. Therefore precise 
measurements of bubble size spectra and their dependence 
on wind speed are necessary. 
To investigate the properties of air bubble distributions, we 
have developed an optical sensor which images bubbles on a 
CCD camera. This allows for a instrument which causes only 
low flow disturbances. The need to place the measurement 
volume as far away from the instrument as possible and to 
image bubbles in a size range of 20 um to 1000 um with a 
suitable resolution causes a narrow depth of field. Therefore, 
blur in the images is introduced in a natural way, causing only 
bubbles near the focal plane to be imaged sharply. With in- 
creasing distance from the focal plane (subsequently denoted 
by ’depth’) the images of the bubbles become blurred. With 
our technique this blur is used to determine their depth and, 
consequently, their three-dimensional position in real world 
coordinates. From these data, both the measuring volume as 
well as the size distribution is determined. 
This paper will describe the sensor and the image processing 
techniques used to determine the size distributions from the 
image sequences. The sensor is also suitable for other types 
of particles in gases or liquids, provided they are of uniform 
shape and their refractive index is different from that of the 
surrounding medium. 
194 
2 DEPTH-FROM-FOCUS BASED 
CONCENTRATION MEASUREMENT 
2.1 Introduction to Depth-from-Focus 
The processes of imaging a three-dimensional scene onto a 
two-dimensional sensor at a first glance causes the loss of 
depth information. Obviously depth recovery needs additional 
information. For example, stereo imaging can be used to 
partly reconstruct the distance of objects from two images. 
Another approach is the use of blur in the images for depth 
reconstruction. In contrast to a pinhole camera which images 
every point in object space to an ideal point on the image 
plane, a lens with finite aperture images only object points 
on the focal plane to points on the image plane. Increasing 
distance from this plane then causes an increase of blur in 
the image (Fig. 1). 
Depth-from-focus techniques estimate the grade of blur and 
correlate it with the distance of the objects from the focal 
plane. The depth resolution depends on the change of blur 
with the distance from the focal plane and is therefore related 
to the depth of field of the optics. Thus, a small depth of 
field results in a good depth resolution of a depth-from-focus 
system. 
  
image plane lens 
focal plane 
  
  
  
  
Figure 1: Defocused imaging: an object point at ug is imaged 
to a point at the conjugated image plane at vo. A point at u # uo 
results in a brightness distribution of width o on the image plane. 
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B5. Vienna 1996 
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