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AN ANALOG ADAPTIVE SMART IMAGE SENSOR FOR SPATIO-TEMPORAL 
INFORMATION EXTRACTION 
F. Lavainne*, Y. NI**, P. de Carné*, F. Devos* 
* Institut d'Electronique Fondamentale, Université Paris Sud 
91405 Orsay Cedex, France 
Phone: (33) 1 69 41 78 32; Email: lavainne @axis.ief-paris-sud.fr 
** Institut National des Télécommunications 
9, rue Charles Fourier 
91011 Evry Cedex, France 
Email: ni@galaxie.int-evry.fr 
KEYWORDS: adaptive image sensor, FPN, low level vision, spatio-temporal informations, local edge image. 
ABSTRACT 
This paper presents an analog adaptive image sensor, which can extract spatio-temporal salient informations, without 
fixed pattern noise (FPN) problems. Some low level vision problems can be done with only simple local processing. 
Each pixel of this smart sensor has a photoreceptor (PN junction) and a tiny analog processor. The tiny analog 
processor adjusts constantly by small steps the local sensitivity according to the average incoming lighting level. This 
adaptation process makes the sensor insensible to the slow ambiant light change but very sensitive to fast variations. 
The digital output of this smart sensor can be readout fastly of the sensor without noise-speed problems such as in 
analog imager sensors. Many salient spatio-temporal informations can be extracted by analysing these digital output 
signals. In the natural image sensing conditions, spatial contrast informations can be translated into pure local temporal 
one by trembling movement of the sensor. By sensing these temporal informations, spatial contrast informations, such 
as local edge image, can be extracted without the FPN problem in the neighbour pixels because the spatial informations 
are sensed by the same pixel associated with sensor trembling movement. 
1. INTRODUCTION 
Many obstacles exist for the construction of artificial visual systems as like human eye. A practical method is to use 
some structured light sources so as to simplify vision tasks (Besl,1988). Two structured light types can be used: the 
spatial structured light and the temporal structured light; they can be combined. An interest for the biological 
environment shows that the human visual system is mainly sensitive to the temporal component : an image that remains 
strictly static in front of the retina becomes rapidly invisible; this is due to a biological adaptation process (Yarbus, 
1967). In the natural environment, the temporal structured light is therefore the most interesting. 
Some smart sensors have been studied, but these circuits either use only spatial structured light (Gruss,1991), 
(Ni,1993) either use temporal structured light (Delbrück, 1994) but with an adaptation constant time fixed and not 
programmable. In this paper, we present an adaptive image sensor for the temporal structured light. Each pixel of this 
sensor has a photoreceptor and a tiny analog processor. This tiny analog processor adjusts constantly the local 
sensitivity according to the ambiant light level. It is thus insensitive to the slow variations, but on the other hand very 
sensitive to quick variations, such acts a high pass filter. The speed of adaptation process is programmable. This 
increases the dynamic range of detection. It is thus possible, by making trembling movement to the sensor, to translate 
spatial contrast informations into pure local temporal ones. By sensing these temporal informations, spatial contrast 
informations can be extracted. 
2. PIXEL STRUCTURE 
2.1 Photoreceptor 
There are principaly two types of photoreceptors: discrete photoreceptor and continuous one. The discrete one uses 
some photocharge integration such as that in CCD or in MOS imager. The continuous one uses somewhat current- 
voltage conversion structure (Mead, 1989). The advantage of the discrete configuration is the conservation of short 
light variations, contrary to the continuous configuration (Lavainne, 1994). This can give a better sensitivity in the case 
where perfect synchronization is not possible. : iin 
We have chosen the discrete photoreceptor, a diffused diode on the substrate, because in many applications a good 
synchronization is not possible. The discrete aspect is obtained by a reverse polarisation of the diode, followed by an 
insulation of it from polarisation during a fixed time. 
IAPRS, Vol. 30, Part 5W1, ISPRS Intercommission Workshop "From Pixels to Sequences", Zurich, March 22-24 1995