Radiant energy – Photocells; circuits and apparatus – Photocell controls its own optical systems
Reexamination Certificate
1999-04-29
2001-10-02
Evans, F. L. (Department: 2877)
Radiant energy
Photocells; circuits and apparatus
Photocell controls its own optical systems
C250S201500, C356S602000, C356S003020
Reexamination Certificate
active
06297488
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the field of position sensitive light spot detectors for single or multiple wavelengths (e.g. colour RGB) arranged as points, lines (straight or curved) or grids such as for use in range cameras.
BACKGROUND OF THE INVENTION
The optical measurement of light spot position is used in many robotic control and vision systems. Range cameras, used for three dimensional (3D) imaging, exploit geometric arrangements (such as active optical triangulation), properties of light propagation, and light interference to extract distance shape. Optical triangulation relies on the measurement of the position of a focused light beam impinging on a linear or bi-dimensional sensor.
FIG. 1
illustrates a basic light position measurement system
10
according to the prior art. The system includes a lens
12
for receiving light beams
14
a,b
generated from a projected beam
16
. An optical position sensor
18
receives the light beams
14
a,b
after passing through the lens
12
emanating from the intersection of the projected beam
16
with a reflective surface at
19
a
and
19
b.
Optical triangulation is based on the sine law where the knowledge of two angles of a triangle and one side fully determines its dimensions. As shown in
FIG. 1
, two known angles ∝
1
, and ∝
2
of a triangle
20
(bounded by the projected beam
16
, the collected beam
14
a
and a baseline
22
) are the angles of the projected beam
16
and the collected beam
14
a
relative to the baseline
22
, respectively. The angle ∝
2
of the collected beam
14
a
is measured using the lens
12
and the optical position sensor
18
that measures a length L as the distance between the collected means
14
a,b
on the sensor
18
. The length L is related to the angle ∝
2
and therefore a displacement Z based on the sine law as discussed above.
Many devices have been proposed in the prior art for measuring the position of the collected light beams. The devices generally belong to one of two main groups: continuous response position sensitive detectors (CRPSD) and discrete response position sensitive detectors (DRPSD).
A CRPSD is defined as a class of position sensitive detectors that determinelcalculate the centroid of a light distribution, which may include stray light components in addition to a desired light spot. A DRPSD is defined as a class of position sensitive detectors that samples and analyses the entire light distribution to determine the position of the desired light spot within the light distribution.
CRPSD are generally based on lateral effect photodiodes and geometrically shaped photo-diodes (wedges or segmented) such as disclosed in A. Makynen and J. Kostamovaara,
Linear and sensitive CMOS Dosition sensitive photodetector
, Electronics Letters, Vol. 34 (12), pp. 1255-1256, Jun. 1998 and in A. Makynen et al.,
High accuracy CMOS position sensitive Photodetector (PSD)
, Electronics Letters, Vol. 33 (2
1
), pp.128-130, Jan. 1997.
DRPSD are generally implemented using an array of photosensors where they are read out serially by metal oxide semiconductor field effect transistor (MOSFET) switches or a charge coupled device (CCD) such as disclosed in F. Blais and M. Rioux,
Real
-
Time Numerical Peak Detector
, Signal Process., 11(2), 145-155 (1986).
CRPSD are known to measure the centroid of a light distribution impinging on its resistive surface to a very high level of resolution and speed. However, accuracy is reduced when spurious light is present together with the main light distribution. DRPSD can achieve, with a peak detection algorithm, higher accuracy levels since the distribution is sampled and hence available for processing but at a slower speed relative to a CRPSD.
Consequently, there is a need for a position sensitive detector that provides high resolution and speed, as offered by traditional CRPSD, together with high accuracy under variable light conditions, as offered by traditional DRPSD.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an improved position sensitive light spot detector.
Another object of the present invention is to provide a position sensitive light spot detector capable of determining the centroid of a desired light spot under variable lighting conditions.
In accordance with one aspect of the present invention there is provided an apparatus for detecting a position of a light spot in a light distribution comprising: (a) means for determining a first centroid of the light distribution; and (b) means for determining a second centroid of the light distribution within a reading window defined about the first centroid and within the light distribution, said second centroid being the position of the light spot in the light distribution.
In accordance with another aspect of the present invention there is provided a method of detecting the position of a light spot in a light distribution comprising the steps of: (a) calculating a first centroid of the light distribution; (b) determining a reading window defined about the first centroid and within the light distribution; and (c) calculating a second centroid of the light distribution within the reading window, said second centroid being the position of the light spot in the light distribution.
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F. Blais and M. Rioux, “Real-Time Numerical Peak Detector”,Signal Process, 11(2), Jan. 1986, pp. 145-155.
A. Makynen et al., “High Accuracy CMOS Position Sensitive Photodetector (PSD)”,Electronics Letters, vol. 33, No. 2, Jan. 1997, pp. 128-130.
A. Makynen and J. Kostamovaara, “Linear and Sensitive CMOS Position-Sensitive Photo-detector”,Electronics Letters, vol. 34(12), Jun. 1998, pp. 1255-1256.
“Large Area Sensor Technology—LAE”, Heimann, Heimann GmbH, Weher Koppel 6, D-6200, Wiesbaden, West Germany.
Mäkynen, Anssi et al., “A binary photodetector array for position sensing”, Sensors and Actuators A, CH, Elsevier Sequoia S.A., Lausanne, vol. 65, No. 1, Feb. 1998, pp. 45-53.
Beraldin J. Angelo
Blais Francois
Domey Jacques
Rioux Marc
Evans F. L.
National Research Council of Canada
Smith Zandra V.
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