Position sensitive light spot detector

• US 6,297,488 B1
• Filed: 04/29/1999
• Issued: 10/02/2001
• Est. Priority Date: 04/29/1999
• Status: Expired due to Term

• Current Assignee: National Research Council Canada
• Original Assignee: National Research Council Canada
• Inventors: Domey, Jacques, Blais, Francois, Beraldin, J. Angelo, Rioux, Marc
• Primary Examiner(s): Evans, F. L.
• Assistant Examiner(s): Smith, Zandra V.
• Application Number:
  US09/300,445
• Time in Patent Office: 887 Days
• Field of Search: 250/559.38, 250/201.2, 250/201.5, 250/214.1, 250/208.1, 356/602, 356/600, 356/3.01, 356/3.02, 356/4.01
• US Class Current: 250/201.2
• CPC Class Codes: G01C 3/08 Use of electric radiation detectors
• CPC Class Codes: G01D 5/30 the beams of light being detected by photocells
• CPC Class Codes: G01J 1/1626 Arrangements with two photodetectors, the signals of which are compared
• CPC Class Codes: G01J 1/42 using electric radiation detectors optical or mechanical part G01J1/04; by comparison with a reference light or electric value G01J1/10
• CPC Class Codes: G01J 1/4204 with determination of ambient light solar light G01J2001/4266

Abstract

The present invention provides a method and an apparatus for detecting a position of a light spot in a light distribution that can include stray light components (e.g. from other lasers, ambient lighting etc.) The apparatus includes a continous response position sensitive detector (CRPSD, e.g. lateral effect photo-iode) for determining a first centroid of the light distribution and a discrete response position sensitive detector (DRPSD, e.g. multiplexed array) for determining a second centroid of the light distribution within a reading window defined about the first centroid and within the light distribution. The second centroid represents the position of the light spot in the light distribution. This multiple stage approach exploits the high resolution and speed offered by traditional CRPSDs together with the accuracy under variable lighting conditions offered by traditional DRPSDs.

Claims

1. 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.

2. The apparatus of claim 1, wherein the means for determining the first centroid of the light distribution includes:

(i) a continuous response position sensitive detector having N photo-elements for receiving the light distribution and producing a first and a second photo-current; and

(ii) means for calculating the first centroid based on the first and second photo-currents.

3. The apparatus of claim 2, wherein the first centroid is defined by:

$P=\frac{I_2-I_1}{I_1+I_2},P\in <br><br>\left[-1,1\right]$

where P represents the centroid and I₁, and I₂ represent the first and second photo-currents respectively.

4. The apparatus of claim 2, further comprising:

(i) an analog-to-digital converter for converting the first centroid into a digital representation; and

(ii) an adder for shifting the digital representation of the first centroid by a predetermined amount based on the reading window.

5. The apparatus of claim 4, wherein the means for determining the second centroid includes:

(i) a discrete response position sensitive detector for reading M photo-elements, wherein M represents a size of the reading window and M is less than N; and

(ii) a circuit for detecting the second centroid based on determining a peak of the digital representation of the first centroid.

6. The apparatus of claim 2, wherein the continuous response position sensitive detector is a transparent lateral effect photo-diode having a first sensor and a second sensor whereby the light distribution passes through the first sensor to the second sensor.

7. The apparatus of claim 2, wherein the continuous response position sensitive detector is a split sensor having a pair of spaced apart wedges to reduce thermal noise whereby the light distribution impinges the pair of spaced apart wedges and wherein each wedge generates one of the respective first and second photo-currents.

8. The apparatus of claim 5, wherein the continuous response position sensitive detector is transparent and overlays the discrete response position sensitive detector.

9. The apparatus of claim 5, further comprising a beam splitter for splitting the light distribution between the continuous response position sensitive detector and the discrete response position sensitive detector, wherein the continous response position sensitive detector and the discrete response position sensitive detector are separated.

10. The apparatus of claim 5, further comprising a diffraction grating for directing the light distribution between the continuous response position sensitive detector and the discrete response position sensitive detector, wherein the continuous response position sensitive detector and the discrete response position sensitive detector are separated.

11. An apparatus for detecting a position of a multiple wavelength light spot in a light distribution comprising the apparatus of claim 1 for each wavelength and means for computing a first estimate of the centroid of the multiple wavelength light spot.

12. The apparatus of claim 11, wherein the multiple wavelength light spot is composed of a red (R,r) wavelength component, a green (G,g) wavelength component, a blue (B,b) wavelength component and wherein the means for determining the first centroid of the light distribution includes:

(i) a continuous response position sensitive detector having N photo-elements for receiving the light distribution and producing a first and a second photo-current; and

(ii) means for calculating the first centroid based on the first and second photo-currents.

13. The apparatus of claim 12, wherein the first centroid for each wavelength component (R,G,B) is defined by:

$P=\frac{I_2-I_1}{I_1+I_2},P\in <br><br>\left[-1,1\right]$

where P represents the centroid and I₁ and I₂ represent the first and second photo-currents respectively for each wavelength component (R,G,B) where p_r, p_g and p_b represent light spot positions for each wavelength component and I_r, I_g and I_b represent wavelength intensities value for each wavelength component and where I_{r,g, or b}=I₁+I₂.

14. The apparatus of claim 13, wherein the means for computing includes a weighted estimator for receiving the wavelength intensities and I_r, I_g and I_b and the light spot positions p_r, p_g and p_b and generating the first estimate of the centroid of the multiple wavelength light spot, said first estimate being used as input to the means for determining the second centroid.

15. The apparatus of claim 14, wherein the first estimate of the centroid of the multiple wavelength light spot is calculated using:

$p=\frac{\frac{1}{{\mathrm{\tau <br><br>}}_g^2}}{\sum <br><br>}<br><br>\text{\hspace{1em}<br><br>}<br><br>p_g+\frac{\frac{1}{{\mathrm{\tau <br><br>}}_r^2}}{\sum <br><br>}<br><br>\text{\hspace{1em}<br><br>}<br><br>p_r+\frac{\frac{1}{{\mathrm{\tau <br><br>}}_b^2}}{\sum <br><br>}<br><br>\text{\hspace{1em}<br><br>}<br><br>p_b,\text{\hspace{1em}<br><br>}<br><br>\mathrm{where}<br><br>\text{\hspace{1em}<br><br>}\sum <br><br>=\frac{1}{{\mathrm{\tau <br><br>}}_g^2}+\frac{1}{{\mathrm{\tau <br><br>}}_r^2}+\frac{1}{{\mathrm{\tau <br><br>}}_b^2}$

where p is the first estimate, p_g. p_r, p_b represent the position of light spots, and τ

_r², τ

_g², τ

_b² represent a variance of the three light spot positions p_g, p_r, p_b.

16. The apparatus of claim 15, wherein the variance of the three light spot positions are obtained using:

${\mathrm{\tau <br><br>}}_g^2=K_g<br><br>I_g^2,{\mathrm{\tau <br><br>}}_r^2=K_r<br><br>I_r^2,{\mathrm{\tau <br><br>}}_b^2=K_b<br><br>I_b^2$

where K_g,r,b represent a width of the reading window.

17. 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.

18. The method of claim 17, wherein step (a) is determined by impinging a first detector with the light. distribution and obtaining a first and a second photo-current where the first centroid is calculated by:

$P=\frac{I_2-I_1}{I_1+I_2},P\in <br><br>\left[-1,1\right]$

where P represents the first centroid and I₁ and I₂ represent the first and second photo-currents respectively.