High speed topography measurement of semi-diffuse objects
First Claim
1. A system for rapidly measuring the topography of a semidiffuse target surface comprising:
- a projection source means comprising a laser producing a pulsed plane polarized light beam having a substantially uniform intensity cross section, said light beam passing through a mask, said mask having apertures to form a grid pattern, said light beam through said grid pattern projecting sharply defined bright and dark areas on the semi-diffuse target surface;
a video camera means comprising a polarizing filter, image forming optics and a CCD (charge coupled device), said camera forming on the CCD an image of the grid pattern projected on said target surface, the polarizing filter blocking specular reflected light and passing diffuse reflected light from said image on the target surface;
a focussing means whereby said projection source and the video camera are mounted on a common platform, said platform being movable as to maintain focus and positioning of the grid pattern on the CCD;
a master clock producing low jitter constant frequency pulses;
a frequency divider receiving pulses from said master clock and delivering pulses at a reduced repetition rate to the CCD so that a single pixel readout occurs for each of the reduced repetition rate pulses, said readout resulting in an output voltage from the CCD proportional to pixel light intensity;
a master clock counter giving a count of all pulses from said master clock over a time period corresponding to a complete readout of the pixels of the CCD, said count precisely identifying the pixel location on the CCD;
an analog circuit assembly consisting of a differentiator, zero crossing detector, and gate circuit, said assembly enabling the capture of the master clock count at an instant when a voltage peak corresponding to a light intensity peak is outputted by the CCD and when said voltage peak is above a threshold value;
a digital register, into said register is stored the master clock count at the instant an enabling pulse is issued by said gate circuit;
an analog to digital converter producing a digital equivalent of the CCD output voltage, said digital equivalent being stored in said digital register simultaneous with storage of said master clock count, said converter being used to provide a feedback signal to automatically control movement of said platform;
a digital computing means, either in an application specific integrated circuit (ASIC) or computer based, performing in real time the functions of temporary storage, averaging, interpolation, error correction, topography calculation and displays.
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Abstract
Forming an image on a CCD of a grid pattern projected on a target surface and detecting the positional coordinates of the image by correlating voltage peaks in the CCD output voltage during pixel readout synchronized with the count of pulses from a master clock. Topography is calculated by knowing the equations of the rays comprising the projection pattern and the rays from the determined CCD positional coordinates and solving for the 3D coordinates formed by the intersections of the two sets of rays. Means are provided for: blocking the specular component of reflected light from the target surface while passing the diffuse component; forming the rays of the projected grid pattern so that fluctuations in the target surface position do not cause measurement error; adequate focussing of the full grid pattern on the CCD; differentiating the CCD output voltage and detecting a zero crossing for precise timing of count capture at the instant of a peak. In a preferred embodiment, the target surface is a cornea undergoing PRK and a laser projection source pulsed at a high enough rate to provide real time differential topographical display and feedback to the photoablation means. Alternate embodiments include: using the uv photoablating laser as also the projection source in conjunction with a uv sensitive CCD; an adjunct CCD type sensor in the 8 micron region for monitoring corneal sector temperatures during PRK concurrent with the topography measurement.
45 Citations
20 Claims
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1. A system for rapidly measuring the topography of a semidiffuse target surface comprising:
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a projection source means comprising a laser producing a pulsed plane polarized light beam having a substantially uniform intensity cross section, said light beam passing through a mask, said mask having apertures to form a grid pattern, said light beam through said grid pattern projecting sharply defined bright and dark areas on the semi-diffuse target surface; a video camera means comprising a polarizing filter, image forming optics and a CCD (charge coupled device), said camera forming on the CCD an image of the grid pattern projected on said target surface, the polarizing filter blocking specular reflected light and passing diffuse reflected light from said image on the target surface; a focussing means whereby said projection source and the video camera are mounted on a common platform, said platform being movable as to maintain focus and positioning of the grid pattern on the CCD; a master clock producing low jitter constant frequency pulses; a frequency divider receiving pulses from said master clock and delivering pulses at a reduced repetition rate to the CCD so that a single pixel readout occurs for each of the reduced repetition rate pulses, said readout resulting in an output voltage from the CCD proportional to pixel light intensity; a master clock counter giving a count of all pulses from said master clock over a time period corresponding to a complete readout of the pixels of the CCD, said count precisely identifying the pixel location on the CCD; an analog circuit assembly consisting of a differentiator, zero crossing detector, and gate circuit, said assembly enabling the capture of the master clock count at an instant when a voltage peak corresponding to a light intensity peak is outputted by the CCD and when said voltage peak is above a threshold value; a digital register, into said register is stored the master clock count at the instant an enabling pulse is issued by said gate circuit; an analog to digital converter producing a digital equivalent of the CCD output voltage, said digital equivalent being stored in said digital register simultaneous with storage of said master clock count, said converter being used to provide a feedback signal to automatically control movement of said platform; a digital computing means, either in an application specific integrated circuit (ASIC) or computer based, performing in real time the functions of temporary storage, averaging, interpolation, error correction, topography calculation and displays. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12)
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13. A method for performing high speed topography measurements of a semi diffuse target surface comprising the steps of;
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aligning a light source to project through an apertured mask a cross sectionally modulated grid pattern; projecting said grid pattern on said target surface; imaging, using an objective lens, said grid pattern on a CCD with sufficient resolution to achieve a desired topography accuracy; setting a pulse repetition rate of a master clock at a rate higher than the rate at which light intensity at successive pixels is measured; reading out a voltage from the CCD proportional to light intensity of each pixel of the CCD, the step of pixel readout being performed at a submultiple of the pulse rate of the master clock, the step of pixel readout producing an output voltage as a function of time; counting the pulses from the master clock to produce a count during the readout of the pixels of the CCD, resetting said count to zero upon completion of the readout of all of the pixels of the CCD; capturing said counts precisely when peaks are detected in said output voltage, said peaks corresponding to peaks of light intensity; storing said captured counts in memory, correlating the counts with two dimensional coordinates on the CCD corresponding to said detected peaks; calculating three dimensional coordinate points corresponding to the topography of the target surface from said two dimensional coordinates. - View Dependent Claims (14, 15, 16, 17, 18, 19)
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20. A method for topography determination of a semi-diffuse surface comprising the steps of:
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directing a plane polarized monochromatic pulsed through a mask to project a grid pattern on said surface; placing a plane polarizing filter rotated at right angles to the plane of polarization of the monochromatic light source thereby blocking specular light while passing diffuse light emanating from said projected grid pattern; imaging said pattern on a CCD of a video camera by means of an objective lens; initiating a readout of pixels comprising the image on the CCD, said initiating also being the point in time of initiating a count of pulses from a master clock; storing said count when a peak voltage out of the CCD corresponding to the readout of a light intensity peak occurring on a pixel or between two pixels is detected; correlating said count with a two dimensional coordinate point on said CCD; calculating the three dimensional topography of said surface by means of an algorithm that uses the fore knowledge of the point of origin and direction cosines of a first set of rays comprising the projected grid pattern, said algorithm using said two dimensional CCD coordinates to form a second set of rays, this second set of rays passing through a second point, said second point being the center of said objective lens, said second point and said two dimensional CCD coordinates forming a second set of direction cosines; solving for the intersection points in three dimensional space, said intersection points being the desired topography.
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Specification