US 5,870,196 AGranted

Optical three-dimensional profilometry method based on processing SPECKLE images in partially coherent light, and interferometer implementing such a method

AssigneeEuropean Community

InventorsAlfredo Lulli, Alfredo Carlo Lucia, Marco Franchi

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Abstract

An optical three-dimensional profilometry method in which are recorded SPECKLE images formed by the interference, due to different optical paths, of the rays reflected/diffused by a rough object and the rays generated by a beam source. The method presents a number of processing steps including the steps of digitizing a first SPECKLE image, modifying the optical path difference, and digitizing a second SPECKLE image recorded after modifying the optical path difference. The difference in the modulus of the corresponding pixels of the first and second digitized images is determined to generate a difference image which is subsequently analyzed. A first arbitrary value is assigned to all the difference image pixels exceeding a given threshold value, and a second arbitrary value is assigned to all the difference image pixels of a value below the threshold. The binary image so generated is processed to generate an artificial image in which all the binary image pixels equal to the second arbitrary value are assigned a zero value, and all the binary image pixels equal to the first arbitrary value are assigned a current processing step value. The processing steps are repeated cyclically for a predetermined number of steps to generate a depth map presenting a number of pixels. Each pixel represents a grey level corresponding to the last processing step in which the threshold was exceeded. The depth map contains three-dimensional information relative to the profile of the rough object under examination.

Claims9

Dependent claims

An optical three-dimensional profilometry method based on processing SPECKLE images in partially coherent light, the method comprising the steps of:

generating a light beam;
splitting said beam and directing a first portion of the beam to a viewer;
directing a second portion of the beam onto a rough object for examination, and collecting the rays reflected and diffused by the object by means of said viewer;
acquiring, with said viewer, a first SPECKLE image formed by interference between the rays of said first portion and the rays from said object;
digitizing the first SPECKLE image and converting it into a first matrix of pixels, wherein each pixel corresponds to a grey level;
modifying the geometric path difference of the paths travelled by the rays of said first portion and the rays of said second portion;
digitizing a second SPECKLE image acquired following said modification of the geometric path difference and generating said digitized second SPECKLE image defined by a second matrix of pixels;
comparing the first digitized SPECKLE image with the second digitized SPECKLE image, determining the difference in the modulus of the corresponding pixels of the first and second images and generating a difference image;
analyzing said difference, assigning a first arbitrary value to all the pixels in the difference image exceeding a threshold value, assigning a second arbitrary value to all the pixels in the difference image representing a value below or equal to said threshold, and generating a binary image;
and
using said binary image to generate an artificial image by assigning a current processing step value solely to the pixels in the binary image representing a value equal to a first arbitrary value;
the value assigned to the other pixels in the binary image in the previous processing step being left unchanged;
said method also comprising a cyclic repetition of a predetermined number of said processing steps, terminating a data acquisition step upon said predetermined number of processing steps being reached, and generating a final matrix comprising a number of pixels, each representing a value corresponding to the last processing step value at which said threshold was exceeded;
said final matrix defining a depth map containing three-dimensional information related to the profile of the rough object under examination.

11 more elements

An interferometer for three-dimensional profilometry based on processing SPECKLE images in partially coherent light said interferometer including:

light beam generator and a filter modifying the coherence length of the rays of said beam;
a splitter facing said beam generator and receiving an incoming beam produced by said generator;
a reflector facing said beam splitter and intercepted by a first portion of said incoming beam travelling through said beam splitter, said reflector comprising a position regulator for adjusting the distance between said reflector and said beam splitter by moving said reflector in successive steps of a constant quantity;
a viewer cooperating with said beam splitter and receiving said first portion reflected by said reflector;
said beam splitter also directing a second portion of said incoming beam onto to rough object for profile examination, to direct the rays reflected and diffused by the object towards said viewer;
an electronic recorder connected to said viewer and which provides for acquiring a SPECKLE image formed by interference between the rays of said first portion and the rays from said object;
and
a control unit for sequentially processing a number of SPECKLE images to obtain three-dimensional information related to the profile of the rough object under examination.

5 more elements

Dependencies are inferred from the claim text.