Plenoptic zoom with optimised range

Abstract

An optical system includes an objective of the zoom type and a depth-estimating optical detecting unit, the depth-estimating optical detecting unit including a matrix array of micro-lenses and a matrix-array detector, the matrix array of micro-lenses being arranged so that the image of the focal plane of the zoom is focused by the matrix array of micro-lenses on the plane of the matrix-array detector. The optical system calculates, for a first focal length of the zoom and for a given object, the estimated distance of this object and the measurement uncertainty in this estimation, depending on the first focal length and the estimated distance and, this estimated distance being known, optimizes by allowing at least one second focal length of the zoom to which a lower measurement uncertainty in this estimation corresponds to be determined.

Claims

1. An optical system including an objective of a zoom type and a depth-estimating optical detecting unit, said depth-estimating optical detecting unit including a matrix array of micro-lenses and a matrix-array detector, the matrix array of micro-lenses being arranged so that an image of a focal plane is focused by the matrix array of micro-lenses on the plane of the matrix-array detector, wherein said optical system is configured to calculate at least: for a first focal length of a first zoom and for a given object, the distance of the object being in the field-depth interval of the first focal length, a first estimated distance of this object and a measurement uncertainty in this estimation, the measurement uncertainty being a distance as a function of said first focal length and said first estimated distance, and for a second focal length of a second zoom and for the same given object, the distance of the object being in the field-depth interval of the second focal length, a second estimated distance of this object and a second measurement uncertainty in this second estimation, the second measurement uncertainty being a second distance as a function of said second focal length and said second estimated distance of the object, a best estimated distance being the one that gives the smallest measurement uncertainty, and wherein the image of the focal plane is focused by the matrix array of micro-lenses on the plane of the matrix-array detector based on the best estimated distance.

2. The optical system as claimed in claim 1, wherein the optical system is further configured to calculate a loop of iterations allowing an optimal focal length to which a minimum measurement uncertainty in said estimation corresponds to be determined, each iteration including at least one change of focal length, and the estimated distance corresponding to a new focal length and the measurement uncertainty in this estimation to be calculated.

3. The optical system as claimed in claim 2, wherein the loop of iterations is carried out in a constant zoom focal plane.

4. The optical system as claimed in claim 2, wherein the loop of iterations is carried out in a variable zoom focal plane.

5. The optical system as claimed in claim 2, wherein the loop of iterations is carried out at constant zoom aperture.

6. The optical system as claimed in claim 2, wherein the loop of iterations is carried out at variable zoom aperture.

7. The optical system as claimed in claim 1, wherein the first and the second zooms are continuously variable focal lengths.

8. The optical system as claimed in claim 1, wherein the first and the second zooms are multi-focal objectives.

9. The optical system as claimed in claim 1, wherein the depth-estimating optical detecting unit is an infrared detecting unit.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will be better understood and other advantages will become apparent on reading the following nonlimiting description and by virtue of the appended figures, in which:

(2) FIG. 1 shows a fixed-focal-length light-field camera according to the prior art;

(3) FIG. 2 shows the measurement uncertainty in an estimated distance as a function of the distance for a known fixed-focal-length light-field camera;

(4) FIG. 3 shows the measurement uncertainty in an estimated distance as a function of the distance for a light-field camera including objectives of increasing focal length;

(5) FIG. 4 shows, for each preceding objectives, the minimum measurement-uncertainty range as a function of distance;

(6) FIGS. 5 and 6 show a light-field zoom according to the invention in two extreme focal-length configurations;

(7) FIG. 7 shows an enlarged view of the receiving portion of the preceding light-field zoom; and

(8) FIG. 8 shows, for a light-field zoom of continuously variable focal length, the minimum measurement uncertainty as a function of the estimated distance.

DETAILED DESCRIPTION

(9) The subject of the invention is an optical system including an objective of the zoom type and a depth-estimating optical detecting unit, said depth-estimating optical detecting unit including a matrix array of micro-lenses and a matrix-array detector, the matrix array of micro-lenses being arranged so that the image of the focal plane of the zoom is focused by the matrix array of micro-lenses on the plane of the matrix-array detector.

(10) To ensure depth is correctly estimated, it is necessary to know the distortion and field curvature of the zoom for various focal-length values of this zoom.

(11) There are two categories of zoom. On the one hand there are zooms of continuously variable focal length and on the other hand the zooms known as multi-focal objectives. The invention applies to both of these categories.

(12) By way of example, FIGS. 5 and 6 show a light-field zoom Z according to the invention in two extreme focal-length configurations. FIG. 7 shows an enlarged view of the receiving portion of the preceding light-field zoom.

(13) This zoom Z includes two groups of fixed lenses referenced G1 and G2 and three groups of movable lenses D1, D2 and D3. Moving these three groups of lenses in a given way allows both the focal length of the zoom to be modified and the focus on its focal plane to be preserved. Thus, FIG. 5 shows the zoom in a long-focal-length configuration and FIG. 6 the zoom in a short-focal-length configuration. The focal-length ratio is about 7 in the case of this zoom. Other zoom configurations are possible.

(14) Since the zoom according to the invention is a light-field zoom, it includes, where in a conventional zoom the detector is located, a depth-estimating optical detecting unit, said depth-estimating optical detecting unit including a matrix array of micro-lenses MML and a matrix-array detector DM as may be seen in FIG. 7 which shows an enlarged view of the encircled portion of the preceding FIGS. 5 and 6.

(15) As was described above, there is, for each estimated distance, a focal length of the zoom that gives a minimum measurement uncertainty I.sub.MIN. This minimum measurement uncertainty as a function of the estimated distance D.sub.E is shown in FIG. 8. This uncertainty depends on the field-depth limits of the optical system. In this curve, the measurement uncertainty does not exceed 1 meter at 100 meters of distance. The focal length of the zoom changes along this curve. In the case of FIG. 8, the focal length varies by a factor of 8 between the bottom of the curve and the top of the curve. The zoom necessarily has a maximum focal length. Therefore, this curve has a linear aspect provided that it is possible to increase the focal length of the objective. When the focal length reaches its maximum value, the variation becomes greater.

(16) When the user takes a measurement, he does not know, a priori, the distance to be estimated and therefore, he cannot know, beforehand, the focal length of the zoom giving the lowest uncertainty in the measurement of this distance. Thus, the optical system includes:

(17) means for calculating, for a first focal length of the zoom and for a given object, the estimated distance of this object and the measurement uncertainty in this estimation, depending on said first focal length and said estimated distance and,

(18) this estimated distance being known, optimizing means allowing at least one second focal length of the zoom to which a lower measurement uncertainty in this estimation corresponds to be determined.

(19) The means for calculating estimated distance and the measurement uncertainty in this distance employ conventional methods used in light-field cameras, the focal length of the zoom and its aperture being known.

(20) The chosen first focal length may, by way of example, be the smallest focal length of the zoom or its largest focal length or an intermediate focal length. It is possible, this choice of focal length being arbitrary, for the object the distance of which it is sought to measure not to be in the field-depth interval of the adopted focal length; in this case, the measurement is restarted with a larger or smaller focal length depending on the case in hand until a first distance evaluation is obtained in order to initiate the distance-estimating optimization process.

(21) There are various techniques that allow the measurement to be optimized. By way of example, the optimizing means include a loop of iterations allowing an optimal focal length to which the minimum measurement uncertainty in said estimation corresponds to be determined, each iteration including at least one change of focal length, and the estimated distance corresponding to the new focal length and the measurement uncertainty in this estimation to be calculated. It is thus possible to rapidly converge on the optimal focal length. This process may be automated, the optimizing means automatically adjusting the focal length of the zoom to obtain the desired precision.

(22) This loop of iterations may be carried out in a constant zoom focal plane. It is also possible, so as to change the limits of the distances accessible to the zoom, to apply a known defocus and to restart the iteration process with the new limits thus defined.

(23) It is also possible to work at constant zoom aperture so as to facilitate the distance calculation in the loop of iterations. In this case, for a point located in the object field, the number of micro-lenses employed in the measurement remains constant. It is also possible to work at variable zoom aperture so as to improve the measurement uncertainty. In this case, for a point located in the object field, the number of micro-lenses employed in the measurement increases with the aperture.