OPTICAL DEVICE ALLOWING THE ANGULAR EMISSION PATTERN OF A LIGHT SOURCE OF FINITE AREA TO BE RAPIDLY MEASURED

Abstract

A device allowing the angular emission pattern of a source to be measured without mechanical movement comprises, in succession, along its optical axis: a first objective, called the Fourier objective, arranged to form a Fourier surface each point of which corresponds to one direction of observation of the object; a diffuser used in transmission and placed on the Fourier surface; a substance of optical density placed upstream of the diffuser and arranged to attenuate the light backscattered toward the Fourier objective and the areal source; and a video photometer located downstream of the plane of the diffuser and arranged to image the surface of the diffuser.

Claims

1. A device for measuring the angular emission pattern of a light source of finite area, comprising, successively along an optical axis of the device: a first Fourier objective arranged to form a Fourier surface, each point of the Fourier surface corresponding to a direction of observation of the source; a diffusing surface used in transmission and arranged on the Fourier surface; and a video photometer located downstream of the diffusing surface and arranged to image the diffusing surface.

2. The device of claim 1, further comprising a substance of optical density arranged upstream of the diffusing surface and arranged to attenuate light backscattered toward the Fourier objective and the source.

3. The device of claim 1, wherein each point of the Fourier surface corresponds to an angular direction of emission of the source.

4. The device of claim 1, wherein each point of the Fourier surface corresponds to all rays of light emitted by the source, and converging at a point situated at a fixed distance from the source and in an angular direction of observation.

5. The device of claim 1, wherein the optical axis of the video photometer is oriented parallel to a direction normal to the source.

6. The device of claim 1, wherein a density-diffuser pair is formed by the substance of optical density, the substance of optical density being frosted on one side and having an anti-reflective treatment on another side.

7. The device of claim 1, wherein a density-diffuser pair is formed by the substance of optical density and a diffusing film bonded on one side of the substance of optical density and bonded with an anti-reflective treatment on another side.

8. A method for measuring the angular emission pattern of a light source of finite area, the method comprising placing successively, successively along an optical axis: a first Fourier objective forming a Fourier surface, each point of the Fourier surface corresponding to a direction of observation of the source; a diffusing surface used in transmission and arranged on the Fourier surface; and a video photometer located downstream of the diffusing surface and arranged to image the diffusing surface.

9. The device of claim 2, wherein each point of the Fourier surface corresponds to an angular direction of emission of the source.

10. The device of claim 9, wherein the optical axis of the video photometer is oriented parallel to a direction normal to the source.

11. The device of claim 10, wherein a density-diffuser pair is formed by the substance of optical density, the substance of optical density being frosted on one side and having an anti-reflective treatment on another side.

12. The device of claim 10, wherein a density-diffuser pair is formed by the substance of optical density and a diffusing film bonded on one side of the substance of optical density and bonded with an anti-reflective treatment on another side.

13. The device of claim 2, wherein each point of the Fourier surface corresponds to all rays of light emitted by the source, and converging at a point situated at a fixed distance from the source and in an angular direction of observation.

14. The device of claim 13, wherein the optical axis of the video photometer is oriented parallel to a direction normal to the source.

15. The device of claim 14, wherein a density-diffuser pair is formed by the substance of optical density, the substance of optical density being frosted on one side and having an anti-reflective treatment on another side.

16. The device of claim 14, wherein a density-diffuser pair is formed by the substance of optical density and a diffusing film bonded on one side of the substance of optical density and bonded with an anti-reflective treatment on another side.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Other advantages and particularities of the present disclosure will become apparent on reading the detailed description of implementations and embodiments, which are in no way limiting, with reference to the accompanying drawings.

[0035] FIG. 1 describes a state of the prior art dedicated to the far-field angular measurement of a light source.

[0036] FIGS. 2 and 3 respectively give the angular resolution and the size of the diffusing screen of the system of FIG. 1 according to the source-diffusing screen distance for some diameters of sources and some angular apertures of the system.

[0037] FIG. 4 describes a version of the prior art using a diffuser in transmission instead of the diffusing screen of FIG. 1. The system no longer has geometric distortion, because the video photometer 9 can be aligned on the optical axis of the source, but the constraints of FIG. 2 are still valid.

[0038] FIG. 5 describes another version of the prior art that uses a diffusing hemisphere instead of the diffusing screen. The surface of the hemisphere is imaged by a convex mirror 11 and a slightly off-axis video photometer 9. The angular aperture of the system is always large regardless of the size of the sphere, but the angular resolution is always subject to the constraints of FIG. 2.

[0039] FIG. 6 describes a state of the prior art based on a Fourier objective device. The light emitted by the source 1 is collected by a Fourier objective 2 that refocuses each direction coming from the object 1 on a primary Fourier plane 3.

[0040] This primary Fourier plane is reimaged on a secondary Fourier plane 3bis via field lenses 4 and a transfer lens 6. The diaphragm 5 optically conjugated to the measurement zone on the object 1 makes it possible to define the apparent size of the measured zone independently of the direction of observation. Detection is performed by a matrix of detectors located on the secondary Fourier plane 3bis.

[0041] FIG. 7 shows the principle of the present disclosure based on the assembly of FIG. 4 including a Fourier objective 2 that focuses each direction of emission on a diffuser 8 shown flat in the figure, but that can be a surface of revolution. A substance of optical density 12 can be positioned in front of the diffuser to strongly attenuate the stray light backscattered toward the source. The angular measurement is carried out by a video photometer 9 on the other side of the plane of the diffuser.

[0042] FIG. 8 shows a particular configuration of the present disclosure including a particular Fourier objective 2 that collects the light coming from the source by simulating the vision of the source at a distance R (typically 10 cm). Each point of view on the sphere 13 of radius R is refocused on the same type of diffuser/substance of optical density 8, 12. In this particular configuration, the source is seen at a finite distance R, and not at infinity as with a conventional Fourier objective.

DETAILED DESCRIPTION

[0043] Since the embodiments described hereinafter are not limiting in nature, it is possible, in particular, to consider variants of the present disclosure that comprise only a selection from the features that are described, provided that this selection of features is sufficient to confer a technical advantage or to differentiate embodiments of the present disclosure from the prior art. This selection comprises at least one preferably functional feature without structural details, or with only a portion of the structural details if this portion alone is sufficient to confer a technical advantage or to differentiate the present disclosure from the prior art.

[0044] The present disclosure is based on the transmission configuration as shown schematically in FIG. 4.

[0045] It is shown schematically in FIG. 7, where two additional elements are added: a Fourier objective 2 between the emissive source 1 and the diffuser in transmission, which can either be flat or can follow a chosen surface of revolution 3, and possibly a substance of optical density 12 located before the diffuser. These two additional elements bring three decisive advantages for the practical realization of the device.

[0046] The Fourier objective 2 ensures the collection of each beam of light coming from the entire surface of the emissive object 1 and its focusing on the surface of the diffuser. Under these conditions, the angular resolution of the system is no longer defined by the size conditions of the system, but by the intrinsic characteristics of the Fourier objective 2. It can therefore be very good for large sources without the geometric constraints on the distance and the size of the diffuser that were explained previously. The constraints related to the realization of this Fourier objective are also different from those related to the classic Fourier systems shown in FIG. 6. In the latter case, the focusing must be carried out as parallel as possible to the optical axis of the system for all the angles of incidence, which requires complex optics. The location of the focal points is not necessarily a plane, but a surface of revolution. In the case of the present disclosure, the location of the focal points must be on the surface of the diffuser, which can be either a plane or any surface of revolution, but the angles of incidence of each beam of light do not need to be oriented along the optical axis of the system, which eliminates a major constraint on the practical realization of this optic. A deviation of a few tens of degrees is permissible, since the diffuser is able to do its job even for off-axis light beams. Eliminating this constraint means that the Fourier objective can be produced with a limited size and a reasonable cost, even for source sizes of a few cm.sup.2.

[0047] Introducing a substance of optical density before the plane of the diffuser also makes it possible to greatly reduce the stray light backscattered in the Fourier objective 2 and on the source 1. In fact, any backscattered light passes through the substance of optical density 12 twice, whereas the useful light detected by the video photometer 9 passes through it only once. To measure high-power light sources, setting up a substance of optical density does not pose any particular problem, and is even necessary in most cases to avoid saturation of the video photometer. If a substance of optical density of 1.0 is used, for example, the parasitic light backscattered in the system will be of the order of 1%, which will generate a reflected part in the measurement channel of less than 0.005% (untreated optics at a reflection of the order of 5%) that is completely negligible compared to the useful transmitted light for the measurement, which is of the order of 10%. The performance of the system, and, in particular, the angular resolution, are dependent on the quality of the diffuser, which must be very homogeneous and of the lowest possible thickness. A practical way to produce the density/diffuser pair is to use a black glass plate of calibrated thickness that is frosted on one side or with a diffusing film bonded on one of the sides. This plate must be anti-reflective on the other side to limit backscattered light.

[0048] Furthermore, the diffuser optically decouples the collection part constituted by the Fourier objective 2 and the substance of optical density 12, and the reception part constituted by the video photometer 9. As a result, the system becomes very insensitive to the alignment of the source with the optical axis of the system. The angle of incidence on the sensor depends only on the optical system of the video photometer 9 and is therefore completely independent of the position of the source owing to the decoupling produced by the diffuser.

[0049] The present disclosure as described previously measures the far-field angular emission pattern of a source. The Fourier objective 2 collects all the light beams emitted in a given direction and refocuses them at the same point on the Fourier surface where the diffuser is located. The source rays are observed to come from infinity, as is usually the case for this type of characterization.

[0050] In certain particular applications such as laser sources, the angular characterization also aims to ensure that the source complies with certain safety standards such as the IEC60825-1 standard (see IEC, “IEC 60825-1—Safety of laser products—Part 1: Equipment classification,” 1.2 edition (2008)). In this case, for wavelengths between 400 and 1100 nm, the measuring device must simulate the human eye with a recommended observation distance of 10 cm. The conventional Fourier objective (2) can then be designed to observe a virtual spherical surface placed 10 cm from the source, as shown in FIG. 8. The angular data collected by the system over an angular aperture of at least 38.5° are then averaged over 4° corresponding to the angular aperture of the eye at 10 cm for a pupil diameter of 7 mm. The values obtained represent the maximum light collectable at each angle by the human eye and must not exceed a certain radiance to ensure the innocuousness of the source.

[0051] As will be readily understood, the present disclosure is not limited to the examples that have just been described, and numerous modifications may be made to these examples without departing from the scope of the invention as defined by the claims. In addition, the various features, forms, variants, and embodiments of the present disclosure may be grouped together in various combinations as long as they are not incompatible or mutually exclusive.