WO2020075169A1

WO2020075169A1 - Entrance port adapter for illumination measurement device

Info

Publication number: WO2020075169A1
Application number: PCT/IL2019/051103
Authority: WO
Inventors: Shimon Elstein; Efi ROTEM; Raphael Cohen; Mark Ivker
Original assignee: Ophir Optronics Ltd.
Priority date: 2018-10-10
Filing date: 2019-10-08
Publication date: 2020-04-16

Abstract

An adapter for a device to measure illumination, the adapter having a conical shape and a specularly reflective inner surface, and designed to increase the amount of light trapped within the measuring device. The adapter is configured to be positioned at an opening of the illumination measuring device, oriented such that its narrower aperture is directed outward toward the source of illumination, and its wider aperture is situated at the entrance of the measuring device. In some implementations, the measuring device is an integrating sphere. In other implementations, the measuring device is an optical detector having a housing with at least one photodetector element disposed therein. The disclosed adapter is advantageous for measuring a small light source that radiates over a wide angle.

Description

ENTRANCE PORT ADAPTOR FOR ILLUMINATION

MEASUREMENT DEVICE

FIELD OF THE INVENTION

The invention relates to the field of detection and measurement of radiation throughout the optical extent of the electromagnetic spectrum, especially to methods using reflective cones at the entrance aperture of an integrating sphere or other light detector.

BACKGROUND

Integrating sphere systems are generally used to measure light intensity. An integrating sphere is an optical element comprising a hollow spherical cavity with its interior covered with a diffuse reflective coating, which generates a uniform scattering or diffusing effect, and with small apertures for entrance and exit ports. The light to be measured enters the integrating sphere, and is internally reflected many times until the illumination falling on the inner surface of the sphere is uniform. By this means, any spatial and/or angular asymmetry of the illumination to be measured is averaged out, and a detector at the exit port, sampling a small fraction of this light within the integrating sphere, can be used to accurately measure the average integrated input illumination. Alternate methods of measuring optical power are by use of photodiode or thermopile detectors, depending on the beam intensity.

However, both for an integrating sphere and a conventional photo-detector diode installed within a housing, there is a limit to the angle of acceptance of the incident illumination which the detection device can measure, because the physical dimensions at the entrance aperture limit the angle of acceptance of the light which the aperture can input to the device for measurement. In the case of the integrating sphere, the ratio of the wall thickness to the port diameter limits the angular extent of input light which can be measured. The walls of the sphere are commonly made of materials such as PTFE, which has good diffusive reflective properties. However, to avoid translucency and to achieve the high reflectance levels required by integrating spheres, the PTFE wall must be comparatively thick. The sphere is typically encased in a metal housing which also contributes to wall thickness. The thicker the sphere wall thickness, the more limited is the illumination acceptance angle of the device. Increasing the diameter of the input port is not desirable because of the adverse effects it has on integrating sphere performance, including decreasing the accuracy of measurements and necessitating calculation of correction factors for these sensitive, calibrated measuring devices.

In the case of the photodetector housing, it is both the wall thickness and the relative physical dimensions of the input aperture window as compared to the lateral dimensions of the detector chip itself, that limit the angular extent of input light which can be measured. In many types of photodetectors, such as photodiodes, the light sensitivity varies with angle of incidence, being significantly less sensitive for light incident at large angles.

Reflective cones have been widely used to accept light from a large aperture and concentrate it into a smaller aperture, such as that at the entrance port of an integrating sphere or detector package. This configuration is commonly employed to effect an increase in the effective aperture of a small detector or the entrance to the integrating sphere, by concentrating light incident thereon. In US Patent 8,008,572 to U. Ortabasi, for“Integrating Sphere Photovoltaic Receiver employing Multi-junction Cells”, use of such a cone is shown, and it appears logical to use a large aperture to funnel and concentrate light into a smaller aperture, increasing the input or power going into the detector. However, such an arrangement does not in fact, solve the problem of measuring the total emitted intensity of a light source having a wide angle of illumination, and is not useful for directing the illumination from such a light source into the integrating sphere or other detector.

Typically, such entrance angle limitations are disadvantageous for the measurement of light from sources having a large angular spread of their illumination field, and there therefore exists a need for an optical measuring device, which overcomes at least some of the disadvantages of prior art devices and methods, especially to allow the widest possible acceptance angle at the entrance aperture for measurement of incident illumination.

The disclosures of each of the publications mentioned in this section and in other sections of the specification, are hereby incorporated by reference, each in its entirety. SUMMARY

The current disclosure describes a novel entrance aperture configuration for an optical measurement device such as an integrating sphere or a detector having a photodetector installed therein, which increases the angular extent of light accepted by the device. The entrance aperture configuration comprises a port adaptor in the form of a reflective cone, manufactured of a thin material, and whose inner surface has high specular reflectivity at the wavelengths to be measured. The cone is oriented such that light is collected from the surroundings by the small aperture of the cone and is directed into the integrating sphere via the larger aperture. The smaller aperture of the port adapter faces outward to receive the light, and the larger aperture faces the interior of the integrating sphere or other detecting device, to deliver the light thereto. The necessity for a specular reflective inner surface, as contrasted to a diffusive reflective inner surface, is that a diffuse component to the reflectivity will offset the advantages provided by the cone geometry. Diffusively reflected light will not adhere to the well-defined reflective paths determined by the geometry. It will spread out in all directions and not all will enter into the integrating sphere to be measured. In an extreme case, if the surface were to be perfectly diffuse, then roughly 50% of the light impinging on the inner surface of the reflective cone would be scattered in the backwards direction and lost.

This novel configuration is counter-intuitive to the conventional prior art entrance cone configuration of accepting light at the larger aperture of a cone, and concentrating it down through the smaller aperture of the cone. For an integrating sphere or other detector to absorb as much light as possible, it would appear, at first sight, to be most logical to use a funnel- shaped light collector, with the wider opening collecting the light, as described above and as shown in US 8,008,572, where a reflective cone is used to accept light from a large aperture and concentrate it into a smaller aperture. However, from an analysis of the geometrical properties of such cones and of the behavior of light rays entering them, this common conception appears to be false; in fact, an inverted funnel with a narrower input aperture, as described in this disclosure, is more effective at capturing as wide an acceptance angle as possible for input into the optical measurement device.

The novel configuration of the present disclosure can thus be used to solve the problem of the reduction in the acceptance angle of incident illumination because of the limitations imposed by the wall thickness of the entrance aperture of the measurement device. By using a cone, having its narrower end facing the environment from where the illumination is to be measured, simple geometric ray plotting shows that the acceptance angle can be substantially enlarged, compared with the acceptance angle which would arise from a prior art entrance cone, having its larger opening facing the incident illumination. This is in contrast to the use of the cone configuration shown in US 8,008,572, in which light, at close to a glancing angle of incidence to the device aperture, entering an entrance cone having its large aperture directed towards the ambient illumination, may be reflected back out of the cone at its first impingement, or, if it does enter the cone, even at the second impingement, as will be shown in the detailed description section herein below. The extent of illumination acceptance depends on the number of similar reflections from the inside of the cone, which itself is dependent on the angle of the cone.

The devices described in the present disclosure are operative to increase the effective angular acceptance of the device, thereby improving the device’s ability to measure light from a wider range of incidence angles. This mode of use provides significant advantages specifically when it is required to measure the total light output of light sources that are physically small and have large angular extents, up to very close to 180°. For such sources, the light must be positioned very close to the input port in order for the large angle rays to be accepted by the integrating sphere. Often, such very wide angle sources are small in active size but situated on a large surface having certain reflective properties, such as an LED on a circuit board. As any reflective surface located close to the source can affect the operation of the sphere, it is advantageous to match the port size to the size of the source to be measured.

Furthermore, for highest integration efficiency, the ratio of entrance aperture to total sphere inner surface area should be kept low, ideally <5%. The current disclosure port configuration would allow use of relatively smaller entrance apertures for a given sphere, thus enabling use of smaller integrating spheres or detectors, while still maintaining the <5% preferred criterion. Use of smaller photodiode and other detectors provide a number of advantages over larger ones including lower noise, faster response time, lower cost and greater uniformity. Moreover, the smaller the port used with a given detector, the less the level of extraneous light that will enter from the outside and interfere with the accuracy of the measurements. Reflective conical structures having geometric and illumination directional properties similar to those of the novel cones of the present disclosure, in that they emit light at a larger aperture, are also commonly used to decrease the angular extent of light exiting the aperture. Such cones, generally of ellipsoidal shape, are used, for instance, in illumination fixtures for propagating light from a small dispersive light source over a region to be illuminated. However, in these applications, there is no small aperture at all, and the light does not thus enter the conical element at its small aperture, but is generated within the confines of the conical structure at its smaller end, and exits the larger aperture in a more collimated form. Such cones are thus constructed and operative in an entirely different configuration to that of the novel cone configuration of the present disclosure.

In this disclosure, the novel cone configuration is generally described in association with the entrance aperture of an integrating sphere, as an exemplary light measuring device. However, it is to be understood that an integrating sphere is only one example of the use of the presently described devices, and that the entrance aperture cone configuration described herewithin can also be applied to other optical measurement devices having apertures through which the light must pass before impingement on the measurement element, such as photodiodes, thermopiles and other light detectors.

There is thus provided in accordance with an exemplary implementation of the devices described in this disclosure, a device for measurement of illumination, comprising a body having in its wall, an opening for receiving illumination, and at least one photodetector adapted to measure the illumination; and a conical element having a high reflectivity, specularly reflective inner surface, and having two apertures at opposite ends, the first aperture having a smaller cross sectional area than the second aperture. The conical element is positioned at the opening of the body, in a direction with its first aperture facing the illumination to be measured, and its second aperture facing the body, such that the conical element is configured to collect the illumination and to direct it into the body for measurement by the photodetector. The one or more photodetectors may be mounted in the wall of the body, or may be disposed remotely, and connected thereto by means of one or more optical fibers embedded in the walls of the body. In some implementations, the body is an integrating sphere having a diffusively reflective internal surface. In other implementations, the body is an optical detector having a housing with the at least one photodetector element disposed therein. The conical structure may have straight or curved surfaces, and an elliptical shape or a polygonal shape. The plane of the first aperture of the conical element may be positioned radially at a distance level with or more remote than the outer surface of the wall of the body, whereas the plane of the second aperture of the conical element is then positioned radially at a distance level with or radially inward from the inner wall of the body. In some implementations, the body has in its wall more than one opening for receiving illumination, each opening being fitted with a conical element.

BRIEF DESCRIPTION OF THE FIGURES

The present invention will be understood and appreciated more fully from the following detailed description, taken in conjunction with the drawings in which:

Figure 1 schematic shows a prior art integrating sphere, demonstrating how the entrance aperture and acceptance angle are limited by the thickness of the sphere wall;

Figs. 2 and 3 show different methods of increasing the effective angle of acceptance of a light detection device, both having disadvantages. Fig. 2 shows a thin walled entrance aperture, while Fig. 3 shows a large diameter entrance aperture;

Figure 4 shows a prior art photodiode detector in its housing and the acceptance angle of light which enters through the entrance aperture;

Figure 5 shows a prior art integrating sphere with a reflective cone adapter oriented as shown in US Patent 8,008,572;

Figure 6 illustrates the optical properties of the prior art reflective cone of Fig. 5;

Figure 7 schematic shows the novel port adapter device of the present application, illustrating how light from a broad acceptance angle is captured by a cone at its smaller aperture and directing the captured light towards a larger aperture and into the detector; and

Figure 8 illustrates an enlarged view of the novel port adapter and the manner in which it interfaces with the integrating sphere and broadens the acceptance angle at the entry aperture. DETAILED DESCRIPTION

Reference is first made to Figure 1, illustrating light 6 entering a prior art integrating sphere 4 through the entrance aperture 1. The acceptance angle 8 is defined by the limiting rays 10 and 11, which determine the maximal angle of acceptance of the light entering the sphere. The received light is integrated by the diffusively reflective inner surface of the sphere 5 and sensed by one or more detectors 12, generally disposed in the wall of the sphere, which relay a signal proportional to the optical intensity to an output 13. As an alternative, the detector may be positioned remotely from the integrating sphere, and the internal illumination collected by a fiber optical element (not shown) embedded in the wall of the sphere. The acceptance angle 8 of the sphere is limited by the thickness 7 of the material of the sphere’s shell. For the purpose of maximizing the acceptance angle of light at the entrance aperture, the wall thickness of an integrating sphere should be made as thin as possible. Integrating spheres are commonly manufactured from the polymer polytetrafluoroethylene (PTFE). Due to the comparative flexibility of PTFE, and its translucent nature, the PTFE shell has to be made comparatively thick, and it is difficult to achieve large angular acceptance of light entering the integrating sphere, since the thickness of the skin of the sphere limits that angle. The thicker the material of the shell, the smaller the angle of acceptance, thus allowing light incident from a smaller field of view into the sphere and restricting the accuracy of the measurements obtained therefrom. This limitation of wall thickness 7 preventing light from entering the sphere 4 at the broadest possible acceptance angle, is especially problematic in applications having physically small light sources with a wide angular extent.

Two solutions to increasing the effective angle of acceptance 8 of the device are illustrated in Figures 2 and 3. One solution is to decrease the thickness of the integrating sphere wall, as shown in Figure 2, thereby increasing the effective angle of light acceptance that can be accepted by the sphere. This solution is problematic because of the above-mentioned minimum thickness required to achieve low absorption or transmission through the wall, and the need to maintain the integrating sphere structural integrity. Another solution to increasing the angle of light acceptance, is to increase the entrance aperture 2 of the sphere, as shown in Figure 3. This model is also unacceptable because the larger the opening to the sphere, the greater the reflective noise introduced from background light sources, such as the reflective surface on which a wide angle, small light source is situated. Such background reflective noise decreases the accuracy of the measurements, and in prior art devices, necessitated calculation of a correction factor to be applied to the measurements. Furthermore, the larger the input aperture, the larger the chance of illumination escaping from the integrating sphere and not bring measured by the detector. Thus, neither proposal solves the problem of limited acceptance angle, as illustrated in Fig. 1.

Reference is now made to Fig. 4, which illustrates how the problem of the limited angle of acceptance 8 expresses itself also for the case of light impinging on a photodetector element 14 mounted in its housing 15. Illumination strikes the detector from within a range of angles 8, as illustrated by limiting rays 10 and 11, this range being limited by the thickness of the cover of the housing 15, and other geometrical factors.

Reflective cones have been used in prior art to modify either the angle of acceptance of light illuminating a specific target, or to concentrate light impinging on a detector. In these applications, reflective cones are used to accept light from a large aperture and concentrate it into a smaller aperture. This configuration is commonly employed to effect an increase in the effective aperture of a small detector or the entrance to an integrating sphere that is located opposite the smaller, distal aperture. In such applications, the cone concentrates the light impinging on a detector disposed remotely from the illumination source relative to the cone. Physical laws of conservation of energy limit the angular extent of the light that can be accepted at the larger entrance aperture. Thus, this cone adapter is useful in applications that require concentration of incident light from a large spatial area, but not for those intended to capture as much of the incident light from as wide an angle as possible.

Reference is now made to Fig. 5 which illustrates the previously mentioned prior art solution for concentrating light 6 impinging on the entrance aperture 1 of an integrating sphere 4, as disclosed in US Patent 8,008,572. This reference discloses a reflective cone 2A with its wider opening facing the light source and narrower opening facing the entrance aperture of the detector 4, in this case, an integrating sphere. The effect of this cone is to concentrate the light from a large spatial area so that it will pass through an aperture of smaller dimensions at the entrance to the integrating sphere. However, it concomitantly effectively narrows the angle of illumination 8 at the entrance aperture.

The optics of the reflective cone positioned to concentrate the light entering an integrating sphere, as described in US 8,008,572, are shown in Figure 6. The internal angle of the cone and the orientation of the cone are such that the acceptance angle of light 8, defined by limiting rays 10-10' and 11-11’ received by the integrating sphere 4 via its entrance aperture 1 is significantly narrower than that shown in the original prior art examples without such a cone, as shown in Figures 1 and 4. This is because such a cone acts as a concentrator for light intended to enter the integrating sphere entrance aperture 1, and such a concentrator results in a reduction of the acceptance angle, rather than the desired increase in the acceptance angle. A phenomenological explanation of this result is shown by observing the optical path of two rays entering the cone at angles beyond the marked acceptance angle 8. In Fig. 6, the ray marked 16, designated by the short dashed line, enters the cone at an angle slightly more obtuse than that of a ray 11- 1 G at the outer limit of the acceptance angle of the cone. After impinging on the highly reflective wall of the cone at the point marked 18, and depending on the angle of the cone, it does not exit the cone into the measurement device 4, but is reflected back to a second point on the cone inner surface, marked 19, from which it is again reflected, but this time back out of the cone, and never enters the entrance aperture 1 of the measurement device 4. A second exemplary ray 17, designated by a thin solid line, entering the entrance cone at an angle even further from the limit of the acceptance angle than the ray 16, is shown to make a single impingement on the cone wall at the point 20, and is then reflected back out of the cone in the general direction from which it entered. Thus, the prior art cone, as shown in Fig. 6, instead of increasing the acceptance angle over a device not having such a cone, actually decreases the acceptance angle.

Reference is now made to Figs. 7 and 8, which illustrate how the reflective cone port adapter of the present disclosure actually increases the acceptance angle of the entrance port of a light measuring device, thereby overcoming the limitations arising from the constraints of the thickness of the wall of the device at the entrance aperture. Fig. 7 shows the reflective cone 2B of the present disclosure, oriented such that the narrower aperture is positioned outwards of the wider aperture. The light 6 enters the cone shaped port adapter through the narrower aperture and exits through the wider aperture to enter the integrating sphere. This orientation thus affords the widest possible angle of acceptance 8 of the illumination at the entrance aperture 1 to the measurement device. The light source 6 in this figure, is shown radiating over almost 180 degrees. The inner surface 3 of the reflective cone 2A is specularly reflective and reflects all light entering the cone toward the wider opening of the cone. Although the cone is most conveniently constructed as a right circular conical structure, it is to be understood that any other similar structure may be used on condition that the entrance aperture of the illumination is smaller than the exit aperture to the measuring device. Thus, the cone can also be ellipsoidal in shape, or having a polygonal shape, or a curved shape, or any other shape which may have the essential properties of transferring the illumination from the small aperture to the large one, and of having a highly specularly reflective inner surface.

Reversed cones, such as that shown in Fig. 7, have been used at the entrance of spectroradiometers, for measuring luminous flux and other light indices. However, the internal characteristics of such spectroradiometer entrance cones are different from the present port adapter, in that such measuring ports have a highly diffusive internal surface, the inside of the cone being essentially an extension of an absorbing integrating sphere. By contrast, an essential feature of the present port adapter is that its inner surface is highly specular reflective to reflect all light toward the entrance aperture of the integrating sphere.

Reference is now made to Figure 8, in which the current modified entrance to the integrating sphere is shown in the context of the original entrance aperture 1 to the sphere 4 as illustrated in Figure 1. The inverted cone adapter 2B is oriented with the narrower aperture facing outward and the wider aperture connecting with and attaching to the integrating sphere 4 at its entrance 1. The wall of the cone has a specular reflective inner surface 3. Such an adapter approaches the ideal entrance aperture of a detector, i.e., one having zero thickness of the shell wall, as illustrated in Figure 2.

One feature of this conical port adapter is that the narrow aperture may extend beyond the outer edge of the sphere shell, such that the acceptance angle is determined only by the cone itself, and the acceptance angle can approach 180°. Such a cone input port is advantageous for measuring a small light source that radiates over 180 degrees, such as an LED device.

It is appreciated by persons skilled in the art that the present invention is not limited by what has been particularly shown and described hereinabove. Rather the scope of the present invention includes both combinations and subcombinations of various features described hereinabove as well as variations and modifications thereto which would occur to a person of skill in the art upon reading the above description and which are not in the prior art.

Claims

CLAIMS We claim:

1. A device for measurement of illumination, comprising:

a body having an opening in its wall for receiving illumination, and at least one photodetector adapted to measure illumination falling on a portion of the internal surface of the wall; and

a conical element having a high reflectivity, specularly reflective inner surface, and having a first aperture and a second aperture at its opposite ends, the first aperture having a smaller cross sectional area than the second aperture,

wherein the conical element is positioned at the opening, in a direction with its first aperture facing the illumination to be measured, and its second aperture facing the body, such that the conical element is configured to collect the illumination and to direct it into the body for measurement by the at least one photodetector.

2. A device according to claim 1 wherein the body is an integrating sphere having a diffusively reflective internal surface.

3. A device according to claim 1 wherein the body is an optical detector having a housing with the at least one photodetector element disposed therein.

4. A device according to any of the previous claims, wherein the conical structure has straight or curved surfaces.

5. A device according to any of the previous claims, wherein the plane of the first aperture of the conical element is positioned radially at a distance level with or more remote than the outer surface of the wall of the body.

6. A device according to any of the previous claims, wherein the plane of the second aperture of the conical element is positioned radially at a distance level with or radially inward from the inner wall of the body.

7. A device according to any of the previous claims, wherein the conical structure has an elliptical shape or a polygonal shape.

8. A device according to any of the previous claims, wherein the body has in its wall more than one opening for receiving illumination, each opening being fitted with a conical element.

9. A device according to any of the previous claims, wherein the at least one photodetector is mounted in the wall of the body.

10. A device according to any of the previous claims, wherein the at least one photodetector is mounted remotely from the wall of the body, the device further comprising at least one optical fiber for conveying light from the internals surface of the body to the at least one photodetector.