GB2543539A

GB2543539A - Optical imaging system and method

Info

Publication number: GB2543539A
Application number: GB1518646.3A
Authority: GB
Inventors: Jeremy Marsh Richard
Original assignee: Individual
Current assignee: Individual
Priority date: 2015-10-21
Filing date: 2015-10-21
Publication date: 2017-04-26
Also published as: GB201518646D0

Abstract

An optical imaging system and method is proposed for detecting an image of an object in a target range of wavelengths. The optical imaging system comprises: an optical arrangement adapted to receive light from the object, the received light having a first range of wavelengths and to generate a two-dimensional image of the object from the received light. The optical imaging system also comprises a dispersive element 3 such as a diffraction grating or a prism adapted to disperse the light of the two-dimensional image, the dispersed light having the first range of wavelengths. A light blocking arrangement 6, such as an opaque plate having an aperture or an optical filter, is adapted to partially block the dispersed light so as to only transmit a portion of the dispersed light having a second range of wavelengths which is a subset of the first range of wavelengths. An image sensing apparatus 7 is adapted to receive the portion of the dispersed light. The dispersive element 3 may be rotatable.

Description

Optical Imaging System and Method BACKGROUND

This invention relates to optical imaging, and more particularly to detecting an image of an object in a predetermined range of wavelengths.

DESCRIPTION OF THE RELATED ART

Narrow spectral bandwidth images are usually produced in two differing methods. The first produces an image of the object onto a 2D detector via an imaging means. Between the object and the detector an optical component is introduced which transmits only a certain (e.g. narrow) range of wavelengths. Typically this optical component will be a bandpass filter. In order to see the object over a different range of wavelengths a different filter must be introduced. This limits the number of different ranges and the bandwidth of each range that can be viewed by the number of different filters within the instrument.

Other systems view a small part of the total field of view at any one time. This is commonly done with a slit whose length is considerably longer than its width. An image of the object is formed onto the slit which selects the portion of the image to be imaged. This one-dimensional line (or strip) image is then dispersed by optical means, in a direction orthogonal to the slit long axis, to produce an image onto a detector. The resultant image is that of the object along one axis of the detector and the spectrum of each point in that line along the detector second axis. The resultant image is therefore only a one-dimensional section or part of the two-dimensional image that was formed onto the slit. Consecutive one dimensional sections or parts may then be sequentially captured and combined to form a two-dimension image.

SUMMARY OF THE INVENTION

In an attempt to overcome the limitations of existing instruments, embodiments of the present invention provide a two-dimensional image at a detector that may be an entire (e.g. complete) image of the object as seen only within a predetermined wavelength range or ranges (e.g. target range(s)).

Embodiments of the present invention may allow the range and bandwidth of the imaged object to be smoothly variable rather than incrementally variable. Multiple images may therefore be provided, thus enabling chromatic analysis of an object for example. For example, each image may be a 2-D image of the object at a different wavelength.

Embodiments may therefore allow the viewing by an observer of the entire object at a specific (e.g. target) wavelength. Furthermore, embodiments may enable the target wavelength (or target range of wavelengths) to be varied in a smooth (e.g. continuous manner). In this way, an observer need not be limited to viewing the object a predetermined set of allowable wavelength (or wavelength ranges), but instead the allowable wavelengths (or wavelength ranges) may be infinite in number (due to being variable over a continuous range of values) Unlike prior art approaches, which only form one-dimensional sections or strips of an image (for all wavelengths) on a detector, proposed embodiments form the entire image on a detector but at a predetermined wavelength (or reduced wavelength range).

Embodiments may relate to an imaging spectrograph which includes optical components that produce an intermediate 2D image of the object within the instrument. This intermediate image may be in the space occupied by an optical component that can disperse light of differing wavelengths such as a diffraction grating. Further optical components may be employed in order to select only a range of wavelengths (e.g. a subset of wavelengths from the range of wavelengths of the dispersed light) which may then be imaged on a 2D detector, such as a charge coupled device (CCD) for example. The resultant image may therefore comprise a direct 2D image of the object at a selected subset range of wavelengths, and this 2D image may be formed in a single instant of time (and not assembled in a sequential manner from a series of individually captured one-dimensional sections/lines over an image formation time period, like prior art approaches).

Embodiments may provide an optical instrument or arrangement which is adapted to instantaneously produce an entire image of a scene on a 2D detector, wherein the image is of substantially the same wavelength range across the whole image and whose central wavelength may be varied across a continuous range of wavelength values. A whole 2D image may therefore be processed simultaneously, to yield one or more monochromatic images in real time, each 2D image having specific bandwidths and wavelength ranges.

An intermediate image of the scene may be produced within (or within the vicinity of) a dispersive element or arrangement. An aperture may be used to select a range of wavelengths from the dispersive element/arrangement such that all points of the image at the detector are of the same wavelength range (e.g. within a second, subset of the range of wavelengths making up the dispersed light) and whose central wavelength can be varied in a continuous fashion by changing the relative positions of the dispersing element/arrangement and the aperture.

By way of example, an entrance aperture may be employed in conjunction with an exit aperture to define the spectral bandwidth of the image. Also, one or more optical components may be added in front of, or behind, either of the apertures, so as to change a field of view of the scene for example.

According to an aspect of the invention there may be provided optical imaging system adapted to detect an image of an object in a target range of wavelengths, the optical imaging system comprising: an optical arrangement adapted to receive light from the object, the received light having a first range of wavelengths, and to generate a two-dimensional image of the object from the received light, the generated two-dimensional image comprising light having the first range of wavelengths; a dispersive element adapted to disperse the light of the two-dimensional image, the dispersed light having the first range of wavelengths; and image sensing apparatus adapted to receive a portion of the dispersed light, the portion of the dispersed light having a second range of wavelengths, wherein the second range of wavelengths is a subset of the first range of wavelengths.

Embodiments may therefore employ a concept of firstly forming a two-dimensional image of an object or scene, the two-dimensional image consisting of a first range of wavelengths. The two-dimensional image may then be dispersed by light dispersing means so as to provide dispersed light consisting of the first range of wavelengths. The dispersed light may then be passed through an optical system so that only a subset of the first range of wavelengths (e.g. a second range of wavelengths which is a sub-range of the first range of wavelengths) exits the optical system so as to be incident on light detecting or imaging means, such as an image sensor. In this way, spectrally dispersed light of a two-dimensional image consisting of a first range of wavelengths may be provided, and then this dispersed light of the two-dimensional image may be selectively filtered so as to provide a two-dimensional image fern FROM filtered light consisting of a second range of wavelengths, the second range being a subset of the first range.

The dispersed light may be chromatically dispersed into two or more wavelengths of light, each wavelength of light being spatially separated from one another. Embodiments may therefore disperse light of the two-dimensional image in a spatial manner so that differing wavelengths of light are physically spaced from each other.

By way of example, the dispersive element may comprise at least one of the following: a transmission diffraction grating, a reflective diffraction grating and a prism. Other dispersive optical elements may be employed so as to disperse light from the object.

Further, the dispersive element may be adapted to be rotatable about an axis perpendicular to the prevailing direction of the dispersed light so as to adjust the second range of wavelengths received by the image sensing apparatus. The second range of wavelengths received by the image sensing apparatus may therefore be adjusted by moving (i.e. rotating) the dispersive element. Embodiments may therefore be adapted to modify the second range of wavelengths received by the image sensing apparatus by physically moving the dispersive element. Such movement of the dispersive element may be affected and/or controlled by an electrical and/or mechanical arrangement, and this may done is response to an input control signal supplied from a user and/or a control system, for example.

In alternative embodiments, the image sensing apparatus may be movable relative to the dispersive elements to adjust the second range of wavelengths received by the image sensing apparatus.

Embodiments may comprise a light blocking arrangement adapted to partially block the dispersed light so as to only transmit the portion of the dispersed light in the second range of wavelengths. For example, the light blocking arrangement may comprise an output light blocking element formed from opaque material, the output light blocking element being adapted to partially block the dispersed light so as to transmit the portion of the dispersed light in the second range of wavelengths. An output light blocking element may, for instance, comprise an optical filter that is adapted to only pass a subset of incident light/wavelengths. Alternatively, or additionally, the output light blocking element may comprise an opaque plate having one or more apertures.

According to another aspect of the invention, there may be provided an optical imaging apparatus for generating an image of an object in a target range of wavelengths, the optical imaging system comprising: an optical arrangement adapted to receive light from the object, the received light having a first range of wavelengths, and to generate a two-dimensional image of the object from the received light, the generated two-dimensional image comprising light having the first range of wavelengths; a dispersive element adapted to disperse the light of the two-dimensional image, the dispersed light having the first range of wavelengths; and a light blocking arrangement adapted to partially block the dispersed light so as to only transmit a portion of the dispersed light, the portion of the dispersed light having a second range of wavelengths, wherein the second range of wavelengths is a subset of the first range of wavelengths.

The apparatus may be adapted to be coupled to an image sensing apparatus so as to provide the transmitted portion of the dispersed light to the image sensing apparatus. The image sensing apparatus may comprise a portable image capture device, such as a mobile phone, portable computing device, tablet computer, digital camera, film camera, smartphone, or personal digital assistant.

Thus, embodiments may provide ‘add-on’ apparatus that can be employed in conjunction with a camera (which provides image sensing apparatus/means). By way of example, an embodiment of such ‘add-on’ apparatus may be adapted to be connected to the camera of a mobile phone (or smartphone). In this way, the built in camera of the mobile phone may be used as the imaging device, and a software application on the mobile phone may be employed to process the sensed light (e.g. process the image data). Alternatively, an embodiment may be adapted to be attached to a camera body (i.e. a camera without a lens) so as to employ the camera’s light sensing means. Such an embodiment may, for example, be similar in size and shape to a camera lens, and be adapted to attach to the camera using the lens fitting arrangements (e.g. the standardised bayonet lens fitting). This may allow for a standard DSLR or film camera to be converted into a hyperspectral camera.

According to another aspect of the invention, there may be provided a method for generating an image of an object in a target range of wavelengths, the method comprising: receiving light from the object, the received light having a first range of wavelengths; generating a two-dimensional image of the object from the received light, the generated two-dimensional image comprising light having the first range of wavelengths; dispersing the light of the two-dimensional image so as to provide dispersed light having the first range of wavelengths; and partially blocking the dispersed light so as to only transmit a portion of the dispersed light, the portion of the dispersed light having a second range of wavelengths, wherein the second range of wavelengths is a subset of the first range of wavelengths.

BRIEF DESCRIPTION OF THE DRAWINGS

An example of the invention will now be described by referring to the accompanying drawings, in which:

Figure 1 illustrates a schematic of an imaging system according to a first embodiment of the invention;

Figure 2 shows an imaging system according to a second embodiment of the invention, where the dispersive element is a reflective diffraction grating;

Figure 3 shows an imaging system according to a third embodiment of the invention;

Figure 4 illustrates an imaging system according to a fourth embodiment of the invention; and

Figure 5 shows an imaging system according to a fifth embodiment of the invention, identifying the inclusion of a lens in front of an aperture.

DETAILED DESCRIPTION

Embodiments of the imaging system described herein may include an element for collecting light from an object, imaging that light into a space occupied by a dispersive element, means for selecting light of a defined wavelength range and reimaging the object onto a detector.

It will be understood that the figures shows only those rays that lie within the plane of the dispersion and that rays outside of that lane are omitted for clarity.

It will be understood that the image device 7 may be any device such as a charge couple device (CCD), a CMOS device of other light sensitive/reactive devices that can convert incident light into an electrical signal. In addition, it will be understood that such a device can be connected to a computer and that this computer may process the image to allow display and storage of the images. In addition other processing may take place including image coordinate manipulation to correct for aberrations such as distortion.

Figure 1 shows an imaging system according to a first embodiment. Figure 1 identifies a general system which uses a dispersive element 3 to separate incident light into different wavelengths. The light 8 from the object enters the system through an aperture 1. An optical system 2 images the object close to or at the dispersive element 3. Light exiting from the dispersive system comprises a first range of wavelengths and is imaged by an optical system 5 onto the detector 7. A subset 9 of the light exiting the dispersive system (e.g. a second, sub-range of the first range of wavelengths) passes through the aperture 6, whilst light of different wavelengths 4 (outside of the second range of wavelengths) does not pass through the aperture (e.g. is blocked).

In this way an image of certain wavelengths and bandwidth is produced on the detector 7. Here, the certain wavelength(s) and bandwidth consist of the subset 9 of the light exiting the dispersive system (i.e. the second range of wavelengths which is a subset of the first range of wavelengths.

The dispersive element 3 can be a reflective diffraction grating, a transmission grating and can be either flat or have optical power. In addition, it can be either a ruled grating or a holographic grating. Other dispersive components such as glass wedges could also be used.

All those dispersive elements described may introduce an angular dispersion in the light passing through the element, wherein the angle of dispersion is some function of wavelength. Elements 5 and 6 of the optical subsystem are adapted to allow separation of the required wavelengths as shown by rays 9 from those outside of the required spectral range 4. In other words, the optical subsystem, modifies the dispersed light (from the dispersive element 3) so that only a subset of dispersed light is incident on the detector 7. Re-arrangement (e.g. relative movement) of the elements 5 and 6 may therefore modify the subset of dispersed light that is incident on the detector 7.

As we wish to produce an image at the detector where all points within that image have similar spectral content it is required that light of only the required wavelength range passes through the aperture 6. It may therefore be preferred that the light from all object points enters within the dispersive system 11 at the same angle to the optical axis.

Referring to figure 2, an imaging system according to a second embodiment of the invention may now be described. It is noted that light of other wavelengths which are not shown are blocked by the aperture 6. Aperture 6 therefore only passes light of a predetermined range of wavelengths, the predetermined range being a subset of a first range of wavelengths provided by a dispersive element dispersing light from an object.

Light 8 from the object passes through aperture 1. The shape of aperture 1 may be circular, elliptical or rectangular. The extent of the aperture in the plane of the dispersion has a direct effect on the angular extent 25 of the rays at the grating. Lens system 2 which may be a single or multiple elements is positioned such that the principle rays from each object point are parallel to the optical axis. This is commonly known as a telecentric image space and is produced when the aperture 1 is in the rear focal plane of lens system 2.

An image of the object will be produced by lens system 3. This image is formed between lens system 2 and lens system 5. It is not a requirement that the image is formed at the grating 10 and the actual position will be determined by the optical performance of the system. In the example shown in figure 2 the image is close to the grating 10 though it is understood that there may be advantages in having the image some distance from a surface to reduce the effects of dust and other defects on the grating which may appear in the image at the detector 7.

The extent of the aperture 1 in a direction orthogonal to the plane of dispersion is chosen to maximise the light throughput whilst limiting any aberrations introduced. It is well known that optical system when operating with object points off their axis of rotation can result in distortion of the image. Lenses 2 and 5 produce an image of the aperture 1 at the aperture 6. Distortion can cause the image of a slit that is curved. The effect of such curvature is to have slightly different wavelengths passing through aperture 6 for different positions along the length of aperture 1 thereby increasing the bandwidth of the image at 7. By having either of both apertures 1 and 6 curved the effect of having different wavelengths passing through different parts of the aperture can be reduced.

Lens system 5 and aperture 6 combine to select only a defined range of wavelengths that can form an image at the detector 7. Aperture 6 is placed close to the rear focal plane of lens system 5 such that the principle ray from all object points pass through the centre of aperture 6. Wavelengths other than those that are required to form an image will not be passed through the aperture 6 (i.e. blocked).

By rotating the grating 10 about an axis perpendicular the plane of dispersion the wavelength range selected by (e.g. the range of wavelengths passing through) aperture 6 can be varied and images corresponding to these wavelengths will be received by the detector 7.

Lens 5 produces an image of the object and detector 7 is placed at that image plane. As can be seen from figure 2 the intermediate image does not lie in a plane orthogonal to the optical axis of lens 5 due to the effect of the diffraction grating 10. In such a case it is well known that rotating the detector about an axis perpendicular the plane of dispersion can improve the image quality.

Aperture 1 is effectively imaged onto aperture 6 and the dimensions of the apertures should be in relation to the magnification of the optical system defined by lenses 2 and 5.

Figure 3 shows an imaging system according to a third embodiment of the invention whereby lenses 2 and 5 are replaced by a single lens 12. In particular, the grating is used close to the Littrow condition whereby the incident and diffracted light are at similar angle to the grating. Light from the grating 10 will emerge after the optical system 12 to be close to collimated assuming the object light 8 was close to collimated. Aperture 6 selects light of a certain wavelength range and the optical system 3 produces an image onto the detector 7

The use of lens systems 2, 5 and 12 can be designed to be achromatic over a range of wavelengths. However it is difficult to design such systems to cover the spectrum from the UV through the visible to the infrared. In order to correct for the change in focal lengths with wavelength the optical systems 2 and 5 can be moved relative to the apertures 1 and 6.

Figure 4 shows an imaging system according to a fourth embodiment of the invention. Lens systems 2 and 5 are replaced by mirrors 15 and 16 and an additional lens system 13. Mirrors are achromatic and the distances between the apertures 1 and 6 and the mirrors 15 and 16 will not need to change with wavelength.

Light from the object passes through aperture 1 and mirror 15 produces an image close to the grating 10. The distance from the aperture 1 to the mirror 15 is set such that the rays within the plane of dispersion are telecentric. Grating 10 is rotated to select a specific range of wavelengths that will be directed towards the aperture 6 by mirror 16. It should be noted that the focal lengths of mirrors 15 and 16 are not required to be the same though the design may perform better with similar focal lengths.

Mirror 16 produces an image of the object at infinity. Lens system 13 produces an image on the detector 7. The position of the image may change with wavelength depending on the design of lens system 13 and the position of the detector 7 with respect to lens system 13 may therefore be adjusted to compensate. In addition the detector 7 may be rotated about an axis orthogonal to the plane of dispersion as the grating 10 angle changes.

With reference to Figure 5, a fifth embodiment of an imaging system according to the invention may be described. Figure 5 shows the inclusion of lens 20 in front of the aperture 1. This allows imaging of a close object 21.

Alternative embodiments may omit an image sensing apparatus for receiving the dispersed light. Such embodiments may be thought of as image generating apparatus that can be used in conjunction with conventional image sensing apparatus (such as a camera). For example, an embodiment of such ‘add-on’ apparatus may be adapted to be coupled to the camera of a mobile computing device that has a camera (e.g. a mobile phone, portable computing device, tablet computer, digital camera, smartphone, or personal digital assistant), or to a conventional film camera. Where a mobile computing device is employed with the proposed embodiments, a software application may be implemented to process the sensed light (e.g. process the captured image data).

For the purpose of use with conventional image sensing apparatus, embodiments may be adapted to be attached to a camera body using a conventional lens fitting/mount.

Claims

1. An optical imaging system adapted to detect an image of an object in a target range of wavelengths, the optical imaging system comprising: an optical arrangement adapted to receive light from the object, the received light having a first range of wavelengths, and to generate a two-dimensional image of the object from the received light, the generated two-dimensional image comprising light having the first range of wavelengths; a dispersive element adapted to disperse the light of the two-dimensional image, the dispersed light having the first range of wavelengths; and image sensing apparatus adapted to receive a portion of the dispersed light, the portion of the dispersed light having a second range of wavelengths, wherein the second range of wavelengths is a subset of the first range of wavelengths.

2. The optical imaging system of claim 1, wherein the dispersed light is chromatically dispersed into two or more wavelengths of light, each wavelength of light being spatially separated from one another.

3. The optical imaging system of any of claims 1 or 2, wherein the dispersive element comprises at least one of the following: a transmission diffraction grating, a reflective diffraction grating and a prism.

4. The optical imaging system of any preceding claim, wherein the dispersive element is adapted to be rotatable about an axis perpendicular to the prevailing direction of the dispersed light so as to adjust the second range of wavelengths received by the image sensing apparatus.

5. The optical imaging system of any preceding claim, further comprising: a light blocking arrangement adapted to partially block the dispersed light so as to only transmit the portion of the dispersed light in the second range of wavelengths.

6. The optical imaging system of claim 5, wherein the light blocking arrangement comprises at least one of: a light blocking element formed from an opaque material; and an opaque plate having an aperture.

7. The optical imaging system of any preceding claim, further comprising: an input light blocking element formed from opaque material, the input light blocking element being adapted to partially block light received from the object so as define a field of view of the dispersive element.

8. The optical imaging system of claim 7, wherein the input light blocking element further comprising at least one optical component adapted to change the field of view of the dispersive element.

9. The optical imaging system of any of claims 7 or 8, wherein the input light blocking element comprises an opaque plate having an aperture.

10. A method of detecting an image of an object in a target range of wavelengths, the method comprising: receiving light from the object, the received light having a first range of wavelengths; generating a two-dimensional image of the object from the received light, the generated two-dimensional image comprising light having the first range of wavelengths; dispersing the light of the two-dimensional image so as to provide dispersed light having the first range of wavelengths; and receiving only a portion of the dispersed light at image sensing apparatus, the portion of the dispersed light having a second range of wavelengths which is a subset of the first range of wavelengths.

11. An optical imaging apparatus for generating an image of an object in a target range of wavelengths, the optical imaging system comprising: an optical arrangement adapted to receive light from the object, the received light having a first range of wavelengths, and to generate a two-dimensional image of the object from the received light, the generated two-dimensional image comprising light having the first range of wavelengths; a dispersive element adapted to disperse the light of the two-dimensional image, the dispersed light having the first range of wavelengths; and a light blocking arrangement adapted to partially block the dispersed light so as to only transmit a portion of the dispersed light, the portion of the dispersed light having a second range of wavelengths, wherein the second range of wavelengths is a subset of the first range of wavelengths.

12. The apparatus of claim 11, wherein the apparatus is adapted to be coupled to an image sensing apparatus so as to provide the transmitted portion of the dispersed light to the image sensing apparatus.

13. The apparatus of claim 12, wherein the image sensing apparatus comprises a portable image capture device, such as a mobile phone, portable computing device, tablet computer, digital camera, film camera, smartphone, or personal digital assistant.

14. A method for generating an image of an object in a target range of wavelengths, the method comprising: receiving light from the object, the received light having a first range of wavelengths; generating a two-dimensional image of the object from the received light, the generated two-dimensional image comprising light having the first range of wavelengths; dispersing the light of the two-dimensional image so as to provide dispersed light having the first range of wavelengths; and partially blocking the dispersed light so as to only transmit a portion of the dispersed light, the portion of the dispersed light having a second range of wavelengths, wherein the second range of wavelengths is a subset of the first range of wavelengths.

15. An optical imaging system substantially as herein described with reference to the accompanying figures.