Patent Description:
An imaging system, such as camera, captures light emission from an object scene and uses the captured light to construct spatial and chromatic representation of the object scene at an image plane. The image may be recorded by a detector or light-sensitive media. Such imaging systems may be characterized by their operating space and their performance within the operating space.

An imaging system's operating space may include, for example, angular field of view, working distance, and spectral bandwidth. An imaging system's performance may include, for example, spatial resolution, relative illumination across the image plane, and system sensitivity to low light conditions.

When an image of an object is formed by an imaging device, such as a camera, the influence of the imaging device on the optical information can be described by various parameters. For example, the image of a point source will be altered according to the imaging device's point spread function (PSF). The PSF characterizes how an imaging device alters the fine details in an object scene when constructing an image scene. An image exhibits aberrations that are brought about by the device and are not otherwise part of the object. More generally, the image field resolution and contrast will be determined by an imaging device's modulation transfer function (MTF). Both the PSF and the MTF may exhibit wavelength dependencies, system aperture geometry dependencies, and aberration dependencies; i.e., MTF may be different for different wavelengths and different for different aperture geometries, and may depend also on the extent to which the final wavefront is diffraction limited or aberration-limited.

The PSF, the MTF, and other such parameters of real imaging systems account for and include diffraction effects and aberration effects. For example, if an aberration is introduced in an imaging system, both the MTF and the PSF may change, decreasing image quality. A system that is aberration limited across the whole field of view may show improved performance when the aperture is reduced. In such a system, however, one wavelength may be predominantly responsible for off-axis performance deterioration.

Some imaging systems exhibit more aberrations off-axis than on-axis and may exploit vignetting to control off-axis aberrations that would otherwise adversely affect image quality. Vignetting involves selectively stopping peripheral rays from reaching the image plane. For example, coma can be reduced by preventing some rays associated with off-axis field positions from reaching the image plane. These rays can be blocked in regions before and/or after the system aperture stop. The rays may be blocked by insertion of a limiting (vignetting) aperture or by under-sizing a lens that is not located at the system aperture stop. However, in systems that image more than one wavelength where different wavelengths have different intensities, such vignetting may reduce too much light at a low intensity wavelength, so that an image for the low intensity wavelength may not be discernible. Related prior art can be found in <CIT>, <CIT>, <CIT> and <NPL>.

The invention is directed to a multi-channel wide field imaging system according to independent claim <NUM> and a method of imaging multi-channel wide field light according to independent claim <NUM>.

Preferred embodiments are set-out in the dependent claims.

Features will become apparent to those of skill in the art by describing in detail exemplary embodiments with reference to the attached drawings in which:.

Example embodiments will now be described more fully hereinafter with reference to the accompanying drawings; however, they may be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey exemplary implementations to those skilled in the art.

"Cross-talk" refers to light that is directed toward, and detected by, an incorrect sensor. The light has followed the correct geometric path, but not the correct chromatic path.

"Fully filtered light" refers to light including only the wavelengths intended for the downstream sensor(s). "Partially filtered light" refers to light including primarily the wavelengths intended for the downstream sensor(s), but with detectable, but acceptable, levels of light not intended for the downstream sensor(s). Due to cost, alignment requirements, and other issues associated with achieving fully filtered light, depending on the particular scenario, partially filtered light may be employed. Light reflected by a filter may be fully filtered while the transmitted beam is not, or vice versa. Also, light that was deemed fully filtered at one junction may be split again at a subsequent junction, after which the outputs may or may not be fully filtered in their subsequent respective paths.

"Parasitic light" refers to any light incident upon a detector that should not be there, regardless of how it was produced and regardless of what path it took.

"Chief ray" refers to a ray launched from a point on the object such that it crosses the system's optical axis at the aperture stop. Each object point emits one chief ray and each chief ray carries a broad range of wavelengths.

"Marginal ray" refers to a ray launched from a point on the object such that is passes through the optical system's aperture at the extreme limit of the aperture. Each object point emits marginal rays that, when they pass through the system stop, scribe a perimeter whose shape and size is equivalent to the stop shape and size. Stopping down the system reduces the acceptance angle subtended by a set of marginal rays emitted at the object plane.

"Lens" is any element with optical power.

An optical system designed for the human eye may be optimized for the visible spectrum, with particular weight given to the middle, or green, part of the spectrum. However, imaging systems, in which an electronic image sensor is located in an image plane, may operate over a much larger spectral region that may include visible, ultraviolet, near infrared regions, and so forth. The design of an imaging system becomes more complex as its operating bandwidth increases.

In particular, imaging systems that operate over wavelength ranges from different sources, e.g. directly from an illumination source, reflected from or transmitted by an object, direct observation, and so forth, may deal with image signal intensities that vary widely for different wavelength ranges. For example, an image signal in a specific wavelength range and having a relatively large intensity may benefit from some beam correction to improve the image, but an image signal in a wavelength range having a relatively low intensity may not be able to afford loss of light such a correction imposes.

An imaging system according to an embodiment may provide: (a) wavelength-dependent channels that facilitate simultaneous imaging of signals in different spectral regions and having intensities that differ by orders of magnitude; (b) as-detected resolution and contrast that do not vary across the field of view; and (c) chromatic cross-talk eliminated across some or all of the sensors.

In accordance with embodiments, the following constraints inform an optical design for use in such imaging system:.

In view of the above, in accordance with embodiments, one or more of the following design aspects may be incorporated into an imaging system:.

<FIG> illustrates a schematic view of a multi-channel imaging system <NUM> of an analysis system <NUM> in accordance with an embodiment not being part of the invention. The analysis system <NUM> may also include an illumination source and associated optics <NUM>, which provide illumination to a target which is then imaged by the multi-channel imaging system <NUM>. Generally, the illumination source may include wavelength range(s) for visualization of the target (not limited to visible wavelengths) and wavelength range(s) for additional information about the target. Details of specific examples of this general system, as well as specific optical elements, will be discussed in detail with respect to <FIG>, <FIG> and <FIG>.

The multi-channel imaging system <NUM> includes a common objective lens <NUM> and a beam splitter region <NUM>. The beam splitter region <NUM> may include at least one dichroic beam splitter. The beam splitters in region <NUM> divide the beam from the common objective lens <NUM> into multiple, here three, channels, and may include more than one element. These three channels illustrate different types of typical channels that may be employed in the multi-channel imager <NUM>.

A first channel <NUM> receives light of a first wavelength band and includes optical elements <NUM> and a first channel aperture <NUM>, collectively, a first channel optical system, and an image sensor <NUM>. The aperture <NUM> may be before, among, or after the optical elements <NUM> depending on the optical design.

A second channel <NUM> receives light of a second wavelength band and includes optical elements <NUM> and a second channel aperture <NUM>. The light of the second wavelength band is then split by a beamsplitter <NUM> to create multiple sub-channels, here two sub-channels, each receiving sub-bands of the second wavelength band. A first sub-channel <NUM> receives partially filtered light from the beamsplitter <NUM> and includes additional optical elements <NUM> and a sensor <NUM>. A second sub-channel <NUM> receives partially filtered light from the beamsplitter <NUM> and includes additional optical elements <NUM> and a sensor <NUM>. All elements in the second channel <NUM> other than the sensors <NUM>, <NUM> collectively form a second channel optical system. The second channel aperture <NUM> may be before, among, or after the optical elements <NUM> depending on the optical design.

A third channel <NUM> receives light of a third wavelength band and includes optical elements <NUM> and a third channel aperture <NUM>. The light of the third wavelength band is then split by a beamsplitter <NUM> to create multiple sub-channels, here two sub-channels, each receiving sub-bands of the third wavelength band. A first sub-channel <NUM> may include a rejection filter <NUM> to eliminate any unwanted wavelengths that were transmitted by the beamsplitter <NUM> to the first sub-channel <NUM>. The first sub-channel <NUM> then transmits the fully filtered light to additional optical elements <NUM> and a sensor <NUM>. A second sub-channel <NUM> receives partially filtered light from the beamsplitter <NUM> and includes additional optical elements <NUM> and a sensor <NUM>. All elements in the third channel <NUM> other than the sensors <NUM>, <NUM> collectively form a third channel optical system. The third channel aperture <NUM> may be before, among, or after the optical elements <NUM> depending on the optical design.

As indicated in <FIG>, the beamsplitter <NUM> is provided in an optimal region for wavelength dependent beam splitting. When the beam splitter <NUM> includes a dichroic element, the optimal region may be realized when <NUM>) field dependent ray bundles transmitted by the common objective lens <NUM> have near parallel cones and <NUM>) a marginal ray having the largest cone angle is within the acceptance angle for the beamsplitter <NUM>.

<FIG> illustrate constraints on the rays that occur in the region where the dichroic beam splitter <NUM> is inserted for splitting wavelength bands. <FIG> illustrates the position and direction for the chief rays after the common objective lens <NUM>, here illustrated as a triplet. The remaining lenses ("channel optics") and the sensor in <FIG> are in respective channels. Ray <NUM> indicates the chief ray for an on-axis field position and Rays <NUM> and <NUM> indicate the chief rays for extreme off-axis field positions, with remaining chief rays being between Rays <NUM> and <NUM>. By having the chief rays for all field positions be near-parallel in the region where the dichroic is inserted, the dichroic reduces or eliminates reflection or transmission dependencies associated with object field positions (such as may cause a variation of hue at the image plane). Increased parallelism may be achieved by increasing the distance between the lens elements before and after the dichroic beam splitter <NUM>. In other words, the chief rays become more parallel as the space between the common objective lens <NUM> and the beam splitter <NUM> increases. This may result in more power in the common objective lens <NUM>, here a negative triplet, and a larger, i.e., in diameter, first lens element in the channel optics, here a doublet.

As may be seen in <FIG>, the chief rays <NUM>, <NUM>, <NUM>, from the common objective lens <NUM>, are made sufficiently parallel that the ray bundles are effectively similar enough with respect to the dichroic beam splitter <NUM> specifications. The dichroic beam splitter <NUM> is specified according to various reflection and transmission characteristics across various wavelength ranges. However, another set of parameters concern incident beam angle and cone half angle.

Referring to <FIG>, a comparison is now made for equivalent marginal rays. The upper marginal ray for the upper field position is labeled 1a. The upper marginal ray for the lower field position is labeled 2a. Their angles relative to the optical axis are labeled 1b and 2b respectively. The larger of these two angles is equal to or less than the defined half cone angle for the incident beam specification for the dichroic beam splitter <NUM>.

The prescription in Table <NUM> below provides an example solution for the design form meeting the constraints noted above. Other solutions may be provided, as multiple solutions will fulfill the same requirements of the design form as summarized above in the Overview.

Herein, the specific prescription of Table <NUM> is referenced for the purpose of illustrating the form. Other variations of this form exist. Lens design and filter selection may be simplified or become more complex if the operating space and / or the performance requirements change.

<FIG> illustrates a wide field imaging system <NUM> according to the invention. The wide field imaging system <NUM> includes a common objective lens <NUM>, a beam splitter <NUM>, a first channel <NUM>, a second channel <NUM>, and a third channel <NUM>. The prescription of Table <NUM> may be used with this embodiment.

Light from an object field enters the common objective lens <NUM>, here illustrated as a triplet lens. The beam splitter <NUM>, e.g., a dichroic element, splits the light into two wavelength bands. In this particular example, the beam splitter <NUM> reflects a first wavelength range to the first channel <NUM>, and transmits a second wavelength range to the second and third channels. In this particular embodiment, the first wavelength range is more intense than the second wavelength range.

The first channel <NUM> may include a mirror <NUM> for redirecting the light towards lenses <NUM>, an aperture stop <NUM>, and a detector <NUM>.

The second and third channels <NUM>, <NUM> are split respectively by another beam splitter <NUM>, e.g., a dichroic element, into longer and shorter wavelength ranges within the second wavelength range. A rejection filter <NUM> may be positioned before the beam splitter <NUM> in case some light of the first wavelength range was transmitted by the beam splitter <NUM> into the second and third channels.

Each of the second and third channels <NUM>, <NUM> may include a mirror <NUM>, <NUM> for redirecting the light towards a detector <NUM>, <NUM>, optical elements <NUM>, <NUM>, and an aperture stop <NUM>, <NUM>. Each of the second and third channels may include rejection filters <NUM>, <NUM>.

When a target being imaged by the imaging system <NUM> has been illuminated by laser light and fluoresces in response to the laser light, e.g., the imaging system is used in open field surgery or clinical assessments, the first wavelength range may include visible light and the laser light, and the second wavelength range may include fluorescent light. The first wavelength range has shorter wavelengths and brighter light than the second wavelength range, e.g., the second wavelength range may include near infrared (NIR) light that is one or more, e.g., four or five, orders of magnitude fainter than light in the first wavelength range. The second wavelength range may be split into shorter NIR for the second channel <NUM> and longer NIR for the third channel <NUM>.

As noted above in the Overview, the optical elements may have identical lens prescriptions within the channels. As used herein, identical lens prescriptions mean that the lens materials and lens geometries are the same regardless of associated wavelength range or channel. However, air to glass coatings may be identical or may be wavelength range dependent.

As shown in <FIG>, light from the common objective lens <NUM> and split by the beam splitter <NUM>, is incident on lenses <NUM>, <NUM>, and <NUM>, and passes through respective aperture stops <NUM>, <NUM>, <NUM> before being incident on respective image sensors <NUM>, <NUM>, <NUM>. In the particular design shown herein and detailed above in Table <NUM>, lenses <NUM> and <NUM> are doublets with the aperture stop being before the doublet <NUM>. The faint signal in the second and third channels may have a relatively larger aperture stop, e.g., an f/<NUM> stop, while the stronger signal may have a smaller aperture stop, e.g., an f/<NUM> stop. The final lens <NUM> may be a negative meniscus lens. In other words, an open area of the aperture stop may be larger for the lower intensity light than an open area of the aperture stop for the higher intensity light.

The channel lenses <NUM>, <NUM>, and <NUM> may have a net positive power such that, in combination with the common objective lens <NUM>, each optical system will have a retro-focus form, i.e., the forward group has negative power, the rear group has positive power, and the back focal length exceeds the effective focal length. Further, the channel lenses <NUM>, <NUM>, and <NUM>, in combination with the common objective lens <NUM>, may be image space telecentric, such that images produced at the respective sensors <NUM>, <NUM>, <NUM> may have a same size regardless of axial color or the axial position of best focus at the sensors. Ray cones approaching the sensor have the same angle of incidence and angular subtense, i.e., the image space is telecentric or near-telecentric, and the beam passing through the system is unvignetted, everywhere in the image plane, the image is evenly illuminated.

<FIG> illustrate the spatial frequency versus MTF and through-focus MTF for the NIR, laser reflectance, and visible light in the imaging system <NUM> of <FIG>. As may be seen therein, the design specified by the parameters in Table <NUM> allows channels to operate f/<NUM> or higher, with a full field view of <NUM> degrees, and in the spectral region from <NUM> to <NUM>. The prescription may be modified and optimized to include other wavelengths, other fields of view, other f-numbers, and so forth, which satisfy the conditions outlined above in the Overview.

<FIG> illustrates a wide field imaging system <NUM> according to an embodiment not being part of the invention. The wide field imaging system <NUM> includes a common objective lens <NUM>, a beam splitter <NUM>, a first channel <NUM>, and a second channel <NUM>. The prescription of Table <NUM> may be used with this embodiment. The first channel includes a mirror <NUM>, optical elements <NUM>, a first aperture stop <NUM>, and a sensor <NUM>. The second channel includes a rejection filter <NUM>, a mirror <NUM>, optical elements <NUM>, a second aperture stop <NUM>, and a sensor <NUM>.

When a target being imaged by the imaging system <NUM> has been illuminated by laser light, backscattered light will form a random interference pattern, i.e., a speckle pattern. When there is movement in the target, the speckle pattern changes. Here, the first wavelength range may include visible light and the second wavelength range may include laser light. Here, the beam splitter <NUM> separates the light from the common objective lens <NUM> into the visible wavelength range for the first channel <NUM> and the laser wavelength range for the second channel <NUM>. The aperture stop <NUM> in the first channel may have a simple geometry, i.e. a circle, square, or polygon, as is generally the case, while the aperture stop <NUM> in the second channel may have an aperture for use with speckle imaging, e.g., a Fourier aperture.

<FIG> illustrates a wide field imaging system <NUM> according to an embodiment not being part of the invention. The wide field imaging system <NUM> includes a common objective lens <NUM>, beam splitters <NUM>, <NUM>, <NUM>, <NUM>, <NUM> a first channel <NUM>, a second channel <NUM>, a third channel <NUM>, a fourth channel <NUM>, a fifth channel <NUM>, and a sixth channel <NUM>. The prescription of Table II may be used with this embodiment.

The imaging system <NUM> may image fluorescence, laser speckle, laser reflectance, and white light. A first beam splitter <NUM> may transmit high flux light and reflect low flux light.

The high flux light is incident on a second beam splitter <NUM>, which may or may not include a dichroic element, that reflects the visualization wavelengths into the first channel <NUM> including lenses <NUM>, an aperture stop <NUM>, and a sensor <NUM>. The second beam splitter <NUM> transmits the laser reflectance and laser speckle light to a third beam splitter <NUM>, which reflects the laser reflectance light to the second channel <NUM> and transmits the laser speckle light to the third channel <NUM>. The second channel <NUM> includes lenses <NUM>, an aperture stop <NUM>, and a sensor <NUM>. The third channel includes lenses <NUM>, an aperture stop <NUM>, and a sensor <NUM>. A first lens of the second and third channels <NUM>, <NUM> may be before the third beam splitter <NUM>, such that the first lens is shared, i.e., may be a common lens to both channels <NUM>, <NUM>.

The low flux light is incident on a fourth beam splitter <NUM> that reflects visible fluorescence light to the fourth channel <NUM> and transmits NIR fluorescence light. The fourth channel includes lenses <NUM>, an aperture stop <NUM>, and a sensor <NUM>. A fifth beam splitter <NUM> reflects the shorter wavelength NIR fluorescence light and transmits the longer wavelength NIR light. The fifth channel <NUM> includes lenses <NUM>, an aperture stop <NUM>, and a sensor <NUM>. The sixth channel includes lenses <NUM>, an aperture stop <NUM>, and a sensor <NUM>. A first lens of the fifth and sixth channels <NUM>, <NUM> may be before the fifth beam splitter <NUM>, such that the first lens is shared, i.e., may be a common lens to both channels <NUM>, <NUM>.

By way of summation and review, one or more embodiments may provide wavelength-dependent channels that facilitate simultaneous imaging of spectral regions having intensities that differ by order(s) of magnitude; as-detected resolution and contrast that are substantially constant across the field of view, such that superimposition is useful; and/or reduce or eliminate chromatic cross-talk across some or all of the sensors.

Claim 1:
A multi-channel wide field imaging system (<NUM>) comprising:
a common objective lens (<NUM>);
a first dichroic element (<NUM>) configured to split light from the common objective lens (<NUM>) into a first wavelength range and a second wavelength range;
a second dichroic element (<NUM>) configured to split the light of the second wavelength range received from the first dichroic element (<NUM>) into longer and shorter wavelength ranges within the second wavelength range;
a first image sensor (<NUM>) configured to receive the light in the first wavelength range via a first channel (<NUM>);
a second image sensor (<NUM>) configured to receive the light in the shorter wavelength ranges within the second wavelength range via second channel (<NUM>);
a third image sensor (<NUM>) conf igured to receive the light in the longer wavelength ranges within the second wavelength range via a third channel (<NUM>);
wherein the first channel (<NUM>) comprises a first channel lens system (<NUM>)configured to receive the light of the first wavelength range from the first dichroic element (<NUM>);
wherein the second channel (<NUM>) comprises a second channel lens system (<NUM>)configured to receive the light in the shorter wavelength ranges within the second wavelength range from the second dichroic element;
wherein the third channel comprises a third channel lens system (<NUM>)configured to receive the light in the longer wavelength ranges within the second wavelength range from the second dichroic element; and
a rejection filter (<NUM>) for light of the first wavelength range transmitted by the first dichroic element into the second and third channels, the rejection filter being located between the first dichroic element (<NUM>) and the second dichroic element (<NUM>);
wherein the first wavelength range and the second wavelength range have intensities that differ by at least one order of magnitude and the second wavelength range has a lower intensity than the first wavelength range.