Objective for optical imaging systems   

The invention relates generally to the field of endoscopic imaging and more specifically, to the field of optical design of compact high collection power endoscopic objectives for fluorescence and white light imaging.

Fluorescence imaging is used to highlight molecules and structures not otherwise visible under white light illumination. By administering a molecular contrast agent to a patient, disease processes can be specifically labeled for visualization during clinical examination. In concert with white light imaging, fluorescence imaging captures movies of anatomy with tissue specific information, and provides the clinician with a macroscopic visualization of biology in its intact and native physiological state. It holds promise as a way for real time guidance for tumor resection, sentinel lymph node mapping, vasculature and tissue perfusion imaging, as well as early detection of colorectal cancer.

However, many technical challenges are still present. One of the challenges is the specificity and affinity of the contrast agent. With respect to the physics of imaging, a challenge is light attenuation in living tissue. Yet another challenge is the sensitivity of the imaging instrument at low light conditions.

Light attenuation in tissue, is related to the spectroscopic properties of the biological medium and the optical properties of the fluorescent contrast agent, or fluorophore. Shifting the emission wavelength of the fluorophore from the visible to the deep red or near infrared (NIR) improves visualization by providing better rejection of ambient light and deeper penetration depth into tissue. For example, in addition to water, the tissue constituents that dominate absorption of light in the visible and NIR are hemoglobin, bilirubin, and lipids, which have absorption minima in the red to NIR. Moreover, there is a substantial decrease in tissue scattering in the NIR relative to visible wavelengths. The reduced absorption and scattering (collectively known as the attenuation coefficient) results in less light attenuation and thus deeper penetration. Imaging in the NIR minimizes background autofluorescence, as most of the endogenous fluorescent species (e.g., collagen, elastin, NAD(P)H) emit in the visible spectrum.

The required sensitivity of the system depends on whether a targeted or non-targeted agent is used. Some clinical procedures do not require specific molecular targeting. For example, during cholecystectomy, an uncommon but potentially serious complication with the procedure is injury to the common bile duct. For this application, a targeted contrast agent, such as methylene blue, can be used to highlight the bile duct to give the surgeon guidance during the procedure. Such agents can be introduced with relatively high concentration, and are not limited by the local update of the dye. Other non-targeted applications include sentinel lymph node mapping, and highlighting of vasculature and tissue perfusion.

However, imaging of targeted agents, requires higher sensitivity to detect low levels of the agent. Regardless of the dose orally, intravascularly, or otherwise administered to the patient, local concentrations of the contrast agent can be on the order of tens of nmol/L.

The imaging instrument sensitivity is determined by collection efficiency, illumination power density at the sample, and detector sensitivity. The entrance pupil diameter (EPD) of the primary optics, which determines the numerical aperture (NA), impacts collection efficiency. In an endoscopic imaging system, the EPD is normally 0.2 mm. At 25-100 mm working distance, the NA may be on the order of 0.002-0.008, resulting in low collection power. The illumination power can be partially increased to compensate for that loss in collection efficiency, but only up to the point of maximum permissible exposure (MPE), dictated by ANSI-Z-136.1. Another practical consideration is limiting the excitation light source to a Class III device (<500 mW exposure in the NIR) to avoid the use of laser interlocks and personal protective equipment. However, to illuminate a wide field of view of 120°, the irradiance at the sample provided by a Class III excitation source can be very low (<2.5 mW/cm2). On the detector side, the majority of video endoscopes use ¼ inch or ⅙ inch detectors, which have small pixels, resulting in very low sensitivity.

Conventional endoscope objectives use a retrofocus objective designed to cover large fields of view (FOV). Generally, a small entrance pupil diameter (EPD) is used to maintain large depth of field, and to reduce aberrations at large field. Although this is normally adequate for white light imaging, it is not optimized for fluorescence imaging, where the typical fluorescence signal strength from stained tissue is substantially weaker than the white light reflectance image. It is advantageous for an objective intended for dual use of white light and fluorescence imaging to improve collection power, while maintaining a small overall diameter, large FOV, and high optical resolution for both visible and near infrared wavelengths.

It is therefore desirable to provide a compact endoscope objective with high collection power for multi-mode endoscope systems.

In one embodiment, a wide angle hybrid refractive-diffractive endoscope objective is provided. The objective comprises a negative meniscus lens having a first surface and a second surface; a stop adjacent to the negative meniscus lens; a positive lens adjacent to the negative lens and having a first surface and a second surface; and a hybrid refractive-diffractive element adjacent to the positive lens and having a first surface and a second surface, wherein one of the first surface, or the second surface comprises a diffractive surface, wherein the objective has an effective focal length in a range from about 0.8 mm to about 1.6 mm.

In another embodiment, a wide angle hybrid refractive-diffractive endoscope objective is provided. The objective comprises a negative meniscus lens having a first surface and a second surface; a stop adjacent to the negative meniscus lens; a positive lens adjacent to the negative lens and having a first surface and a second surface; and a hybrid refractive-diffractive element adjacent to the positive lens and having a first surface and a second surface, wherein one of the first surface, or the second surface comprises a diffractive surface, wherein the objective has an effective focal length in a range from about 0.8 mm to about 1.6 mm, and wherein the objective is employed in an endoscope, a handheld probe, or a borescope.

In yet another embodiment, a system for optical imaging is provided. The system comprises an optical endoscope comprising: one or more illumination sources for producing a visible light and an excitation light, wherein the excitation light is configured to induce luminescence in an specimen; an objective for directing the visible light and the excitation light in a direction of the specimen, wherein the objective comprises: a negative meniscus lens having a first surface and a second surface; a stop adjacent to the negative meniscus lens; a positive lens adjacent to the negative lens and having a first surface and a second surface; a hybrid refractive-diffractive element adjacent to the positive lens and having a first surface and a second surface, wherein one of the first surface, or the second surface comprises a diffractive surface, wherein the objective has an effective focal length in a range from about 0.8 mm to about 1.6 mm; a single detector in operative association with the one or more illumination sources for detecting luminescent light and visible light emitted from the specimen; and a signal processor for processing signals corresponding to the endoscope.

These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:

FIGS. 1-2 are cross sectional views of endoscope objectives, in accordance with embodiments of the present technique;

FIG. 3 is a graphical representation of aberrations occurring for the objective of FIG. 2,

FIG. 4 is a graphical representation of a modulation transfer function for the objective, in accordance with embodiments of the present technique;

FIG. 5 is a cross sectional view of an endoscope objective, in accordance with embodiments of the present technique; and

FIG. 6 is a schematic diagram of endoscope employing the objective of the present technique.

DETAILED DESCRIPTION

Embodiments of the invention relate to a wide angle hybrid refractive-diffractive endoscope objective for optical imaging systems. In certain embodiments, a hybrid refractive-diffractive element is employed to increase the entrance pupil diameter (EPD) of a wide angle fisheye lens, thus achieving higher collection power and maintaining small overall diameter. The entrance pupil diameter (EPD) of the objective may be greater than about 0.6 mm. As used herein the term “entrance pupil diameter” means the diameter of a virtual aperture that defines the area at the entrance of the optical system that can accept light rays, such that the rays that pass through the pupil are able to enter the optical system and pass through the optical system through the exit. Collection power of a lens is proportional to the square of the EPD, therefore, the objective of the invention has about 9 to 25 times higher collection power than conventional endoscope objectives which have EPD of about 0.2 mm. In certain embodiments, the objective is adapted to capture fluorescence signal for visible or near infrared wavelengths, or both.

In certain embodiments, the wide angle hybrid refractive-diffractive endoscope objective employs a negative meniscus lens having a first surface and a second surface; a stop adjacent to the negative meniscus lens; a positive lens adjacent to the negative lens and having a first surface and a second surface; and a hybrid refractive-diffractive element adjacent to the positive lens and having a first surface and a second surface, wherein one of the first surface, or the second surface comprises a diffractive surface, wherein the objective has an effective focal length in a range from about 0.8 mm to about 1.6 mm. As used herein, the term “adjacent to” encompasses instances where a gap may exist between the two elements that are being referred to as being adjacent to, that is, the two elements may not be in physical contact. In one embodiment, the stop is located adjacent to the second surface of the negative meniscus lens. In one embodiment, the positive lens is disposed adjacent to the stop such that the first surface of the positive lens is closer to the stop. In one embodiment, the hybrid refractive-diffractive element is disposed adjacent to the positive lens.

In certain embodiments, the objective may have an effective focal length in a range from about 0.8 mm to about 1.6 mm. As used herein the term “effective focal length” refers to the distance from the principal point to the focal point. The focal length of an optical system is a measure of how strongly the system converges (focuses) or diverges (defocuses) light. A system with a shorter focal length has greater optical power than one with a long focal length; that is, it bends the pencil of rays more strongly, bringing them to a focus in a shorter distance. For applications, such as endoscopy where it is desirable to have a large angular field-of-view, it is desirable to have small effective focal length. In one embodiment, the effective focal length of the hybrid refractive-diffractive element is greater than about 1 mm. In certain embodiments, a full field of view (FFOV) of the endoscope is in a range from about 60 degrees to about 170 degrees.

Typically, for endoscope objective, chromatic aberration is minimized by using a cemented doublet element. However, the correction for chromatic aberration using a cemented doublet is not very effective when the EPD is larger than 0.5 mm. Such a cemented doublet is also difficult to manufacture and results in a large objective thickness. It would be advantageous to improve chromatic aberration without the use of a cemented doublet.

To substantially increase the collection power of the endoscope, while maintaining a wide FFOV, as well as large depth of field and compactness, better correction for chromatic aberration is required. In certain embodiments, the wide-angle hybrid refractive-diffractive endoscope objective exhibits an improved correction for chromatic aberration while maintaining a wide FFOV, large depth of field and compactness due to the fact the effective Abbe number for a diffractive surface is negative as compared with a conventional achromatic doublet uses glass element where the Abbe number is positive. In these embodiments, the hybrid refractive-diffractive element uses a combination of refractive and diffractive surfaces to effectively correct for chromatic and spherical aberration. The refractive surface may be a spherical or an aspherical surface.

In one example, the chromatic aberration of the light introduced by a refractive portion of the lens is at least about 150 nm range may be corrected by a diffractive portion, which is incorporated into the lens, thereby providing an improved hybrid refractive-diffractive lens for the objective. In addition, the hybrid refractive-diffractive element enables the endoscope to minimize and correct chromatic aberration, coma, astigmatism and Petzval field curvature.

The hybrid refractive-diffractive element may take different forms as illustrated, for example, in FIGS. 1-2 and 5. In one embodiment, the aspherical surface of the hybrid refractive-diffractive element is expressed by the Equation 1.

z = ch 2 1 + ( 1 - ( 1 + k )  c 2  h 2 ) + Ah 2 + Bh 6 + Ch 8 + Dh 10 Equation   1

where c is the curvature of the curved surface measured along the radial coordinate h from a point on an optical axis, and k is the conic constant of the aspherical surface, and A, B, C and D are higher order coefficients.

In one embodiment, the diffractive surface of the hybrid refractive-diffractive element is expressed by the Equation 2.

z = 2  π λ nominal  [ c 1  h 2 + c 2  h 4 + c 3  h 6 ] Equation   2

wherein λnominal is a wavelength in the center of a spectral range, c1, c2 and c3 are phase forms.

The negative meniscus lens, the positive lens, and the hybrid refractive-diffractive element are made from optically transmissive material having an index of refraction of at least 1.29. Suitable optically transmissive material may include a rigid material, such as glass, or a moldable material such as optical plastic, which is cheaper and easier to mold as compared to glass.

While using a glass substrate, the most manufacturable configuration is a flat diffractive surface. Whereas, for plastic molded elements it is feasible to make a curved diffractive surface. A curved diffractive surface provides additional freedom in the design for better aberration correction. In one example, a multi-layer diffractive grating may be used to achieve high diffraction efficiency over a large wavelength range. Suitable material for forming curved surface may include any moldable material such as optical plastic (for example, polystyrene, acrylic, cyclic olefin copolymer (COC), polycarbonate) or glass. If the lens element will be molded, a plastic material is more preferable as it is cheaper and easier to mold. Alternatively, if the lens is not to be molded, a diffractive zone pattern may be diamond turned or cut on the lens surface.

Referring now to FIG. 1, an objective 10 is illustrated. In the illustrated embodiment, a negative meniscus lens 12 comprises a first surface 14 and a second surface 16. A positive lens 18 having a first surface 20 and a second surface 22 is disposed adjacent to the meniscus lens. Both the first and second surfaces 20 and 22 of the positive lens 18 are curved surfaces with same or different radii of curvature. A stop 19 may be disposed between the negative meniscus lens 12 and the positive lens 18. A hybrid refractive-diffractive element 26 is disposed adjacent to the positive lens 18. The hybrid refractive-diffractive element 26 has a first surface 28 and a second surface 30. The first surface 28 is a refractive surface, whereas the second surface 30 is adapted for diffractive purposes.

Although not illustrated, the arrangement 10 may also include an optical filter, such as a laser rejection filter or an emission filter, or both. The filter may be disposed adjacent to the hybrid element 26 on the side closer to the diffractive surface 30.

In one example, the effective focal length of the objective 10 is 1 mm, the entrance pupil diameter is 0.8 mm, the F number (FNO) is 1.25, and the full field of view is 150 degrees. The focal length of the negative meniscus 12 is 1.83 mm, focal length of the positive lens 18 is 3.16 mm, focal length of the hybrid element 26 is 2.96 mm. The maximum image height (IH) is 1.50 mm. Exemplary dimensions and spacings are set forth in Table 1.

TABLE 1 Reference No. of Surface Radius (mm) Thickness (mm) Glass Type

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