Abstract
An apparatus for improving contrast in an image captured by an imaging sensor. The apparatus including: an objective optical system positioned in an optical path of illumination light on an object; an image sensor positioned in the optical path such that light from the objective optical system is incident on the image sensor; a device having a variable transparency positioned at a focal plane of the objective optical system; and a processor configured to: detect a bright spot on the image sensor; and control the device to change a transparency of a portion of the device corresponding to the detected bright spot.
Claims
1. A method of improving contrast in an image captured by an imaging sensor, the method comprising: placing an objective optical system in an optical path of illumination light on an object; detecting a bright spot at an image plane; and controlling a device positioned at a focal plane of the objective optical system to change a transparency of the device at a position corresponding to the bright spot on the image plane.
2. The method of claim 1, subsequent to the controlling, further comprising capturing image data at the image plane.
3. The method of claim 1, wherein the detecting and controlled are performed at predetermined intervals.
4. The method of claim 2, further comprising displaying the image data to a user.
5. The method of claim 1, wherein the transparency of the device is controlled so as to be opaque at the position.
6. An apparatus for improving contrast in an image captured by an imaging sensor, the apparatus comprising: an objective optical system positioned in an optical path of illumination light on an object; an image sensor positioned in the optical path such that light from the objective optical system is incident on the image sensor; a device having a variable transparency positioned at a focal plane of the objective optical system; and a processor configured to: detect a bright spot on the image sensor; and control the device to change a transparency of a portion of the device corresponding to the detected bright spot.
7. The apparatus of claim 6, wherein the image sensor comprises a first image sensor, the apparatus further comprising: a beam splitter positioned in the optical path between the device and the first image sensor; and a second image sensor positioned to receive incident light from a reflective surface of the beamsplitter.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] These and other features, aspects, and advantages of the apparatus of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings where:
[0023] FIG. 1 illustrates an imaging system showing the presence of a bright background light from specular surfaces in an object plane in which incident illumination is not shown).
[0024] FIGS. 2A and 2B illustrate spatial filtering techniques used for pass filtering using a mask placed in the spatial frequency plane where FIG. 2A illustrates the mask effective for paraxial rays and FIG. 2B illustrates the mask ineffective for all oblique rays.
[0025] FIG. 3A illustrates an optical system for identifying spatial locations of oblique ray sources focal points.
[0026] FIG. 3B illustrates an optical system where information is coded into an SLM to subtract local or global source of bright background to obtain a high contrast image on an image sensor.
[0027] FIG. 4 illustrates the optical system of FIG. 3B with a feedback path to provide an automated contrast operation.
[0028] FIG. 5 illustrates an alternative embodiment of an optical system for obtaining a high contrast image in the presence of a bright background.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0029] It is understood that imaging can be performed at any suitable frequency and with the appropriate devices. Here for convenience, Applicants refer to the optical imaging system, that uses the visible and near infra-red spectral regions of the electromagnetic spectrum. Any arbitrary object is visible to the imaging system due to any number of physical attributes, such as, reflectivity, scattering or differential phase. The strength of the light intensity originating from the object and that originating from non-object features, are both dependent on the strength of the incident illumination. The goal of any imaging system is to capture as much of the light from the object features while minimizing the background light from entering the imaging system. In practice, it is not possible to reduce one without the other, resulting in poor object visibility due to the bright background. Contrast of the recorded image is further degraded due to the finite resolution of the image detection and recording system. As an example of the image contrast problem, features etched into shiny surfaces, such as silicon wafers, define the boundaries of the object, while flat shiny surfaces are the non-object features giving rise to the background light.
[0030] The background signal can either be of a global specular nature, giving rise to parallel illumination from the entire object surface or can be represented by a mosaic of randomly orientated, small specular surfaces. The latter is more representative of real world practical imaging systems. For example, such surfaces describe human tissue being observed in body cavities or other similar closed enclosures, where illumination light is introduced along the same path as the imaging. Thus, the background signal comprises of groups of oblique rays corresponding to distributions of the mosaic surfaces as illustrated in FIG. 1. Through the imaging system, each group of like surface casts a local bright light spot in the image plane. Superposition of the bright spots originating from the disparate groups of surfaces in the object plane (O) give rise to a composite bright background in the image plane (I). Light intensity from the object features, appears as an intensity modulation, riding on top of a very large background light intensity. In the typical image conditions considered here, the ratio of the modulated signal to the stronger background signal, defined as the signal-to-background ratio (SNB), is much smaller than unity. Under such imaging conditions, the gain of the image detector can be adjusted to avoid saturation, however at the lower gain settings the imaging detector cannot see the modulation, resulting in loss of object information.
[0031] The present methods and devices utilize paradigm shifting approach, illustrated in FIGS. 3A and 3B, which result in high-fidelity imaging under practical illumination conditions. The optical system of FIGS. 3A and 3b is generally referred to by reference numeral 300 and includes an objective optical system 302 having one or more objective lenses.
[0032] Implementation of the proposed approach begins by identifying the object domain origins of the bright regions B, in the image space. With reference to FIG. 3A, a device having a variable transparency, such as a programmable spatial light modulator (SLM) 304, which can be a liquid crystal device, placed in the focal plane F enables discovery of the background sources. With the SLM in 100% transparent mode, a low contrast image provides the initial location of the bright sources. Essentially, any localized concentration of light P1 in the focal plane correlates with a particular set of oblique rays OR1 emanating from the object plane. In the image plane these oblique rays give rise to a local bright region B. Knowledge of the optical image system can be combined with the measured distribution of the bright region B by the image recording device to determine the location of the corresponding pixels at P1 of the SLM. Multiple bright regions can be identified with a single measurement of the intensity by the image recording device. Further improvement in the location of the pixels in the SLM is possible by changing the transparency of all other pixels to zero percent allowing interrogation of individual bright regions. It can be appreciated, that this process can be repeated, leading to a complete mapping of the origins of the background light intensity. As depicted in FIG. 3B, the SLM is controlled through a controller, such as a CPU, to subtract the background light contributions from multiple oblique rays at the locations on the focal plane (F) corresponding to the locations of localized concentration of light (P1). Those skilled in the art will recognize that methods for detecting concentration of light at the SLM and/or at the image sensor are well known in the art.
[0033] Once P1 is detected, the SLM 304 can be controlled to change its transparency at P1 to be partially or completely opaque, as is illustrated at points 306 in FIG. 3B. Thus, as depicted in FIG. 3B, the background intensity has been removed entirely from the image area HI of the image sensor 102, which now has an image with 100% modulation, that is, the highest possible image contrast that can be displayed to the user on the monitor/display 312. The image sensor can be a CCD or CMOS sensor or the like.
[0034] FIG. 4 shows an embodiment, with a feedback path, that allows for continuous tracking and updating of the background signals that are not time stationary. Continuous updating allows for contrast enhancement of video imaging. Thus, a hardware processor 308, such as a CPU, is used to continuously monitor (at predetermined intervals) the presence of localized concentrations of light at the SLM 304 and to control the SLM 304 to change the transparency of a predetermined portion of the SLM 304 sufficient to produce a high contrast image to a degree that is suitable to an end-user. Alternatively or in addition, the processor 308 can monitor the image sensor 102 for low contrast portions and control the SLM 304 accordingly. The optical system 300 can be passive without any user input or, alternatively, the processor 102 can receive user input, through an input device 310 such as a keyboard, mouse, joystick, touchscreen or the like, as to an acceptable level of contrast in the resulting image/video (e.g., low, medium or high contrast images) or to remove control of the SLM altogether to maintain a state of 100% transparency regardless of the presence of bright background. A storage device 314 can also be provided for storing such user input variables and/or a set of instructions for carrying out the methods described above.
[0035] Turning next to FIG. 5, the same illustrates an alternative embodiment of an optical system, generally referred to by reference number 400, in which a second image sensor 402 and beam splitter 404 are added to the embodiment of FIG. 4 (although FIG. 5 does not illustrate the processor, monitor, storage device and input device of FIG. 4, the optical system of FIG. 5 can be similarly configured with the same). In the optical system 400 of FIG. 5, an image of the object (e.g., body tissue) is formed on both the first image sensor 102 and the second image sensor 402 by virtue of the beam splitting properties of the beam splitter 404. Those skilled in the art will appreciate that surface 406 of the beam splitter is such that some of the incident light will pass through to image sensor 102 and some of the incident light will be reflected to the second image sensor 402. In this case, one of the image sensors is used to detect the bright spots (e.g., image sensor 102) in an image and/or video incident thereon and the SLM 306 controlled accordingly as discussed above to produce a high-contrast image/video on the other image sensor (e.g., image sensor 402), which is displayed to the user.
[0036] While there has been shown and described what is considered to be preferred embodiments of the invention, it will, of course, be understood that various modifications and changes in form or detail could readily be made without departing from the spirit of the invention. It is therefore intended that the invention be not limited to the exact forms described and illustrated, but should be constructed to cover all modifications that may fall within the scope of the appended claims.
