Endoscope Light Guide
An endoscope light guide is a rod having an input end and a light emitting end, which is disposed on the illumination window of the distal end of the insertion rod of an endoscope to illuminate an intracavitary region under observation.
The light condensing angle of a light source 10 is determined by the optical characteristics of a source lamp 11, reflector mirror 12 and condenser lens 13, as well as the numerical aperture of a light guide 16 to be connected to the light source 10. By the use of an adaptor with a corrective lens on a connecting light guide connector, the angle of incidence of input illumination light rays from the light source can be adjusted into an optimum angle to optimize the light transmission efficiency and projection of the illumination light through the endoscope illumination windows toward the intracavitary region under observation.
Optical Characteristics
In medical examinations of intracavitary portions or other dark internal regions, an endoscope usually needs a supply of illumination light to a spot under observation. This is achieved by using a concave reflector lamp and a condenser lens to condense light rays toward a predetermined position. In order to transmit this illumination light to the observation spot without substantial losses, an endoscope is typically connected to a light guide which is extended through the entire length of the endoscope and conducts light to the observation spot.
According to the present invention, an endoscopic light guide connector comprises an adaptor with a corrective lens to optimize a light condensing angle of a light source in consideration of a numerical aperture of a light guide to be connected to the light source. Moreover, this adaptor permits to connect various endoscopes with light guides of different numerical apertures to one and the same light source in optimum conditions in terms of transmission and projection of illumination light.
The light guide consists of optical fiber bundles that are formed by embedding single glass fibers into an adhesive material. Depending on the type of adhesive used, the individual fibers can be arranged in different alignment patterns. This alignment pattern determines the position of each individual fiber in the bundle, thereby affecting the spectral properties and angular light distribution of the light guide.
However, this optimization process is not a simple matter because of the manufacturing constraints. Since single glass fibers have to be aligned at a specific angle in order to achieve the required collimation properties, the resulting optical simulation models often are not precise. This in turn causes a reduction in the efficiency of the endoscope system because the illuminance is no longer fully transmitted to the target site.
This loss of efficiency can be attributed to the difference in the spectral properties and angular distribution of the light guide, which may result in a higher loss in transmission. In addition, the spectral properties and angular spread of endoscope light guide the light source may also affect the illuminance. This can be a problem with LED-light sources, which are not yet integrated into rigid and flexible endoscopes.
Transmission Characteristics
An endoscope is a surgical device that enables a physician to examine an area within the body for a specific medical purpose. These devices are usually flexible or rigid and can be inserted through natural orifices or through a percutaneous (percutaneous) incision. They are also used in other medical procedures and have been designed to help in the diagnosis of diseases such as cancer.
In order to obtain an image with minimal light losses, the transmission characteristics of an endoscope are of utmost importance. Specifically, the transmission characteristics of an endoscope must be able to transmit signals with a high signal-to-noise ratio. This is due to the fact that endoscopes are often used to measure small areas of a body that are inaccessible using traditional instruments.
The transmission characteristics of an endoscope can be measured in a variety of ways. One way is by comparing the transmitted signal with a reference waveform. The other method is by determining the attenuation of the transmitted signal.
This is accomplished by comparing the received signal with an attenuation curve and plotting its characteristic line on the graph. A flat characteristic line indicates that signals are properly transmitted.
However, the attenuation of a signal may vary with varying frequencies. This can lead to errors in the transmission of a signal. This can be prevented by evaluating the transmission characteristics of an endoscope in a non-resonant mode.
A non-resonant scanner uses a cantilevered optical fiber and is driven by a voltage source. The cantilevered fiber is excited at a different frequency from its resonance peak to prevent the problems related to resonant scanners. This allows the use of an OCT endoscope with a high sensitivity and without whirling.
The other type of scanner is a semi-resonant scanner that uses a cantilevered fiber and is driven by an intermediate frequency. The cantilevered fiber is excited far from the resonance peak to prevent the sensitivity and nonlinear whirling problems associated with resonant scanners.
This is accomplished by combining a corrective lens with an adaptor that is removably fitted on the fore end portion of the light guide rod 9. The convex lens brings the angle of incidence of input illumination light rays into conformity with the numerical aperture of the light guide 16. Therefore, this ensures a maximum pickup volume of input illumination light and minimum light losses in transmission to the light guide 16 and a wider projection angle of illumination light toward an intracavitary region under observation.
Diffusion Characteristics
The diffusion characteristics of an endoscope light guide are crucial to ensuring the optimal quality of an endoscope image and its correct display on a screen. The guiding of illumination rays through an entire length of the endoscope to endoscope light guide a spot under observation, without substantial losses in transmission, is essential.
In order to achieve this, an endoscope is provided with an illumination optics that guides the input light rays from a light source to the observation spot and captures the reflected light in a mirror to be used for imaging. In the case of rigid endoscopes, the illumination optics is integrated into the body of the endoscope.
Alternatively, flexible endoscopes can use external light sources. These can be small and portable devices that are connected to the endoscope via adapters or couplers, or larger standalone devices that require a connection to a power outlet.
These types of light sources have a much higher light intensity than portable devices and sometimes require a longer endoscope. To transport this light from the light source to the endoscope, these types of endoscopes are usually equipped with fiberoptic light guides that have a certain length and typically are made from glass fibres.
As the light travels from the light source through the fiberoptics, it is coupled to the endoscope through a small condensing cone that can be made of different materials such as glass or plastic. The cone is usually sized in a range of mm.
The cone can be angled to ensure that the light is condensed into a single axis at a given angle. Its shape is adjusted to produce a lens with a given numerical aperture for the particular endoscope with which it is used.
It is important that the condensing cone is aligned correctly for this purpose, since a non-linear geometric distortion can result, in which objects appear larger than their actual size and lines get bended. This is known as barrel distortion and it has to be corrected prior to 3D reconstruction, image registration or other advanced methods that rely on geometric information.
Reflection Characteristics
The reflection characteristics of an endoscope light guide are determined by the geometrical properties of the optics and the critical optical interfaces. In an endoscope, light rays are arranged to condense into an observation spot by means of a concave reflector lamp and a condenser lens. However, the condenser lens is arranged on the surface of the light guide and the illumination light rays have to be transmitted through the entire length of the endoscope in order to achieve collimation at a predetermined position.
In current rigid endoscopes the light collimation is optimized by means of an adhesive layer between single fiber bundles that fully absorbs incident rays. However, the production of these fiber bundles is not an automated process. Therefore, the alignment of individual fibers is not precisely defined and inaccuracies may occur due to the manufacturing constraints.
Furthermore, it is important to have an adequate heat transfer system between the LED and the handle of the device to dissipate the generated thermal energy. As the temperature of an LED mounted at the tip of an endoscope is too high for medical approval, a high-power-LED must be combined with a system to transport the heat to the handle of the instrument (Figure 1).
The most efficient system for achieving this purpose is a high-power-LED as an integrated light source in the handle of an endoscope and an adequate heat transfer system made of heat pipes (Figure 2). This approach leads to a much lower cost than a conventional cold-light source and is also easier to transport from the light source to the endoscope.
Another option is a large standalone light source for longer endoscopes and for procedures that require the observation of the gastrointestinal tract. These systems are usually connected to a power outlet and have a much higher light intensity than the integrated light source in the endoscope.
In this study we simulated the light collimation and spectral properties of an LED based integrated light source in a 3D-optomechanical model. By means of optimizations we improved the efficiency of the resulting light collimation from the typical measured values of about 16%-19% to 21%. Moreover, we were able to demonstrate the advantages of this approach by means of comparison tests on hyperspectral imaging against white light endoscopy, narrow band imaging, autofluorescence imaging and virtual chromoendoscopy.
