Medical endoscopy has become a key technology for the purposes of minimally invasive diagnostic and therapeutic procedures. With the aid of an endoscope, an external observation of a location inside the human body which may be difficult or dangerous to reach or which should be subjected to minimal traumatization associated with the intended medical treatment is facilitated through a viewing channel. A television camera is usually attached to the distal end of an endoscope for image presentation. Of primary importance for the user are image quality, ease of application and safety. This communication is primarily concerned with aspects relating to image quality. First, the optical properties of various types of endocsopes are analysed and their spatial resolution is determined. Second, it is shown that the inherent optical quality in terms of spatial, temporal and contrast resolution of most endoscopes exceeds the performance limits of presently used standard TV systems in part substantially. Accordingly, a special purpose advanced digital television system exhibiting high spatial resolution, furthermore true color and high contrast dynamics has been developed. Since endoscopic images usually are circular, its aspect ratio is quadratic with a spatial resolution of 1000 x 1000 pixels. The framing rate is variable from 1 Hz to 30 Hz noninterlaced. Some design aspects and the performance of the system are shown and further applications are indicated.

An autofocus is a desirable feature of an endoscope, because it relieves the user from performing a task which can be automated and thereby prevents unnecessary interruptions in the work to be performed. Autofocusing is in general best achieved by an active system, i.e., on the basis of a distance measurement. Yet, in handheld medical endoscopes such a method is unsuited because of the added weight associated with the necessary electromechanical components. Autofocusing should rather be performed passively, furthermore, in applications which are particularly critical with respect to safety, e.g., in the eye, a stable and reliable operation in real time and without interruption is necessary. Passive autofocus strategies applied to date and known to the authors lead however to algorithms which are either too slow for a real time implementation and/or are influenced by the structure of the object which is to be brought into focus. Accordingly, a new autofocus procedure has been developed which exhibits a stable and reliable operation in real time under all circumstances of interest. It is based on the squared differences of the intensity of adjacent points in both dimensions of a plane image (Square Plane Sum Modulus Difference, SPSMD) and as such particularly suitable for digital camera systems and real-time needs (typically, 30 evaluations per second on an image of 1024 x 1024 pixels). The SPSMD criterion is more sensitive, has a larger SNR than other focus criteria known to the authors and exhibits in particular no secondary extrema which could adversely affect proper focusing. As it includes intensity differences in both (perpendicular) directions in the image plane, it is essentially independent of image structures.

A novel digital high-definition TV (digital HDTV) system is presented which is adapted to the special characteristics of medical endoscopes. In particular, it has a quadratic image aspect ratio to accommodate round endoscopic images, furthermore, the spatial resolution is 1024 x 1024 pixels thereby approaching the diffraction limit of small endoscopes. It exhibits 24 bits true-color with an adjustable temporal resolution of up to 30 full frames per second in progressive scan.To ensure overall superior performance and high-quality image acquisition and reproduction, complex digital error correction and picture enhancing algorithms are integrated into custom ASICs (Application Specific Integrated Circuit). Additionally, an improved color space transformation is performed in real-time to match the spectral characteristics of the digital camera and the viewing device, allowing for a quantitative judgment of colors.Further features include the conversion of the digital video stream to standard video norms such as PAL or NTSC for recording on analog VCRs, the calculation and evaluation of a focus criterion, which is used to perform passive, stable and reliable auto-focusing and the implementation of an automated illumination control, ensuring proper picture brightness during the whole time of operation.

Real-time motion pictures providing image quality superior to that of standard video in terms of resolution and color fidelity are often required. In many applications, e.g., in biomedicine or machine vision, only cameras with a single charge-coupled device (CCD) sensor can satisfy the stringent space requirements. Reconstruction of the RGB image from the CCD with Bayer color filter array (CFA) requires an extremely high computation effort if reasonable frame rates and artifact-free interpolation quality is to be achieved. An application specific IC (ASIC) performs this task for 1024 by 1024 pixels at 30 frames per second. Pixelwise black-current and white-gain balancing, color space transformation, and focus criterion calculation in real-time are additional functions of this chip.

In endoscopic ophthalmic procedures care has to be exercised that the retina is protected from overexposure. Accordingly, it is advantageous if the endoscope is equipped with a stable and reliable automatic illumination control. To this end, an illumination control system has been devised, which consists of a mechanical iris and a digital control algorithm. The iris is designed such that it influences neither the spectral composition of the lightsource nor its aperture. It is furthermore linear with respect to the light intensity such that a fast control algorithm based on the data of a digital video camera used for observation purposes can be implemented. In order that no false signals are induced from specular reflections caused, e.g., by operating tools held in front of the camera, the control algorithm is designed such that reflections and true overexposure are distinguished from each other. For this purpose, the field of view is subdivided into small sectors and a statistical evaluation is made. The application under realistic conditions shows that the unit provides the user with a well illuminated image while the retina is reliably protected from overexposure.

Present endoscopic systems for routine use are mostly based on Standard Definition TV (SDTV) and analog data transmission for image acquisition and presentation. Accordingly; the intrinsic optical quality of most endoscopes in terms of spatial and dynamic resolution as well as color quality is not fully exploited by this technology.Our newly developed high-definition TV (HDTV) system takes advantage of cutting-edge digital and analog processing stages, combined with real-time digital image processing algorithms, to achieve a substantially improved overall image quality.The system is capable of processing 30 frames per second in progressive scan, each of which consists of 1024 x 1024 pixels in 24-bit true-color mode. To guarantee uniform high-level performance, custom ASIC(a) design has been used and implemented. The pulse generator ASIC is responsible for the CCDb's most optimal working condition and the correction/interpolation ASIC performs an interpolation of the missing color values for each pixel in realtime, as well as offset and gain correction for each pixel, to compensate CCD sensor pixel defects, dark current and illumination irregularities.In addition a focus and illumination control system has been added to achieve optimal sharpness and brightness during the whole time of operation. Algorithms are provided which calculate the necessary parameters to control the illumination and the position-of the focus lenses from a true two-dimensional evaluation of the image content, thus avoiding the need for any user activity.Various configurations for viewing and recording of the video sequence are possible, on particular for storage of single-frame sequences in full resolution and color depth on a PC's harddisk or, if recording over a long period of time is needed, for conversion and storage on a standard VCRc tape.