Endosys

Video Technology

All the video imaging systems use the principle of conversion of light energy into electrical energy so that this electrical signal can be processed to project it onto a TV monitor where it is reconverted into light signal. It should be understood that human eyes can visualise only light images. They cannot view electric signals. Conversion of light energy into electrical energy is required so that the image can be processed to make it sharp and better to view. The device that helps conversion of one form of energy into other is called a Transducer. A transducer converts one form of energy into other but without disturbing the information coded in the energy. Thus, when light is converted into electrical energy, the information coded in it, i.e. image, is still preserved. It is now stored in form of electric signal. This signal can be Analog or Digital in nature. In case of analog signal, the information is stored in wave forms whereas in case of digital signal, it is in 0’s and 1’s. Digital processing and/or digital signals have much more clarity and accuracy than their analog counterparts. When the conversion is made, there are different waveforms that contain respective information regarding the place from which the original light came. When the electric signal is processed and reconverted into light energy, the points that are illuminated on the TV screen are in accordance with the original locations. This enables us to get the original image back.

One such transducer that converts light into electric signal is Charged Coupled Device (CCD). Commonly known as ‘Chip’, the CCD is rectangular in shape and wafer-thin in thickness. It is made up of Silicon. Some parts of this rectangular chip are activated while others are not. These activated parts of Silicon are called as ‘Pixels’. It is pixels that do the actual conversion. The ‘Streets’ among the pixels are inactivated silicon which conduct the converted electrical signal. ‘Activation’ of Silicon is done by doping it with some other material that is sensitive to light. When exposed to light, the activated Silicon gives rise to a electric charge that is proportional to the intensity of light that it has encountered. Different pixels on the same chip encounter different intensity of light. This is because different matter reflects different amount of light which is specific to that matter. The light reflected form different matter spreads in all directions. The pixels falling in line with the reflected light receive it and convert it to proportional electrical signal. Owing to this phenomenon, it is also possible to assign colours to different matter being imaged.

It should be noted that the light that falls on ‘streets’ is wasted. The inactivated silicon does not respond to the light signal. This means that to get a better picture, either the entire chip should be full of active pixels or the light has to be stronger. One way is to use a stronger light source. The other option of having the entire chip activated is not feasible because the electric potential generated by active pixels have to be conducted to the control unit for processing. In absence of ‘streets’ it is not possible to conduct the signal without losing the information regarding location of light and hence the image. To overcome this problem, modern chips come with in-built micro-lenses that focus more light onto the pixels. Each and every pixel is enclosed by a micro-lens that collects the light falling on the periphery of pixel or onto the streets and focuses it onto the pixel. This does not eliminate the wastage of light but it reduces it to a great extent. At the same time it is also true that the greater the number of pixels, the better will be the image. The number of pixels on a chip can be increased by two ways: one is by increasing the size of the chip and other is by increasing the density of pixels on the same chip. Both the options have limitations. The overall size of the chip is limited by the desired size of camera head and the density is limited by need of streets on the chip. The size of the chip can also be not increased because of its cost factor and also that it becomes more and more fragile with increase in size. Dimensions of chip are commonly referred to in terms of its diagonal. Chip is always rectangular in shape and its diagonal is used to denote its size. Thus, when we say that a camera has ½” chip, we mean that the diagonal of its chip is ½” long. The longer the diagonal, the more the number of its pixels.

This chip is contained in ‘Camera Head’. Thus, camera head is the most important component of the camera system. The housing of camera head has threads that screw the coupler/video arthroscope in place. Commercially available camera heads are of different sizes, i.e. C-mount and V-mount. C-mount camera head is 1” in diameter while V-mount camera head is (9/16)”. The image that is picked up by the scope is transmitted to the chip inside the camera head which converts it into the electric signal. This transmission of image from scope to the chip or camera head is done differently with video arthroscopes and direct-view arthroscopes. Video arthroscopes come with in-built threading and focussing controls. The Video arthroscope can just be screwed to the camera head. In case of a direct-view arthroscope, there is no threading on the scope’s eye-piece. To attach this to the camera head, special optical devices know as Couplers are used. Couplers are simple optic devices that fix on one end to the scope and on the other to the camera head. Besides this, they also have focussing attachment onto them that helps the surgeon focus on different depths and hence different parts of the body without moving the scope. To attach to different diameter camera heads, couplers are also available in C-mount and V-mount diameters. Besides this, the couplers are also available in different Focal Lengths. The light that is transmitted from the coupler onto the chip is converted into light signal by pixels on the chip and passed onto the adjoining streets. All the streets of chip combine into one single cable that originates from the other end of head and transfers the signals to camera control unit where the signal is processed. Processing of signal essentially means decoding the signal and making it suitable for acceptance by TV monitor. This decoding means reassembling all the information contained in the signal so that it can be projected onto the TV monitor. This processing can be done using either analog or digital processes. As mentioned earlier, digital processing of the signal always produces better image that analog processing. This is because digital processing is more precise than analog processing. It should be understood that when one refers to a camera as a digital camera, it could be that the signal is converted into digital form or the processing is done digitally or both. On processing the signal is made of amplitude that can be sensed by the TV monitor. This amplified signal is then transmitted to the TV monitor’s video input. Transmission of signal from camera to monitor is done in various formats. These video formats are as listed below: