Image Sensor Terminology
The device which converts light into an electrical signal is called a photo sensor (image sensor). An image sensor is a device consisting of an arrangement of several hundred to tens of thousands of photo sensors, which scans and converts the light entering through the lens to an image. In order to get image, the image sensor owns a function to acquire the signals (called scan or transmission) from many photo sensors, in addition to a photoelectric conversion function. Although photo sensors arranged with photo diodes are mostly used, they are distinguished into CCD image sensors and CMOS image sensors due to the difference in manufacturing method and scanning method. There are line sensors (one-dimensional array) and area sensors (two-dimensional array) depending on the arrangement method of the photo sensors.
CCD Image Sensor
The electrical charge accumulated in the photo sensor (photo diode) of each pixel cannot acquire signal as it is. A CCD image sensor is an image sensor consisting of a single row arrangement of charge-coupled devices called the CCD (Charge Coupled Device), which sequentially transfers the electrical charge between the adjacent devices, and finally the electrical charge is amplified with an amplifier to acquire the signals from each pixel. Although the advantages of a CCD image sensor are the high sensitivity, less variation in a single amplifier, and low noise, etc., there are also disadvantages, such as the high cost due to its complicated structure, and a large quantity and high voltage are required to transfer the electrical charge.
CMOS Image Sensor
A CMOS (Complementary Metal Oxide Semiconductor) image sensor is an image sensor which sequentially acquires the signals by a switching circuit, after the electrical charge accumulated in the photo sensors (photo diodes) are amplified by an amplifier installed for each pixel. Initially there were disadvantages, such as low sensitivity, and high variations and noise because an amplifier is installed for each pixel, however, as the improvement of the photo sensor and re-design of the circuit, a CMOS image sensor now can also acquire image with high definition, as the same with CCD image sensor. The main feature of the CMOS image sensor is the low cost due to its simple structure, where the photo sensor and circuit can be produced in the same manufacturing process, and this manufacturing process is used for many types of ICs, including memory and CPUs. Furthermore, since CMOS image sensor can operate with a single power supply, and its high speed and low power consumption are highly praised, the demand is growing rapidly.
Line Image Sensor
The line image sensor consists of a single row arrangement of photo sensors, and is also called a linear image sensor. There are IC type image sensors and CIS (contact image sensors) which consist of photo sensors arranged in the size of the scanning width, due to the difference in the shape and scanning method. IC type image sensors are used for the reduction scanning method, which scans the light collected in the photo sensors arranged in a high density through a reduction lens. On the other hand, CIS is used for the life-size scanning method, which scans the object with a life-size magnification lens. Since a line image sensor can only scan a one-dimensional image, it is necessary to move either the object or the image sensor, however, it is suitable for high resolution scanning because it can create a large number of pixels (tens of thousands of pixels). In addition, a camera equipped with an IC type line image sensor may also be called a line camera.
Area Image Sensor
An area image sensor consists of photo sensors arranged on a plane, which collects the light in these photo sensors using a reduction lens to scan the object at once. Although area image sensors are used for digital cameras, it is called a CCD camera or CMOS camera depending on whether a CCD or CMOS is used.
Refractive Index Distribution Type Lens (Rod lens, Upright life-size magnification lens)
This is a cylindrical lens where the distribution of the refractive index is changed to the radial direction. Since the light incoming from the end surface of the lens propagates following a sine curve, the image of an upright life-size magnification can be acquired by adjusting the length of the lens, therefore, it is essential to use a lens on CIS. Although the disadvantage of this lens is the narrow focus range compared with a reduction lens, there are advantages too, such as no peripheral distortion, and a compact lens can be manufactured because the light path length is shorter, which is suitable for scanning long objects.
Light Guide (Light guide bar, Light guide plate)
This device converts a point light source (LED) to a linear light source, and also enables uniform illumination of the scanning surface of an original copy, etc. The internal reflection of a transparent resin, such as acrylic, is designed so that uniform illuminance can be realized. There are bar shaped guide (light guide bar) and plate shaped guide (light guide plate).