CMOSsensor

CMOS image sensors cost less to produce than CCD image sensors, because existing semiconductor manufacturing equipment can be repurposed for their production. Unlike CCD sensors that use high-voltage analog circuits, CMOS sensors employ a smaller digital circuitry that uses less power, and are in principle free from smear (vertical white streak in the image taken under bright light) and blooming (corruption of images such as white spots). Since a logic circuitry can be built into the chip during the manufacturing process, CMOS sensors with an on-chip image processing circuit are being developed for such applications as image recognition and artificial vision, and some devices are already being put to practical use.

Which of the following is the correct definition of the focal length of a lens? [A] The focal length of a lens is the distance between the center of the lens and one of its centers of curvature. [B] The focal length of a lens is the distance between its two foci. [C] The focal length of a lens is the distance between the center of the lens and one of its foci. [D] The focal length of a lens is the distance between its two centers of curvature.

CMOSimage sensortutorial

This leaves us with option (C). The focal length of a lens is the distance between the center of the lens and one of its foci. This is correct, so option (C) is our final answer.

A CMOS image sensor is a semiconductor that takes images of objects by extracting light, captured by a lens, as electrical signals for each pixel.

CMOSimage sensorPDF

So, what is the focal length then? The focal length is the distance from the center of the lens, which we’ve marked with this pink dashed line, to one of the focal points. This distance needs to be measured along the optical axis. Now that we know what the focal length is, we can choose the correct answer from the list of options.

We can rule out options (A) and (D) straightaway because these both mention the centers of curvature of the lens. Although the lens does have two centers of curvature, neither of them is used to determine the focal length of the lens. So, these options cannot be correct.

When parallel rays of light pass through a convex lens, the lens changes the direction of the rays so that they converge or get closer together. Having passed through the lens, the light rays meet at a point on the optical axis, called the focal point. Because light can pass through the lens in either direction, the lens actually has two focal points, or foci, one on each side.

CMOSimage sensor

In this question, we are asked to choose one of four statements that correctly describes the focal length of a lens. Let’s remember what the focal length is with the aid of a diagram. Let’s draw a thin, convex lens. When we’re thinking about lenses, there are several important things to remember. The side view of a convex lens should look like the overlap between two circles. The centers of these circles are called the centers of curvature of the lens, and the straight line that joins them is called the optical axis.

Camera sensor

A charge-coupled device (CCD) image sensor has an array of capacitors, each carrying an electric charge corresponding to the light intensity of a pixel. A control circuit causes each capacitor to transfer its contents to its neighbor, and the last capacitor in the array dumps its charge into a charge amplifier. The bucket-brigade style of data transfer is characteristic of CCD sensors. In contrast, a complementary metal oxide semiconductor (CMOS) image sensor has a photodiode and a CMOS transistor switch for each pixel, allowing the pixel signals to be amplified individually. By operating the matrix of switches, the pixel signals can be accessed directly and sequentially, and at a much higher speed than a CCD sensor. Having an amplifier for each pixel also gives another advantage: it reduces the noise that occurs when reading the electrical signals converted from captured light.

Which of the following is the correct definition of the focal length of a lens? (A) The focal length of a lens is the distance between the center of the lens and one of its centers of curvature. (B) The focal length of a lens is the distance between its two foci. (C) The focal length of a lens is the distance between the center of the lens and one of its foci. (D) The focal length of a lens is the distance between its two centers of curvature.

Now, let’s look at option (B). The focal length of a lens is the distance between its two foci. This may sound feasible, because we know that the focal length of a lens depends on the position of the focal point. However, the focal length is not measured between the two foci. The focal length is measured between the center of the lens and one of the focal points. So, option (B) cannot be correct.

Nagwa uses cookies to ensure you get the best experience on our website. Learn more about our Privacy Policy

The working principle of a CMOS (complementary metal oxide semiconductor) image sensor was conceived in the latter half of the 1960s, but the device was not commercialized until microfabrication technologies became advanced enough in the 1990s. Image sensors built into today's digital cameras and mobile phones mostly use either the CCD (charge coupled device) or CMOS technology. Both CCD and CMOS are semiconductor devices that serve as "electronic eyes." While they both use photodiodes, they differ in terms of manufacturing process and signal readout method. Although the CCD technology was dominant at first due to superior sensitivity and picture quality, various improvements in CMOS sensors led them to surpass CCD sensors from 2004 onwards in shipment volume.