I have already written posts on the working of film cameras, digital cameras and also on the image sensors. To know more about them click on the links below.

TAKE A LOOK : WORKING OF FILM CAMERAS

TAKE A LOOK : WORKING OF DIGITAL CAMERAS

Digital cameras are mainly classified according to their use, automatic and manual focus, and also price. Here are the classifications.

1. Compact digital cameras

Compact cameras are the most widely used and the simplest cameras to be ever seen. They are used for ordinary purposes and are thus called “point and shoot cameras”. They are very small in size and are hence portable. Since they are cheaper than the other cameras, they also contain fewer features, thus lessening the picture quality. These cameras are further classified according to their size. The smaller cameras are generally called as ultra-compact cameras. The others are called compact cameras.

Here are some features of this camera

• Compact and simple.
• Images can be stored in computer as JPEG files.
• Live preview can be seen before taking photos.
• Low power flashes are available for taking photos in the dark.
• Contains auto-focus system with closer focusing ability.
• Zoom capability.

Although these features are available, their magnitude may be less compared to other cameras. The flashes may be available only for nearby objects. The preview of the picture to be taken will have less motion capability.  The image sensors used in these cameras have a very small diognal space of about 6mm with a crop factor of 6.

Compact Digital Cameras

2. Bridge cameras

Bridge cameras are most often mistaken for single-lens reflex cameras (SLR). Though they have the same characteristics their features are different. Some of its features are

• Fixed lens
• Small image sensors
• Live preview of the image to be taken
• Auto-focus using contrast-detect method and also manual focus.
• Image stabilization method to reduce sensitivity.
• Image can be stored as a raw data as well as compressed JPEG format.

Though they resemble SLR in many ways, they operate much slower than the latter. They are very big in size and so the fixed lenses are given very high zooming capability and also fast apertures. The autofocus or manual focus is set according to our necessity. The image preview is done using either a LCD or an Electronic View Finder (EVF).

Bridge Cameras

3. Digital single lens reflex cameras (DSLR)

This is one of the most high end cameras obtainable for a decent price. They use the single-lens reflex method just like an ordinary camera with a digital image sensor. The SLR method consists of a mirror which reflects the light passing through the lens with the help of a separate optical viewfinder.

Some features of this camera are

• Special type of sensors is setup in the mirror box for obtaining autofocus.
• Has live preview mode.
• Very high end sensors with crop factors from 2 to 1 with diagonal space from 18mm to 36mm.
• High picture quality even at low light.
• The depth of field is very less at a particular aperture.
• The photographer can choose the lens needed for the situation and can also be easily interchangeable.
• A focal plane shutter is used in front of the imager.
• 

  Digital single lens reflex cameras (DSLR)

4. Electronic viewfinder (EVF)

This is just a combination of very large sensors and also interchangeable lenses. The preview is made using an EVF. There is no complication in mechanism like a DSLR.

Electronic View Finder

5. Digital rangefinders

This is a special film camera equipped with a rangefinder. With this type of a camera distant photography is possible. Though other cameras can be used to take distant photos, they do not use the rangefinder technique.

Digital rangefinders

6. Line-scan cameras

This type of cameras is used for capturing high image resolutions at a very high speed. To make this mechanism possible, a single pixel of image sensors are used instead of a matrix system. A stream of pictures of constantly moving materials can be taken with this camera. The data produced by a line-scan camera is 1-dimensional. It has to be processed in a computer to make it 2-D. This 2-D data is further processed to obtain our needs.

Line-scan cameras

I have already described about the working of a camera. Almost all the basics of this post have been explained there. Now let us know more about a digital camera, its working, and also advantages.

The digital camera can be considered as an alteration of the conventional analog camera. Most of the associated components are also the same, except that instead of light falling on a photosensitive film like an analog camera, image sensors are used in digital cameras. Though analog cameras are mostly dependent on mechanical and chemical processes, digital cameras are dependent on digital processes. This is a major shift from its predecessor as the concept of saving and sharing audio as well as video contents have been simplified to earth.

TAKE A LOOK : WORKING OF AN ANALOG CAMERA

Digital Camera Basics

As told earlier, the basic components are all the same for both analog and digital cameras. But, the only difference is that the images received in an analog camera will be printed on a photographic paper. If you need to send these photos by mail, you will have to digitally convert them. So, the photo has to be digitally scanned.

This difficulty is not seen in digital photos. The photos from a digital camera are already in the digital format which the computer can easily recognize (0 and 1). The 0’s and 1’s in a digital camera are kept as strings of tiny dots called pixels.

The image sensors used in an digital can be either a Charge Coupled Device (CCD) or a Complimentary Metal Oxide Semi-conductor (CMOS). Both these image sensors have been deeply explained earlier.

TAKE A LOOK : CHARGE COUPLED DEVICES (CCD)

TAKE A LOOK : CMOS ACTIVE PIXEL SENSOR (CMOS APS)

TAKE A LOOK : CCD V/s CMOS

The image sensor is basically a micro-chip with a width of about 10mm. The chip consists arrays of sensors, which can convert the light into electrical charges. Though both CMOS and CCD are very common, CMOS chips are known to be more cheaper. But for higher pixel range and costly cameras mostly CCD technology is used.

A digital camera has lens/lenses which are used to focus the light that is to be projected and created. This light is made to focus on an image sensor which converts the light signals into electric signals. The light hits the image sensor as soon as the photographer hits the shutter button. As soon as the shutter opens the pixels are illuminated by the light in different intensities. Thus an electric signal is generated. This electric signal is then further broke down to digital data and stored in a computer.

Pixel Resolution of a Digital Camera

The clarity of the photos taken from a digital camera depends on the resolution of the camera. This resolution is always measured in the pixels. If the numbers of pixels are more, the resolution increases, thereby increasing the picture quality. There are many type of resolutions available for cameras. They differ mainly in the price.

• 256×256 – This is the basic resolution a camera has. The images taken in such a resolution will look blurred and grainy. They are the cheapest and also unacceptable.
• 640×480 – This is a little more high resolution camera than 256×256 type. Though a clearer image than the former can be obtained, they are frequently considered to be low end. These type of cameras are suitable for posting pics and images in websites.
• 1216×912 – This resolution is normally used in studios for printing pictures. A total of 1,109,000 pixels are available.
• 1600×1200 – This is the high resolution type. The pictures are in their high end and can be used to make a 4×5 with the same quality as that you would get from a photo lab.
• 2240×1680 – This is commonly referred to as a 4 megapixel cameras. With this resolution you can easily take a photo print up to 16×20 inches.
• 4064×2704 – This is commonly referred to as a 11.1 megapixel camera. 11.1 megapixels takes pictures at this resolution. With this resolution you can easily take a photo print up to 13.5×9 inch prints with no loss of picture quality.
• There are even higher resolution cameras up to 20 million pixels or so.

Color Filtering using Demosaicing Algorithms

The sensors used in digital cameras are actually coloured blind. All it knows is to keep a track of the intensity of light hitting on it. To get the colour image, the photosites use filters so as to obtain the three primary colours. Once these colours are combined the required spectrum is obtained.

For this, a mechanism called interpolation is carried out. A colour filter array is placed over each individual photosite. Thus, the sensor is divided into red, green and blue pixels providing accurate result of the true colour at a particular location. The filter most commonly used for this process is called Bayer filter pattern. In this pattern an alternative row of red and green filters with a row of blue and green filters. The number of green pixels available will be equal to the number of blue and red combined. It is designed in a different proportion as the human eye is not equally sensitive to all three colours. Our eyes will percept a true vision only if the green pixels are more.

The main advantage of this method is that only one sensor is required for the recording of all the colour information. Thus the size of the camera as well as its price can be lessened to a great extent. Thus by using a Bayer Filter a mosaic of all the main colours are obtained in various intensities. These various intensities can be further simplified into equal sized mosaics through a method called demosaicing algorithms. For this the three composite colours from a single pixel are mixed to form a single true colour by finding out the average values of the closest surrounding pixels.

Take a look at the digital camera schematic shown below.

Parameters of a Digital Camera

Like a film camera, a digital camera also has certain parameters. These parameters decide the clarity of the image. First of all the amount of light thatenters through the lens and hits the sensor has to be controlled. For this, the parameters are

1. Aperture – Aperture refers to the diameter of the opening in the camera. This can be set in automatic as well as the manual mode. Professionals prefer automatic mode, as they can bring their own touch to the image.

2. Shutter Speed – Shutter speed refers to the rate and amount of light that passes through the aperture. This can be automatic only. Both the aperture and the shutter speed play important roles in making a good image.

3. Focal Length – The focal length is a factor that is designed by the manufacturer. It is the distance between the lens and the sensor. It also depends on the size of the sensor. If the size of the sensor is small, the focal length will also be reduced by a proportional amount.

4. Lens – There are mainly four types of lenses used for a digital camera. They differ according to the cost of the camera, and also focal length adjustment. They are

• Fixed-focus, fixed-zoom lens – They are very common and are used in inexpensive cameras.
• Optical-zoom lenses with automatic focus – These are lenses with focal length adjustments. They also have the “wide” and “telephoto” options.
• Digital zoom – Full-sized images are produced by taking pixels from the centre of the image sensor. This method also depends on the resolution as well as the sensor used in the camera.
• Replaceable lens systems – Some digital cameras replace their lenses with 35mm camera lenses so as to obtain better images.

Digital Cameras v/s Analog Camera

• The picture quality obtained in a film camera is much better than that in a digital camera.
• The rise of technology has made filming the help of digital techniques easier as well as popular.
• Since the digtal copy can be posted in websites, photos can be sent to anyone in this world.

Since posts on Charge Coupled Device and CMOS Active Pixel Sensor have already been posted, it is time to know their comparison, advantages and disadvantages. Though both of them are equally used in cameras, there are some differences in parameters like gain, speed and so on.

Comparison –  Charge Coupled Device and CMOS Active Pixel Sensor

• Both the devices are used to convert light into electric signals and are used for the same applications.  After converting the signals, they have to be read from each cell. This process is different for both the devices.

• The charge from each chip is taken to the end of the array and then read in a CCD. This is then converted into a digital signal with the help of an analog to digital converter (ADC). The process of reading the signal by CMOS Active Pixel Sensor is done by using transistors and amplifiers at each pixel and then the signal is moved using traditional wires.

Difference – Charge Coupled Device and CMOS Active Pixel Sensor

• CCD image sensors create super quality pictures. They also produce lesser noise than CMOS APS.

• In a CMOS all the transistors are kept right next to each pixel. As a result, all the photons that hit the device actually get scattered by hitting the transistors as well. Thus, the sensitivity of CMOS Active Pixel Sensor is lesser than that of a Charge Coupled Device.

• The design of the CCD sensors is in such a way that they require more power for its operation. If both the devices of equal reception are taken, the CCD is considered to consume almost 100 times more power than its equivalent CMOS Active Pixel Sensor.

• All the devices have been using Charge Coupled Device devices far more than CMOS Active Pixel Sensor. As a result a vast study has been done on CCD devices. So, they are more mature and also tend to have higher quality pixels.

Active Pixel Sensor (APS) is an image sensor, made up of an array of pixel sensors. In these pixel sensors, each pixel sensor consists of a photo detector and an amplifier. Out of these APS, the most notable is the CMOS Active Pixel Sensor (CMOS APS). CMOS APS has great applications in cameras and also DSLRs. It is called so as it is manufactured by the CMOS process. This type of image sensor is very similar to that of a Charge Coupled Device (CCD). They are also called active pixel sensor imager and also active pixel image sensor.

The CMOS APS uses a photo detector to detect the light and converts it into electrical signal. This signal is then amplified using several transistors and is then moved using traditional wires.

Introduction of CMOS Active Pixel Sensors

The wide use of CMOS Active Pixel Sensors began during the year 1993. The Jet Propulsion Laboratory developed some prototypes which were later commercialized. After knowing its immense potential in the field high speed, low power motion capture cameras many companies quickly adopted and developed this technology.

During the early 1960’s, before the discovery of active pixel sensors, there were only passive pixel sensors. In this mechanism, the pixels were designed in a 2D structure, with access enable wire shared by pixels in the same row, and output wire shared by column. Each pixel did not have an amplifier and so an amplifier was connected ar the end of each column. High power consumption, greater noise and also slow output were some of its disadvantages. In 1969, the active pixel sensor was first introduced by adding independent amplifiers for each pixel. Thus in 1970, the Charge Coupled Device was invented. Thus they were very useful in the working of cameras. During the early 1990’s the CMOS process was well developed and was considered to be the base for all types of logic devices as well as microprocessors. This further led to the invention of CMOS APS.

Architecture of Active Pixel Sensors

For describing the architecture of a CMOS APS, there are mainly three different parameters. They are

• Pixel

The pixel of a CMOS APS mainly consists of photo detectors like a JFET photogate or pinned photodiode. The whole pixel will be called a 4T (4 transistor) cell. The 4T cell mainly consists of a transfer gate, reset gate, selection gate and also a source follower input transistor connected to the photo detector.

The photo detectors used in this 4T cell were first used for Charge Coupled Devices. But, when it was further connected it to the transfer gate and other components the charge transfer was done at a greater speed and also low noise was generated.

There are 3 transistor (3T) cells used now as well. As they are very simple in fabrication they are more commercially produced.

• Thin Film Transistor APS

APS has applications in the field of X-ray as well. For taking digital X-rays, thin film transistors (TFT) is also used. But, its larger size and low gain makes the number of TFTs limited.

• Array

A 2-D array of pixels is organized into rows and columns. The reset lines are connected to the rows so that when RESET occurs the whole row gets reset. Similar is the case of the select lines. The outputs of each pixel in any given column are tied together. Take a look at the architectural diagram given below.

CMOS active pixel transisitor

Applications

The applications of CMOS APS includes web cameras, motion capture cameras, digital radiography, endoscopy cameras and also X-ray imaging. They are mainly known for their application in filmless cameras.

Charge Coupled Device (CCD) – Definition

CCD was developed in the year 1969 by Willard Boyle and George E. Smith at AT & T Bell Labs. It is a shift register device which can be used for the movement of electrical charge within the device. This movement can be from one area of the device to another and the digital value of the moved charge can be easily found out. When the signals are moved, one at a time from one place to another within the device, the value of the charge can be easily manipulated. There are capacitive bins in the device that allow the movement of charge.

During the invention of CCD there was no means to produce the charge than injecting it. But through repeated experiments, it was later found out that when a sensor like a photoelectric device was connected to it, a charge could be easily produced. This charge could then be given to the CCD for its transfer in the device. This discovery was huge enough as it became the stepping stone to the conversion of ordinary signals into digital signals. The device that is used to capture the images with ordinary cameras and replacing them as a digital storage is called a CCD imager.

Charge coupled Device CCD

Charge Coupled Device (CCD) – Operation

There are mainly two regions of a CCD. They are

1. Photoactive Region

As told earlier, a CCD is used to convert a electrical signal into a digital signal. The photoactive region mainly consists of a capacitor array. These arrays can be one-dimensional or two-dimensional depending on the type of device that uses the CCD. If a line scan camera is used, it introduces a one-dimensional capacitor array. It is called 1D because it captures the image in 1D form, that is, a single slice of the image. 2D is used mostly in video applications. This device captures the image in 2D form. The photoactive region is made out of an epitaxial layer of silicon. It is made by doping a boron ion on a substrate such as p++. Sometimes CCD’s are also implanted with a phosphorus ion so as to give them an n-doping . This is often carried out in devices consisting of n-channels This is done in some areas of the silicon ion causing the movement of photo generated packets across them.

As soon as the silicon layer and substrates are made, a dielectric in the form of a gas oxide (mostly capacitor) is made to grow on top of them. Thus the separately lying gates will lie in a perpendicular angle to the channels. This is because the poly-silicon gates are undergoing chemical vapour deposition and then photolithography. Then the channel stop region and the charge carrying channel is made, and that too parallel to each other.

2. Transmission Region

After the image is projected onto the capacitor array, the control circuit comes into action. This circuit makes the capacitors send the appropriate signal to a shift register. The shift register converts each signal into a voltage sequence. This is later sampled, digitized and then stored in the memory.

With different modes of operation for the CCD, the type of the device will also differ. There are versions of CCD called frame transfer CCD and also peristaltic CCD. In the case of a frame transfer CCD, the gate clocks are used to bias the diode in the reverse as well as forward direction. This is mainly done by the n-doped and p-doped layers. Thus the CCD across or near the p-n junction will get depleted. Thus the charges situated under he gates and also across the channels will be collected and moved.

A peristaltic CCD generates a huge electric field from one gate to the next by providing an additional implant. This implant helps in blocking the charge from the Si/SiO2 interface. Thus the additional driving force created die to this action helps in faster transfer of charge particles.

CCD Diagram

Charge Coupled Device (CCD) – Applications

• Astronomy

CCD’s are used in astronomy because of their high linearity in outputs. CCD is used in all the astronomical Ultra violet and infrared applications. They are also highly efficient in quantum applications. Though the CCD characteristics may be affected by thermal noise and cosmic rays, astronomers have taken several counter measures to reduce this. One method includes the timing of the CCD shutter. With the shutter closed the number of images taken will determine the random noise. After tking the mage with the shutter closed, the result is then differed with the open-shutter image so as to remove the dead and hot pixels.

Application in astronomy also includes a method called drift scanning. This method is mainly used to follow the motion of the sky. This is done by converting a fixed telescope into a tracking telescope with the application of CCD.

• Colour Cameras

For the use of CCD in cameras, a much more advanced for called 3CCD is used commonly. By using 3CCD devices, the colour separation becomes much improved, thus increasing the quantum efficiency as well. The image resolution of a camera completely depends on the CCD chip. When the photons hit the sensor, the sensor counts their number. So, the brighter the image at a given point on the sensor, the larger the value that is ready for that pixel.