In this blog, we will try to understand what is a digital image and how it is formed. We will also learn about image sensors and pixels in this blog.

What is a Digital Image?

A Digital Image is a numeric representation of a two-dimensional image and is made of picture elements called pixels, arranged in rows and columns. These numeric values are the intensity or brightness values that are associated with the pixels.

Now, let’s understand what is a pixel and how the intensity value is calculated?

The concept of pixel is closely related with image sensors. So, by understanding image sensors I hope you will get a feel of what a pixel is.

Image Sensor: It is a device that converts the light energy into an electric signal. The figure below shows a single image sensor.

source: R. C. Gonzalez, R. E. Woods, Digital Image Processing

In digital cameras, these sensors are arranged in the form of a 2D array on a chip. Mostly there is a 1-to-1 correspondence between a pixel and a sensor meaning each sensor produces 1 pixel. Thus, the total number of pixels in an image = total number of sensors on a chip. Sometimes, multiple sensors are used to produce a pixel of information.

8MP camera means there are 8×10⁶ image sensors in the camera chip and if there is a 1-to-1 correspondence between a pixel and a sensor then the image contains 8×10⁶ pixels.

Note: Larger the pixel, better will be the image quality.

How the Digital Image is formed?

There are basically two types of image sensors used in digital cameras: CCD or CMOS. The CCD sensor is a silicon chip that contains a 2D array of photosensitive sites or sensors. Each sensor has a PN diode and a storage cell. Each sensor outputs a pixel value through these steps

1. Light energy or photons(>1.2eV) falls on the silicon layer of the sensor and electrons are released(stored in potential well until the shutter is open).
2. After the camera shutter closes, these electrons are moved to the storage cell (by applying +ve voltage on vertical shift register).
3. These electrons are then transferred to the serial shift register.
4. These electrons are converted into the analog voltage that is amplified using an amplifier.
5. The output voltage signal is immediately converted to a digital signal by means of an analog-to-digital converter (ADC) in digital cameras, either on or off-chip. This digital quantity obtained is the intensity value of a pixel.

These steps are for the CCD sensor. For CMOS sensor, step 3 is not performed and voltage conversion and amplification (Step 4) is done in the sensor itself. This voltage is sent to ADC with the help of switch from each sensor and outputs the intensity value of a pixel.

Repeat these steps for all the sensors on a chip to get the intensity values corresponding to each pixel and thus we get a digital image.

Now, you might have got some feeling about the image sensors, pixel, and digital image. Hope you enjoy reading.

If you have any doubt/suggestion please feel free to ask and I will do my best to help or improve myself. Good-bye until next time.