A digital image is a numeric representation, normally binary, of a two-dimensional image. Before we can intelligently analyze and manipulate images (image processing, computer vision), we need to understand the image formation process. That produced a particular image given a set of lighting properties, surface conditions, camera properties.
How do our eyes work?
Before we discuss about the image formation on digital cameras, we have to first discuss the image formation on human eye. Because the basic principle that is followed by the cameras has been taken from the way that the human eye works (no detail).
Light from the sun, or an artificial light, travels in a straight line, bounces off objects and into our eyes through the pupil. The light then passes though the lens. The lens focuses the light and the image is formed onto the back surface of the eye, the retina. A fatter lens bends light more than a flatter lens. The human eye changes the shape of the lens as we look at far or near objects to keep them in focus. This is called accommodation. The image that is formed is inverted. This image is then interpreted by the brain and that makes us able to understand things. Due to angle formation, we are able to perceive the height and depth of the object we are seeing. You can see more detail in the tutorial of perspective transformation.
How do digital camera work?
Now lets take our discussion back to the image formation on in the digital cameras. The image formation is not due to the chemical reaction that take place, rather it is a bit more complex then this. In the digital camera, a CCD array of sensors is used for the image formation. This CCD is actually in the shape of array or a rectangular grid. It is like a matrix with each cell in the matrix contains a censor that senses the intensity of photon. In the case of digital camera, when light falls on the object, the light reflects back after striking the object and allowed to enter inside the camera.
Introduction of pixels
Since we have already define an image as a two dimensional signal and due to the two dimensional formation of the CCD array , a complete image can be achieved from this CCD array. The value of each sensor of the CCD array refers to each the value of the individual pixel. The number of sensors = number of pixels. It also means that each sensor could have only one and only one value.
The charges stored by the CCD array are converted to voltage one pixel at a time. With the help of additional circuits , this voltage is converted into a digital information and then it is stored. We can assume that is the final of image formation process.
1. Computer Vision: Algorithms and Applications, Richard Szeliski (Book).
2. Image Formation on Camera (Tutorialspoint).