Digital sensors

Digital sensors are the electronic components that capture the images and translate them into data to be stored in the memory card: They are therefore the photosensitive element of the cameras and have now supplanted the various types of film, At least in the most widespread use.

Here we describe what these elements are: you enter a purely technological field. To take photographs in a conscious way is not essential To know how it works: even if the understanding of how to form images today through electronics can be useful and interesting. I Carry some basic concepts.

When It comes to sensor, we immediately associate the value of image resolution, namely the ability to make details: Since the image is recorded by the sensor, let's talk about its total resolution, which is measured in millions of pixels total (so-called Megapixel or MP).

To understand the factors that determine the quality of the images, it is necessary to introduce the concept of photosite which is the unitary part of the sensor (belongs to the hardware category). The Pixel represents a data group describing the chromatic characteristics of the smallest detail of the image (it belongs to the software category).

The sensors are divided into two categories: CCD (Charge-Coupled Device) and CMOS (Complementary Metal Oxide Semiconductor). The manufacturing processes of the two sensors are different, as is the arrangement of the circuits on them: they are always platelets full of photosites that collect and convolete the light.

The light hitting the sensor is stored in every photosite as An electrical charge. From each photon one develops an electron, which generates electrical voltage. The electrical charge is then converted by means of an analog-digital converter (ADC), in digital data.

The sensors have no understanding of the colors of reality: they react to the photons that invest them and they free electrons, without being absolutely sensitive to the color of perceived light. To Obtain the colour, a mosaic filter is placed in front of the sensor, with a red, green and blue motif: The diagram shows the Bayer filter pattern, the most widespread.

The Bayer filter consists of an array of colored dots of the same size as the sensor and with a very precise layout. The ratio are 2 green dots for 1 point or red or blue, because the human eye is more sensitive to the green color: it is precisely in that area of the visible spectrum that we can recognize a greater number of details and nuances.

So the light crosses the filter, it hits the sensor and it records the brightness value of a single color: if we were to remove the Bayer filter, our sensor would record black and white images.

To reconstruct all the values RGB (red, green and blue) comes into play the demosaicing: An algorithm that allows to reconstruct, thanks to interpolation techniques, all the colors of the image starting from the data supplied by the sensor. Bilinear interpolation allows you to detect missing channel values at a point, starting from adjacent points: if the Red and Green values fail in the Blue point, they are calculated from the adjacent Red and Green Dots, through the software interpolation of Missing color points (and exploiting as much as possible the fact that the green pixels are in double quantity compared to the red and blue ones).

One of the things to keep in mind is that the sensors respond to the light in a linear manner. That is, they return an electrical charge directly proportional to the amount of light received: graphically corresponds to a straight line. The problem is that the human visual system is much more advanced and "accommodating": we do not see suddenly the highlights or deep shadows disappear. Our response, and that of films, is not linear and this allows us to see the details in a wider tonal range than that of the digital sensor.

Between the acquisition of the data of an image and its storage in the memory card there are other passages: each manufacturer uses a particular technology, aimed at correcting data acquisition errors and obtaining the best possible result. An insight into sensor technology in the article CCD and CMOS Sensors.