How a scanner works
What does a scanner actually do? It records the analogue data of a picture through sensors and converts it into digital data with an analogue-digital converter.
The principle is easier than it sounds: an original picture, for example a photograph, is illuminated with a light source. The reflected light is passed on to an optoelectronic line sensor. This works via a rod lens in which the light is bundled and stray light is eliminated. Then, these analogue light signals are converted into an analogue voltage by light-sensitive photodiodes. Afterwards the analogue voltage is converted pixel by pixel into a digital value by the analogue-digital converter.
How does the scanner determine the different colours? – Scanning with colour filters
To determine the different colours, the scanner can work with colour filters. By preceding colour filters, it is possible to scan the primary colours (red, green and blue) separately. This process is often supported by software-based additive colour mixing. This filtering method requires a separate scan for each basic colour. However, there are also processes that only require a single scan. For example, with the following two.
Scanning with coloured or white light source
A scanner can work with different light sources. If your device works with a white light source, your photo is illuminated with white light. The reflected light is guided through a prism, that splits the light into its colour components. The different colour components are then detected by adjacent optical sensors. When using a white light only a single scan is required.
If the scanner works with coloured and switchable light sources, only one scan is needed as well. This is because the colour separation is given by the light sources themselves. During a measurement, the sensors only measure the light of a certain wavelength. The light is often generated by inexpensive light-emitting diodes.
By now, Bayer sensors are used very frequently. With these and other colour sensors, all sensor elements are provided with a fixed colour filter in a specific sequence. This way, the scanner only has to perform a single scan as well.
Scanners can be very different
Now we have explained the basic principle of scanners. But there are a lot of subtle differences between the scanners. Here you can read about the different types.
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