And 'one of the main advantages of CCD sensors. Indeed, it is the characteristic that makes our CCD sensor and a measuring instrument not just a simple "camera" digital.
Linearity means that there is a simple linear relationship between the input value (the electric charge collected in each photoelement) and the output value (the number assigned to each pixel that composes the final image).
There are two important limits defined by the elements of its CCD sensor input and output elements of the CCD camera: the first is the Full Well Capacity photoelements of making up the sensor while the second is the type of analog to digital converter (or ADC Analog to Digital Converter) used by the CCD camera.
E 'so easy to see that there are two levels of saturation : the first is the ability to collect individual photoelements electrons that make up the CCD, the second is given by the resolution (in bits) of the analog-digital converter.
To fix ideas, always consider the example of the CCD camera sensor with Apogee Ap7p SITE supplied Astronomical Observatory of Cavendish. The size of 24 microns square photoelements have a Full Well Capacity of about 300000 electrons as the analog to digital converter has a resolution of 16 bits, corresponding to 65535 ADU (2 ^ 16).
If we build a graph with the x-axis input data, the number of electrons contained in photoelement, and the ordinate the output value, the number of ADU of the corresponding pixel of the image, these two limits are represented by the saturation two blue lines, respectively (x = 300000) and red (y = 65535).
A CCD camera is not professional but good quality present a curve similar to the linear graph above where the green lines is precisely the section where the camera behaves in a linear fashion while the orange, which starts at around 250,000 photoelectrons captured by photoelement, is the non-linear stretch which is lost by the photometric quality of the chamber. We note that the same graph gives us a couple of notes: we know from analytic geometry that the equation of a line has an expression:
Y = mX + c where
m is the slope, or the slope of the line and c is the so-called known term, namely ntercetta with the y-axis. Well, the slope m (equal to the tangent of the angle that the line of linearity underlying the x-axis) is no more che l'inverso del gain, mentre c è l'offset della camera CCD (che nel caso della Apogee Ap7p è posto a circa 3080 ADU).
Notiamo immediatamente un'altra cosa importante: il tratto non lineare inizia prima che la curva di linearità raggiunga uno qualsiasi dei due livelli di saturazione: dunque l'operatore non ha nessuna avvertenza o segnalazione di quanto sta avvenendo. Per questo è importantissimo determinare in modo sperimentale le coordinate del punto P.
