Una ulteriore definizione di read noise da
http://theory.uchicago.edu/~ejm/pix/20d ... #readnoise:
Sensor read noise: Photons collected by the sensels (the photosensitive part of a pixel) stimulate the emission of electrons, one for each captured photon. After the exposure, the accumulated photo-electrons are converted to a voltage in proportion to their number; this voltage is then amplified by an amount proportional to the ISO gain set in the camera, and digitized in an analog-to-digital converter (ADC). The digital numbers representing the photon counts for all the pixels constitute the RAW data for the image (raw units are sometimes called analog-to-digital units ADU, or data numbers DN). We'll call this digital number the raw value of the pixel.
In an ideal world, the digital number recorded in the raw data would be directly proportional to the photon count. The constant of proportionality between the number of photons and the corresponding raw value is conventionally and somewhat inaptly called the gain of the sensor (it is not really a gain, rather it is a conversion factor between photo-electron counts and raw values). Since each doubling of ISO doubles the raw value for a given exposure, the gain is inversely proportional to ISO -- doubling the ISO cuts the gain in half, because the same number of photons captured are converted to a raw value twice as big when the ISO is doubled.
In the real world, the raw level does not precisely reflect the photon count. Each electronic circuit component in the signal processing chain -- from sensel readout, to ISO gain, to digitization -- suffers voltage fluctuations that contribute to a deviation of the raw value from the ideal value proportional to the photon count. The fluctuations in the raw value due to the signal processing electronics constitute the read noise of the sensor.
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