The purpose of (saturation-based) ISO sensitivity measurement is to measure the exposure necessary to reach a given sensor's saturation point.
To measure the camera sensor’s ISO sensitivity, we set up the camera body alone (without a lens) on a stand to receive light from a controlled source. The source is positioned far enough away from the camera sensor to ensure good light uniformity on the sensor plane. We then precisely measure the illuminance received by the sensor with a certified lux-meter.
The measurement protocol is modified if the camera has non-removable optics (as is the case with most DSC and bridges). To minimize the influence of the optics on ISO sensitivity and to avoid vignetting, we perform measurements on the image center. We set up the camera on a tripod to receive the light from the noise target with the center patch open. (The center patch luminance is measured with a certified luminance-meter.) We calculate ISO sensitivity as described here.
We obtain the sensor exposure by multiplying the illuminance by the exposure time. As there might be a slight difference between the exposure time announced by the manufacturer and the one used to take the shot, we measure the sensor integration time with a specifically-designed device consisting of a set of LEDs that are lit in sequence at a known, precisely-timed frequency. To perform the measurement, we count the number of lit LEDs on an exposure taken with the camera.
Read more about ISO sensitivity.