Blooming occurs when the charge in a pixel exceeds the saturation level and the charge starts to fill adjacent pixels. Typically CCD sensors are designed to allow easy vertical shifting of the charge, but potential barriers are created to reduce flow into horizontal pixels. Hence the excess charge will preferentially flow into the nearest vertical neighbours. Blooming therefore produces a characteristic vertical streak, e.g. see the image below right.
Blooming can be a nuisance when a strong signal can obscure data from a weak signal of interest especially on an image with a high dynamic range. Blooming is usually less of an issue in spectroscopy applications when the CCD is aligned to be in the same orientation as the spectrograph slit. Any excess charge is due to light from the same wavelength and the blooming only serves to effectively increase the system dynamic range.
Anti-blooming structures - pros and cons
Some sensors are designed with structures built into them to limit blooming - anti-blooming structures. Anti-blooming structures bleed off any excess charge before they can overflow the pixel and thereby stop blooming. However, anti-blooming structures can reduce the effective quantum efficiency and introduce non linearity into the sensor. Therefore, anti-blooming sensors are not recommended for applications requiring very low light or high accuracy measurements.
As an alternative to using anti-blooming sensors, an image can be acquired using accumulation mode. Accumulation mode allows successive scans of shorter exposures to be summed to achieve effectively an exposure which is longer by the number of accumulations acquired. If each of the accumulations has light just below the saturation point to be summed, the dynamic range of the accumulated signal is also increased by the number of accumulations.