Blooming suppression and Smear rejection with CCD machine vision cameras

The new Truesense Imaging (TSI) CCD sensors: the 1080p KAI-02170 Interline CCD image sensor, and the 4 megapixel KAI-04070 Interline CCD image sensor, were designed to provide superior image quality for the most demanding imaging applications.   The Adimec OPAL-2070 and OPAL-4070 CCD machine vision cameras are the latest members of the OPAL series designed around these sensors. The larger pixel size of the image sensor enables gains in light sensitivity, dynamic range, smear rejection, and blooming suppression for applications with difficult image capture conditions or precise image quality requirements.

So what are blooming and smear and when are they a concern…

The charge capacity of a CCD pixel is limited; when a pixel is full, the charge starts to leak into adjacent pixels this process is known as ‘blooming’.  Blooming is an effect where the charge developed on a pixel spills into adjacent pixels and corrupts the scene.  It typically occurs when there are very bright spots in the scene.

Full well capacity defines the amount of charge an individual pixel can hold before saturating and produces blooming. Full well capacity is dependent on the physical area of the pixel and is therefore higher with larger pixels.  Blooming affects the appearance of the image as shown below, and it diminishes the accuracy of the pixel data as information from one pixel is then present in adjacent pixels.

Example of blooming

Smear is generated directly or indirectly in the vertical shift registers (VCCD) in the interline transfer CCD (IL-CCD).  The VCCD is a light-shielded area of the image sensor used to transfer the charge off of the sensor.  Smear typically results from very bright spots in an image and is caused by either:

1. Stray electrons generated under the photodiode area (the light sensitive area) and diffused into the vertical shift registers
2. Stray photons which arrive in the vertical shift registers or
3. Scattered photons, which arrive in the vertical shift registers by multiple internal reflections.   

In modern IL-CCD the smear performance is mostly set by the effectiveness of the VCCD light shield.  As a note, the light attenuation of the light shield is a function of wavelength.  For long wavelengths (say > 700nm) the smear performance is much worse than specified. 

Example of Smear 

With the newest image sensors from TSI, one of the main differences is larger pixel size (7.4 um x 7.4 um), which allows for a larger full well capacity (compared to their previous generation sensors with 5.5 um x 5.5 um pixel size). The full well capacity is increased from 20,000 electrons to 44,000 electrons so there is less incidence of blooming.  This allows for full well (charge capacity) in combination with the same read noise gives higher dynamic range. Also the smear performance parameters are better for the new image sensors with further developments in image sensor design.  These image sensors and subsequent cameras are well suited for outdoor applications where there might be bright spots (sun, reflections off metal objects, etc.), or measurement (metrology) applications such as phase-shift interferometry with extremely high demands on pixel data accuracy.  The previous generation sensors and cameras are still ideal for many applications; because with machine vision “best” is subject to the image capture conditions, the measurement requirements, etc.

For more information on these cameras or how to select the best camera, please contact us or check out our free ePaper: