CCD Point, Cluster, and Column Defects

Kodak assigns a cosmetics “class” to their CCD monochrome imaging detectors based on the number and type of “defects” found on the detector chip. Defects are CCD camera pixels that usually still function but respond to light slightly differently than their neighboring pixels (the surrounding 128 x 128 pixels or +/- 64 columns/rows). The “defects” look lighter or darker than the rest of the pixels that surround them. The amount of difference from its neighbors that classifies a pixel as defective depends on the model of the CCD detector or chip. Defects are of three types: point, cluster, or column.

Point defects: These can be classified as either “dark” or “bright” pixels. A “dark” pixel is one that is more than 6% dimmer than its neighboring pixels when illuminated to 70% of saturation. However, some chip models allow a difference of up to 20% before a pixel is considered dark. A “bright” pixel is one whose dark current exceeds some specified value. This value can range from >3000e/pixel/sec at 25° C up to >5000e/pixel/sec, depending on the chip model.

Cluster defect: This is a grouping of not more than 5 adjacent point defects.

Column defect: This is a grouping of >5 contiguous point defects along a single column, OR a column containing a pixel with dark current >6,000e/pixel/sec (up to >150,000e/pixel/sec with some chips), OR a column that does not meet the minimum vertical CCD charge capacity, OR a column which loses more than 250e under 2Ke illumination (more than 500e with some chips).

In most cases, defects can be removed by software when processing the image. For casual imaging purposes, any class of detector will generally be acceptable. However, the more defects you have to begin with, the more processing time will be required to remove them. For scientific work, however, defects can affect the accuracy of astrometric and photometric measurements, should the object of interest inadvertently fall on a defect. If you are interested in doing this type of work, the better the class of chip, the more accurate your results will be.

Some Kodak multi-megapixel imaging detectors are considered “single class” detectors and are not divided into classes based on the number of defects they might have. In addition, the defect specifications are a bit looser, and tested at 80% saturation. Dark pixels are classified as either major or minor, depending on their departure from the nominal response. Cluster defects can be a group of two to ten contiguous defective pixels, but with no more than two adjacent defects horizontally. Column defects are a group of more than ten contiguous major defective pixels along a single column. In all cases, the specs require that there be at least two non-defective pixels separating any two major defective pixels to facilitate the removal of the defective pixels by your software during image processing.

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