How Point Grey Tests for White Blemish Pixels
Technical Application Note TAN2008013
Revised May 22, 2013
Technical Application Note (TAN2008013): How Point Grey Tests for White Blemish Pixels
This technical application note explains how Point Grey Research identifies and corrects white blemish pixel defects; how to get white blemish pixel information about your camera; and what to do if you are concerned about blemish pixels from your camera.
The suite of quality assurance tests that Point Grey Research performs on its cameras includes specifications for identifying and correcting white blemish pixel defects that originate from the camera imager. White blemish pixels can occur randomly in any imager and are believed to be caused by natural cosmic radiation. In some cases, an incidence of radiation can damage a pixel and cause it to generate a permanently high charge. As a result, the pixel takes on a permanently lit, or ‘glowing,’ appearance. This damage generally occurs after the sensor is manufactured, particularly during shipping and handling. White blemish pixels are sometimes referred to as ‘hot,’ ‘burned’ or ‘bright’ pixels.
In the following image, a single white blemish pixel may be seen near the lower right corner (circled).
The pixel is magnified in the following image:
Testing and correcting white blemish pixels occurs during the unit test phase of camera production. This means that an array of blemish pixels is identified for each individual camera, and the mechanism to correct them is then coded into the camera firmware. Pixel correction, as such, becomes a part of the on-camera data flow for every image frame. The algorithm to correct blemish pixels involves applying the average color or grayscale values of neighboring pixels to the blemish pixel.
The following specifications apply to all cameras when testing for white blemish pixels:
Note that the maximum number of pixels corrected varies by camera model, depending on storage space and sensor size. While Point Grey aims to ship cameras with zero white blemish pixels, in some models the maximum number of pixels corrected may be less than the number allowed by the sensor manufacturer.
To determine if blemish pixel correction is enabled on your camera (the default setting is enabled), and the number of pixels that are being corrected, read the PIXEL_DEFECT_CTRL register 0x1A60h. For more information about how to work with this register, consult the Point Grey Digital Camera Register Reference available from the Downloads page.
You may still encounter issues with blemish pixels, in spite of any correction that is performed on-camera. Keep in mind that blemish pixels can occur spontaneously after they leave Point Grey Research, especially during shipping and handling.
To minimize the effects of any additional blemish pixels, be aware that higher operating temperatures, higher gain settings, and longer shutter times can all contribute to an increased effect of white blemish pixels.
If you remain concerned about white blemish pixel defects on your camera, contact email@example.com.