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Tools - Digital Cameras


Dealing with Thermal CCD Noise


We assume that the reason it took so long for manufacturers of consumer-grade digital still cameras to provide long exposure times and  bulb settings on their cameras is that the problem of thermal noise generated in the CCD was too great. Perhaps the manufacturers were trying to protect us from taking "bad" photos in much the same way that many automatic cameras refuse to take a picture or insist on using a flash when the ambient light is judged too low. While thermal noise is a problem that one must deal with, it can be dealt with successfully.


Thermal noise always exists within a CCD, and the problem increases the longer the sensor is active. Some digital camera manufacturers caution the photographer about using shutter durations greater than a second. Noise manifests itself in the image as white specks. The greater the exposure time, the greater the number of specks will be. An example of an image with much noise is provided below. The image hasn't been resized, but it has been cropped to show the area surrounding the subject of interest, the splash of the milk drop. The shutter was held open for several seconds.



The best strategy to minimize noise is simply to keep the shutter duration as brief as possible. A photo taken with the shutter duration under a second is shown below.



Once thermal noise manifests itself in an image, it can be removed by a couple of methods. One of them is simply to spot out the white specks using the cloning tool in an image-editing program. That method would be very time consuming with the noisy splash photo. A faster method is to use a noise filter that is set to remove spots smaller than a given radius. For the splash photo, the radius was set to 2 and the threshold to 10. The result is shown below.



A disadvantage of using a noise filter is that sharp edges and specular highlights may be softened. Note that this is what happened to the image above. Using an unsharp mask filter can restore some of the lost sharpness as shown below. 



Of course, one should keep in mind that repeated application of filters will cause some loss of information in the image. If the image is to serve as data for an experimental investigation, then editing of the image should be done with caution. The best approach, as already stated, is simply to use the shortest shutter durations possible.


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