The Color of Noise

2006.10.04

Noise is everywhere. It is in our pictures. It has always been there. It is even in other people's pictures. And we were OK with that. After all, it is the way film forms images, by exposing grains of different sizes. The resulting pattern of colored dots is the photograph. Some of the wold's best photographs appear quite noisy with large grains visible all over. Below is a 100 ppi crop from one of National Geographic's best images. With the film grain distinctly visible, it is a truly noisy picture, though that does not stop it from being a great photograph.

WIth digital photography, images are formed by dots called pixels. Each pixel can be one of millions of colors. At typical print sizes, pixel noise is smaller than standard 35mm film noise. Yet, many spend far more time thinking about digital camera noise than they did about film grain. Why does it seem so bothersome? It is because of its color.

Digital camera noise appears as variance in both color and intensity. Film noise, however, is mostly colorless. To our eye, colorless noise seems more natural. Interestingly, the human visual system is more sensitive to changes in intensity than it is to changes in color. Ironically, most digital camera sensors are intensity-only sensors. The exception are sensors designed by Foveon.

Since most digital camera sensors only measure intensity, noise formed in the sensor or in the data path from the sensor to the image processor is independent of color. The color of digital noise comes from the fact that a bayer filter is used to block out different colors from reaching different pixels. This setup is equivalent to coloring noise to match the bayer pattern. This is why digital images show noise as variations in color.

Except in RAW mode, digital cameras process the sensor data to reduce the appearance of noise. Knowing the difference between noise and detail is the difficulty. Evidence shows that camera makers make different compromises on how to process noise out. Since noise can be reduced at the expense of detail, processed image noise levels are not indicative of the quality of a digital camera sensor. There are even multiple cameras which use the same sensor part and show very different noise characteristics in their images.

• Digital Photography