Sensor spectral sensitivities, noise measurements and color sensitivity
This article proposes new measurements for evaluating the image quality of a camera, particularly on the reproduction of colors. The concept of gamut is usually a topic of interest, but it is much more adapted to output devices than to capture devices (sensors). Moreover, it does not take other important characteristics of the camera into account, such as noise. On the contrary, color sensitivity is a global measurement relating the raw noise with the spectral sensitivities of the sensor. It provides an easy ranking of cameras. To have an in depth analysis of noise vs. color rendering, a concept of Gamut SNR is introduced, describing the set of colors achievable for a given SNR (Signal to Noise Ratio). This representation provides a convenient visualization of what part of the gamut is most affected by noise and can be useful for camera tuning as well.
Advances in Camera Phone Picture Quality
A unique digital postprocessing technique compensates for performance problems posed by ever-shrinking pixels
by Dr. Guichard Frédéric, DxO Labs
From Photonics Spectra , November 2007
As camera phones become ubiquitous, consumer demand for a photographic experience similar to that of traditional digital cameras is growing. Coupled with the ready availability of high-definition displays, this need has translated into a requirement for higher-resolution cameras in mobile phones. However, handset design aesthetics impose a much smaller form factor for the miniature camera modules built into hand-sets than can be accommodated by reusing the same technology found in digital still cameras.
One of the most challenging aspects of designing a high-resolution camera for a mobile phone is the limitation on the overall height of the camera, measured from the top of the lens to the back of the camera substrate. The typical target height is 6 mm or less, unless a more expensive folded-optics design is considered. Given the angular acceptance of CMOS image sensor pixels, the maximum-size sensor that can be used with such a thin camera measures approximately 4.5 mm diagonal. To increase the resolution without increasing the height of the camera (or thickness of the phone), more pixels must fit into the array defined by this diagonal size. Using a 2.2-× 2.2-µm-pixel size, 2-megapixel sensors can be used in these thin cameras. To achieve 3.2-megapixel resolution, 1.75 × 1.75-µm-pixel size must be used, and 5-megapixel resolution requires 1.4 × 1.4-µm pixel.