Acres of Diamonds
When Sony can't break the laws of physics, we bend them.
By The Sony Tech Guy | July 21, 2011
The diamond pixel pattern of Sony's ClearVid CMOS sensor is featured on small-chip camcorders including the HXR-NX5U and HVR-Z5U.
Physics can be a harsh taskmaster. It constantly forces image sensor designers to walk the line between high resolution (which requires smaller photosites for a given imager size) and high sensitivity, signal-to-noise ratio and exposure latitude (which all favor larger photosites). In small-format sensors, the challenges are especially severe. While Sony engineers must respect the laws of physics, creative designs can give us a little wiggle room. Sony's ClearVid design rotates the conventional pixel array by 45 degrees and employs diamond-shaped photosites to address the specific challenges of small-format sensors.
Challenge #1: The lens
While Moore's Law enables smaller and smaller image sensors, there is no Moore's Law for lenses. For example, achieving full 1080p resolution on a Super 35mm digital sensor requires a lens that resolves about 41 line pairs per mm. This is relatively easy. Move to the 2/3-inch type sensor common to television broadcast cameras and you'll need a 100 line pairs per mm. That's more challenging. Call it ironic, call it perverse, but the small, affordable 1/3-inch cameras need spectacular lens resolution-184 line pairs per mm-to achieve full 1080p. This performance is seldom achieved in the real world.
Challenge #2: Diffraction
A second stumbling block in small sensors is inherent in the wave nature of light. Diffraction causes light to bend as it passes an edge, such as the blades of a camera's iris. The diffraction becomes worse as the iris stops down and worse at longer wavelengths (the red end of the spectrum). What should appear on the image plane as a point becomes a blurry circle called the Airy Disk. Diffraction is much more likely to cause image blurring on a small sensor because the photosite squares are so much smaller in comparison to the Airy Disk size.
With diffraction, what should appear as a point of light (approximated at left) becomes a series of concentric rings like an archery target. Some 84% of the energy is concentrated in the bull's eye, called the Airy Disk (right). (The white circle, not present in nature, indicates the Airy Disk diameter.)
Airy Disk size versus photosite size for 1920 x1080 imagers. The effect of diffraction is more severe on the smaller photosites of the 1/3-inch sensor. Drawings after Sean T. McHugh, cambridgeincolour.com.
Here's another view of how diffraction softens the image, in this case reducing contrast, as measured by MTF.
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