IMAGE CAPTURE SYSTEM

Not another monochrome sensor with color filters

The image sensor in almost all digital cameras, with the exception of Sigma's, is a color filter array (CFA) sensor. The image sensor itself is monochrome; it detects light intensity, but not color. The CFA overlaying the sensor's light-sensitive photodiodes is a mosaic of red, green and blue (RGB) filters in a checkerboard-like grid. Therefore, each photosite, corresponding to a single pixel, receives just one of the three primary colors.

In this kind of system, a 15MP CFA sensor allots 7,500,000 pixels to green light and 3,750,000 each to red and blue light, respectively. Left as is, this checkerboard pattern would create a strange image, so a process called color interpolation is used to blend in neighboring pixel color information. For example, a green pixel gets color information from adjacent blue and red pixels, and so on.

CFA sensor relies on eye's sensitivity to green

Why does a CFA sensor assign twice as many pixels to green as to red or blue? Because the eye is most sensitive to green light (figure 1). Human vision depends mostly on wavelengths in the green range to perceive fine detail and luminance resolution.

The four charts on figure 2 all have the same contrast. Notice how the green background lets you detect fine detail more easily. Most digital cameras take advantage of this fact of human vision. By capturing relatively more green information, they can get by with much less blue and red information. Thanks to this clever solution, a mere monochrome image sensor can be used to deliver color images of high quality.

So, what's the problem? Though it is true that our eyes are most sensitive to green luminance (sharpness and contrast) information, we also see blue and red chrominance (color) information. But the unequal treatment afforded these colors by a CFA sensor causes a disparity between the luminance resolution and chrominance resolution of the photographic images it generates.

Unique, innovative direct image sensor

Unlike CFA sensors, the image sensor in Sigma digital cameras uses three layers of photodiodes to gather the entire red, green and blue color information of light, forming the world's one and only full color capture system. Since introducing its first-generation digital camera in 2002, Sigma has maintained an unwavering commitment to this unique technology.

The image sensor is made of silicon. When struck by light, silicon absorbs shorter wavelengths (blue) near its surface and longer wavelengths (green, then red) at deeper levels. The direct image sensor takes advantage of this fact, using three layers of photodiodes to capture all colors of light at each pixel location.

All pixels capture full RGB color

This means that a direct image sensor having 15,000,000 pixel locations is able to capture full red, green and blue color information, as is, at each pixel location. In other words, all 15,000,000 pixel locations can respond to all three primary RGB colors transmitted by the lens. There is no need to assign red, green and blue to separate pixel locations, nor is it necessary to fabricate or eliminate color information during image processing. This full color capture system is, in principle, capable of providing equally high luminance resolution and chrominance resolution.

Since typical photographic subjects do contain an enormous amount of color information, Sigma is convinced that removing any discrepancy between luminance resolution and chrominance resolution is an important key to photographic naturalism.

Unencumbered by a low-pass filter

Another thing: color filter arrays are prone to false color patterns (moire). This problem is caused by interference between repeating patterns of fine lines in the subject and the mosaic pattern of the color filter array itself. Moire is typically prevented by installing a low-pass filter in front of the image sensor.

A low-pass filter works by removing the higher frequencies of light that carry fine detail. This does prevent interference and, therefore moire. But it also adversely affects resolution and image sharpness. Facing this tradeoff, some cameras use CFA sensors without a low-pass filter, accepting the inevitable color moire patterns as the price to pay for a sharper image.

Sigma's direct image sensor design doesn't use color filters, so it doesn't need a low-pass filter to prevent the false colors of moire. The direct image sensor captures the sharp image formed by the lens without losing any information. As a leading lens maker with uncompromising standards of photographic image quality, Sigma believes the ideal capture system is one that receives full color information at each and every pixel location.

Pure color and light: nothing added, nothing lost

In CFA sensor-equipped digital cameras, only one color is captured per pixel location. Color interpolation is required to generate a full color image from these single-color pixels. Over the past decades, color interpolation algorithms have greatly improved, so that interpolation errors are rarely a cause for concern in today's digital cameras.

Nevertheless, light information lost in the capture process can never be perfectly reconstructed. This standard approach cannot fulfill the promise of truly natural images. In sharp contrast, Sigma's direct image sensor has no use for color interpolation and doesn't suffer from its associated problems.

Sigma's technology generates naturally faithful color images without needing to add information. This is why the ambience of the scene is so vividly captured in photos taken with a Sigma digital camera. This reflects Sigma's policy of protecting the integrity of image information — keeping the image pure.

SIGMA SD1 Special Contents