Camera Sensors

Pixel pitch (Pixel Size) =  (sensor width in mm / sensor resolution width in pixels) * 1000

CCD : Charge-Coupled Device

CMOS : Complementary Metal-Oxide-Semiconductor - faster than CCD

FSI : Front Illuminated, matrix wiring in front of photocathode layer reduces available light

BSI : Backside Illuminated, matrix wiring behind photocathode layer increasing available light

OLP : Optical Low Pass Filter, hereafter called the Anti-Aliasing Filter, makes an image less sharp - on purpose.

           The pixels on a sensor "sample" an image they are shown when the shutter is opened.  In an image with a strong repetitive pattern, the sensor can misinterpret the pattern, which can result in what is called aliasing (Moiré).
           The solution to this aliasing is to put a filter in front of the sensor that just slightly blurs the image, allowing the pixels to detect the pattern correctly without the aliasing.

           Due to the nature of this filter, the image appears less sharp than it does without it.

Sensors

There are two main types of digital camera sensors : charge-coupled device (CCD) and complementary metal-oxide semiconductor (CMOS). Both technologies use a photoactive capacitor array to capture an image, where the charge accumulated in a given capacitor is proportional to the intensity of light at that location. In order to capture color information, the light must be filtered before it reaches the individual pixels. This is generally done with a color filter array placed over the sensors pixel array.

CMOS uses less power, allows for smaller packages, and is capable of faster readouts, enabling features like 60fps and slow-motion video.
The disadvantage of CMOS over CCD is that the circuitry takes up valuable real estate on the surface of the sensor. This means that for a given sensor size, a CCD will capture more light and offer better low-light performance than a CMOS Sensor.

FSI vs BSI Sensors : Front Side illuminated vs Back Side illuminated.

In FSI, photo diodes are on the bottom in the active silicon layer.  Light passes down through the transparent oxide layer and into the diode, exciting it.  This scheme works well for big pixels, but as sensors began to shrink to improve image resolution, camera designers ran into trouble getting enough light to the diode.
The FSI sensor suffers two problems, first, it can't detect low levels of light due to decreased quantum efficiency (QE).  second, it suffers from crosstalk, which blurs the image by causing the color in one pixel to run into adjacent pixels.
 
BSI flips the sensor, putting the semiconductor layer below the diode.  This reduces the path from lens to diode, eliminating much of the issues seen with FSI. BSI sensors gather more light and produce better quality images.

Stacked Sensor : The next-generation BSI sensor placing the BSI pixel array layer onto a signal processing chip serving as a supporting substrate for the thin BSI array layer:

3-Layer Stacked (Exmor RS) CMOS Image Sensor

The CMOS image sensor (CIS) consists of 3 Layers :
- 90nm generation back-illuminated CIS top chip
- 30nm generation DRAM middle chip
- 40nm generation image signal processor (ISP) bottom chip).

ExmorRS sensor features a RAM chip sandwiched in between the sensor and control circuitry layers which serves as a large and fast buffer to where the sensor can temporarily offload a significant amount of captured data before transferring it to the phone's internal memory for processing. This enables the camera to record super slow-motion videos, as well as stills Predictive Capture. When it detects fast-paced movement, the camera automatically captures a maximum of 4 photos before the shutter button is pressed, and lets the user select the best one afterwards.

Microlens arrays (On Chip Lens, microlenticular arrays or lenslet arrays) are used to increase the optical fill factor in sensors. These tiny lens systems serve to focus and concentrate light onto the photodiode surface instead of allowing it to fall on non-photosensitive areas of the device, where it is lost from the imaging information collected by the sensor.

Color Filter Arrays

An economical and practical way to record the primary colors is to permanently place a filter called a color filter array over each individual photosite. By breaking up the sensor into a variety of red, blue and green pixels, it is possible to get enough information in the general vicinity of each sensor to make very accurate guesses about the true color at that location. This process of looking at the other pixels in the neighborhood of a sensor and making an educated guess is called interpolation.

Interpolation :

The advantages of this method are that only one sensor is required, and all the color information (red, green and blue) is recorded at the same moment. That means the camera can be smaller, cheaper, and useful in a wider variety of situations. The raw output from a sensor with a color filter array is a mosaic of red, green and blue pixels of different intensity.

Demosaicing :
Demosaicing can be performed in different ways. Simple methods interpolate the color value of the pixels of the same color in the neighborhood. For example once the chip has been exposed to an image, each pixel can be read. A pixel with a green filter provides an exact measurement of the green component. The red and blue components for this pixel are obtained from the neighbors. For a green pixel, two red neighbors can be interpolated to yield the red value, also two blue pixels can be interpolated to yield the blue value.

This simple approach works well in areas with constant color or smooth gradients, but it can cause artifacts such as color bleeding in areas where there are abrupt changes in color or brightness especially noticeable along sharp edges in the image. Because of this, other demosaicing methods attempt to identify high-contrast edges and only interpolate along these edges, but not across them.

Other algorithms are based on the assumption that the color of an area in the image is relatively constant even under changing light conditions, so that the color channels are highly correlated with each other. Therefore, the green channel is interpolated at first then the red and afterwards the blue channel, so that the color ratio red-green respective blue-green are constant. There are other methods that make different assumptions about the image content and starting from this attempt to calculate the missing color values.

Digital cameras use specialized demosaicing algorithms to convert this mosaic into an equally sized mosaic of true colors. The key is that each colored pixel can be used more than once. The true color of a single pixel can be determined by averaging the values from the closest surrounding pixels.

Bayer Filter:

A Bayer filter mosaic is a color filter array (CFA) for arranging RGB color filters on a square grid of photosensors.
The most common pattern of filters used currently is the Bayer filter pattern.

This pattern alternates a row of red and green filters with a row of blue and green filters. The pixels are not evenly divided -- there are as many green pixels as there are blue and red combined.

This is because the human eye is not equally sensitive to all three colors. It's necessary to include more information from the green pixels in order to create an image that the eye will perceive as a "true color."

The repeating 2×2 grid used in the bayer filter

RGBW Bayer Filter 2x2 pattern size

X-Trans filter :

The X-Trans CMOS's 6 x 6 pixel pattern also ensures that colour reproduction is better than that achieved with an ordinary Bayer array.

The X-Trans filter is claimed to provide better resistance to color moiré than the Bayer filter, and as such they can be made without an anti-aliasing filter.

This in turn allows cameras using the sensor to achieve a higher resolution with the same megapixel count. The arrangement of these pixels is also said to provide grain more like film.

The "X-Trans" CMOS sensor uses an irregular pattern of pixels (similar to that found onSilver halidefilm) in order to reduce moiré without the need for an AA filter.
This same irregular pattern ensures that all horizontal and vertical lines of pixels contain at least one R, G and B pixel whereas Bayer array sensors do not have R and B in some lines resulting in false colour reproduction."
In practical terms - the removal o650f the AA filter (which is a filter, another thing between the light and a sensor) means you get "more photons" available to hit the sensor, meaning you get more light available in your shots.

Old analogue film didn’t get moiré because the crystals in film and photo paper aren’t even in size and placement.  X-Trans is an attempt to simulate the randomness of film and reduce moiré. The problem with Bayer is it’s too even and is suceptible to moiré.

The repeating 6×6 grid used in the x-trans filter

Foveon X3 sensor :

The Foveon X3 sensor is an image sensor for digital cameras, designed by Foveon, Inc. (now part of Sigma Corporation) and manufactured by Dongbu Electronics.

It uses an array of photosites, each of which consists of three vertically stacked photodiodes, organized in a two-dimensional grid. Each of the three stacked photodiodes responds to different wavelengths of light; that is, each has a different spectral sensitivity curve.

This difference is because different wavelengths of light penetrate silicon to different depths. The signals from the three photodiodes are then processed, resulting in data that provides the amounts of three additive primary colors, red, green, and blue.

No OLPF is needed as in the Foveon Sensor, there are three layers, one for each color, thereby inherently preventing the problems that the OPLF is designed to fix.

Silver Halide film

Moiré is not an issue with traditional film due to the irregular size and layout of the particles.
 

The irregular size and layout of film


High Frequency Dust Filter :

Most systems use a piezo crystal ultrasonic vibration of the IR filter in front of the sensor. They vibrate at around 35-50K Hz. Olympus invented it, but now Leica, Panasonic, Canon, and Nikon use similar systems.

Other manufacturers use sensor shifting. The sensor itself is shaken at about 100 Hz, but the length of travel is much further. Konica Minolta developed it. Sony and Pentax now use this method.

Both systems usually include a coating that is negatively charged, just as most dust is. This causes them to repel each other.

Dust shaken off the sensor adheers to a sticky surface below the sensor.

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