In the case of the original front-side illuminated (FSI) sensor design, all the wiring and circuitry necessary for storing, amplifying, and transferring pixel values runs along the borders between each pixel. This means light has to travel through the gaps to reach the photodiode beneath.

When photons enter the photosite, they hit a light-sensitive semi-conductor diode, or photodiode, and are converted into an electrical current that directly corresponds to the intensity of the light detected.

In many cases, such as photographing on a smartphone, that is the end of the process. However, most mirrorless cameras have the ability to save images in RAW format, providing photographers with more options.

The electric field of a light wave vibrates perpendicularly to the direction of propagation as shown in Figure 1. Since the electric field is a vector quantity, it can be represented by an arrow that has both a magnitude (or length) and a direction of orientation. This orientation direction is the polarization of the light. There are basically three polarization states: linear, circular, and elliptical. These terms describe the path traced out by the tip of the electric field vector as it propagates in space. Figure 1 shows a snapshot in time of a linearly polarized wave. Although the electric field alternates direction (or sign), it stays confined to a single plane. Therefore, sitting at a fixed point in z as time passes, the arrow tip would oscillate up and down along a line. The angle (θ) of this line with respect to some reference set of axes completely specifies this linear polarization state. For circular polarization, the electric field vector tip forms a helix or corkscrew shape. For a fixed point in z, the vector would rotate in time, like the second hand on a watch. Circularly polarized light can be either left-handed or right-handed, depending on the clockwise or counterclockwise nature of the rotation. Elliptical polarization is the most general case of polarization. It is the same as circular polarization but with unequal major and minor axes (for circular polarization, these are equal).

For example, the X-Trans CMOS 5 HS stacked sensor found in FUJIFILM X-H2S enjoys four times the reading speed of its predecessor and 33 times the reading speed of the original X-Trans CMOS sensor featured in X-Pro1.

One way to prevent moiré is by adding an optical low-pass filter to the sensor. Another is to use a different color filter array.

With stacked sensors, these processing chips have been added to the back of the sensor, essentially creating a ‘stack’ of chips sandwiched together.

Common instances in which moiré can be seen are when photographing brick walls from a distance, fabrics, or display screens. If the pattern being photographed misaligns with the grid created by the color filter array, strange effects appear, as illustrated in Figure 3.

Polarizers are used to filter the input polarization, increase its purity, or separate orthogonal components of a linearly polarized beam. However, a polarizer cannot convert the polarization state of the input light into a different polarization state. For this type of modification, an optical component known as a waveplate or retarder is required. To understand its operation, it is important to know that any polarization state (not just linear) can be decomposed into orthogonal components. The difference between the polarization states then results from the phase difference between the orthogonal components. Linear polarization possesses components that are in-phase, i.e., no phase difference, but have different amplitudes depending on their angle. Circular and elliptical polarization components possess a phase difference of π/2 or a quarter of a wavelength (circular polarization has the same amplitudes for the different components while elliptical has different amplitudes). Consequently, in order to convert one polarization to another, the phase difference between the two components must be controlled. This can be accomplished by sending a polarized beam into a birefringent crystal such that the o-wave or e-wave each experience a different phase delay. The operation of a waveplate and a summary of how quarter and half waveplates convert one polarization state to another are shown in Figure 4. An important case of polarization conversion is shown on the right side of Figure 4. A half waveplate can rotate the angle of a linearly polarized beam to any other angle, which can be used for rotating a vertically polarized laser beam to obtain horizontal polarization. Furthermore, proper combinations of waveplates and polarizers can be used to form optical systems that allow for variable attenuation of a laser beam or for isolating a laser cavity from spurious reflections.

The door is now open for huge future advances, equipping CMOS sensors with capabilities that simply weren’t possible only a few years ago.

As covered above, a single pixel can only record a single value. But if you zoom into a digital image, each individual pixel can contain a mixture of colors, rather than just the red, green, or blue allowed by the color filter array.

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By stacking them in this way, the distance the pixel values have to travel is drastically reduced, resulting in much faster processing speeds.

An optical low-pass filter – also known as an anti-aliasing filter – is a filter placed in front of a camera sensor to slightly blur the fine details of the scene being exposed, thereby reducing its resolution to a level below that of the sensor.

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Digital cameras are everywhere – from high-end professional equipment used by the media to everyday smartphone cameras, webcams, and even doorbells. At the heart of every single one is a digital camera sensor, also known as an image sensor. Without this vital piece of technology, digital cameras as we know them today simply would not exist.

The reason there is a higher frequency of green filters is because the filter array has been designed to mimic the human eye’s higher sensitivity to green light.

But what are camera sensors and how do they work? We aim to outline the basics behind the most common type of camera sensor and explain how this ever-crucial technology has evolved.

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This was a major problem in the early days of digital photography when sensor resolutions were lower. However, with sensors now enjoying much higher resolutions, moiré is less common.

Polarizers rely on birefringent materials and, since the index of refraction is complex, these materials can exhibit a polarization-dependent absorption and refraction. The first polarizers were based on selective absorption of incident light and are usually denoted as dichroic polarizers. Typical materials used for this anisotropic absorption are stretched polymers or elongated silver crystals; their operation is shown in Figure 3. The strongly absorbing axis of the material is placed perpendicular to the desired output polarization such that the undesired polarization is strongly absorbed. A different type of polarizer is based on the anisotropic refractive indices of a birefringent crystal such as calcite. A birefringent crystal will produce an o-wave or e-wave depending on the axis of the crystal to which the polarization component is aligned. These waves experience different refractive indices and will possess different critical angles for TIR, resulting in one polarization component being reflected while the other is transmitted (see Figure 3). By placing two calcite prisms back-to-back to form a rectangular optic, the transmitted beam will follow the same direction as the incident beam. The gap between these prisms can either be air or an optically transparent cement, depending on whether a high damage threshold or large acceptance angle, respectively, is desired.

While there are a number of different types of camera sensor, by far the most prevalent is the complementary metal-oxide semiconductor (CMOS) sensor, which can be found inside the vast majority of modern digital cameras.

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As the name suggests, a RAW file contains the raw image data before any demosaicing has taken place. This allows photographers to demosaic images using external software such as Capture One.

You may have also noticed the inclusion of a color filter in Figure 1. The reason for this is that pixels detect light, not color, so a camera sensor by itself can only produce black & white images.

The image processor is able to read these digital signals collectively and translate them into an image, because each pixel is assigned an individual value, depending on the intensity of light it was exposed to.

Until the introduction of the stacked sensor, CMOS sensors operated on a single layer. This meant the signal readouts from each pixel had to travel along strips of wiring all the way to the outside of the sensor before they were processed.

Different types of software use distinct demosaicing algorithms, each offering unique aesthetics. An obvious advantage of this is that photographers can choose their personal preference, but the benefits of creating in RAW format extend much further.

Sensor resolutions have risen dramatically since the 16-megapixel X-Trans CMOS sensor in X-Pro1, making it less likely for moiré to occur. As a result, optical low-pass filters have all but disappeared – though increased image sharpness is not the only potential advantage of the X-Trans color filter array.

Figure 5: Cross section of a front-side illuminated vs back-side illuminated CMOS sensor. For illustrative purposes only.

A polarizer is an optical component whose transmission depends strongly on the incident polarization of the light. Polarizers typically filter linear polarization, so an ideal polarizer would transmit 100% of one polarization component while rejecting all of the orthogonal component (see Figure 3). In practice, a portion of the undesired polarization will be transmitted. The transmittances of the target polarization and the undesired polarization through the polarizer are measured (by simply rotating the polarizer by 90 degrees) and the extinction ratio is defined as the ratio of these transmittances. The difference between a polarizer and a Brewster plate is that the former results in strong polarization-dependent transmission while the latter does not (only the reflection is highly polarized).

Using a less uniform pattern helps reduce moiré, eliminating the requirement for an optical low-pass filter and in turn creating sharper images.

Precise control of polarization behavior is necessary to obtain optimal performance from optical components and systems. Characteristics such as reflectivity, insertion loss, and beamsplitter ratios will be different for different polarizations. Polarization is also important because it can be used to transmit signals and make sensitive measurements. Even though the light intensity may be constant, valuable information can be conveyed in the polarization state of an optical beam. Deciphering its polarization can reveal how the beam has been modified by numerous material interactions (magnetic, chemical, mechanical, etc.). Sensors and measurement equipment can be designed to operate on such polarization changes. For these reasons, optical components capable of filtering, modifying, and characterizing a light source's polarization are valuable. Such polarization control can be accomplished by exploiting the reflection, absorption, and transmission properties of materials used in these components. The physical phenomena that enable polarization control, as well as the key components that exploit them, are discussed below.

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The Bayer filter array (see Figure 2) is made up of a repeating 2×2 pattern in which each set of four pixels consists of two green, one red, and one blue pixel. This equates to an overall split of 50% green, 25% red, and 25% blue.

To minimize the amount of light bouncing off this circuitry, a microlens is placed on the top of each pixel to direct the light into the photodiode and maximize the number of photons gathered.

During the compression process, a large amount of tonal and color information read by the sensor is lost. Less information means lower quality and, in turn, restricted freedom to edit.

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By removing the obstruction caused by the circuitry, a greater surface area can be exposed to light, allowing the sensor to gather more photons and subsequently maximize its efficiency.

A color filter array is a pattern of individual red, green, and blue color filters arranged in a grid – one for every pixel. These filters sit on top of the photosites and ensure that each individual pixel is exposed to only red, green, or blue light.

Incoherent light sources such as lamps, LEDs, or the sun typically emit unpolarized light, which is a random superposition of all possible polarization states. On the other hand, the output light from a laser is typically highly polarized, that is, it consists almost entirely of one linear polarization. Analyzing laser polarization is easier if it is decomposed into two linear components in orthogonal directions. In this way, depicting the polarization can be done using the standard symbols shown in Figure 1. The upper part of the table lists the symbols generally used for unpolarized, vertically polarized, and horizontally polarized light. For the graphic shown in the figure, the vertical direction would be along the y-axis while the horizontal direction would lie along the x-axis. When a plane of incidence is specified (see lower part of table in Figure 1), the polarization components acquire specific designations. S-polarization refers to the component perpendicular to the plane while P-polarization refers to the component in the plane. Examples of the depictions of linearly polarized light are illustrated in the remaining figures of the section.

This signal is amplified on-pixel, then sent to an analog-to-digital converter (ADC), which converts it into digital format and sends it to an image processor.

Learn more by exploring the rest of our Fundamentals of Photography series, or browse all the content on Exposure Center for education, inspiration, and insight from the world of photography.

Made up of approximately 55% green, 22.5% red, and 22.5% blue filters, it creates similar proportions of red, green, and blue pixels as the Bayer array. But it uses a more complicated 6×6 arrangement, comprised of differing 3×3 patterns.

Like any technology, camera sensors have come a long way in the past decade alone, and look to continue this development into the future.

As you can see in Figure 1, because the conversion and amplification processes happen on-pixel, the transistors, wiring, and circuitry have to be included in the spaces between each photosite.

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With the move to back-side illumination enabling much higher resolutions and stacked sensors increasing readout speeds so significantly, recent developments amount to nothing short of a revolution in CMOS camera sensor technology.

As a result, RAW files contain a wider dynamic range and broader color spectrum, which allows for more effective exposure correction and color adjustments.

While the basic operation of the CMOS sensor has remained fundamentally the same throughout its history, its design has evolved to maximize efficiency and speed.

File types such as JPEG and HEIF are designed to make image files easily portable, so significant compression takes place to achieve the smallest possible file sizes.

At the most basic level, a camera sensor is a solid-state device that absorbs particles of light (photons) through millions of light-sensitive pixels and converts them into electrical signals. These electrical signals are then interpreted by a computer chip, which uses them to produce a digital image.

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This is done automatically by the camera’s built-in processor, which then turns it into a viewable image file format such as JPEG or HEIF.

Additionally, the less uniform pattern is closer to the random arrangement of silver particles on analog photographic film, which contributes to Fujifilm’s much-loved film-like look.

A CMOS sensor is made up of a grid of millions of tiny pixels. Each pixel is an individual photosite, often called a well (see Figure 1).

The answer is a process called demosaicing, in which a demosaicing algorithm predicts the missing color values for an individual pixel based on the strength of the color recorded by the pixels that surround it.

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What’s more, without the problem of obstructing light entering the sensor, it’s possible to keep stacking additional chips, offering huge potential for future developments.

Although the effects of the filter are so slight that they are invisible to many everyday photographers, blurring inevitably equates to a reduction in sharpness. This is undesirable for many professionals, and is one of the reasons Fujifilm developed the X-Trans color filter array.

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The way in which polarized light interacts with an optical material can enable selective filtering of the polarization or conversion of the incident polarization state to a different one. This polarization control relies on a material's optical properties to respond differently depending on the polarization of the incident light. A material that exhibits birefringence, or different refractive indices for different input polarizations, is said to be anisotropic. This anisotropy affects the transmission and absorption properties of light and is the primary mechanism used in polarizers and waveplates as discussed below. However, even isotropic materials (same index for different polarizations) can enable polarization selection via reflection. The Fresnel equations describe the change in reflectivity as a function of angle of incidence. For a linearly polarized beam, both S- and P-polarizations exhibit different changes in reflectivity versus incident angle. There is an incident angle known as Brewster's angle (θB) at which P-polarized light is transmitted without loss, or exhibits zero reflectance, while S-polarized light is partially reflected. This angle can be determined from Snell's law to be θB = arctan(n2/n1). Figure 2 shows this response when light is incident from air onto a dielectric material where θB ≈ 56°. This polarization-selective reflectivity is exploited in laser cavities to produce strongly polarized light and for fine tuning of the output laser wavelength.

As its name suggests, the back-side illuminated (BSI) sensor flips this original design around so the light is now gathered from what was its back side, where there is no circuitry.

Every vertical and horizontal line in an X-Trans CMOS sensor includes a combination of red, green, and blue pixels, while every diagonal line includes at least one green pixel. This helps the sensor reproduce the most accurate color.