Sensors

November 4th, 2010 Andrei

Inside our eyes there are cone-shaped cells, which are sensitive to some colors, called “primary colors”: red, green and blue. The other colors that we perceive are just combinations of these primary colors. In photography, the red, green and blue corespondents of light expose the corresponding chemical layers of color film. The Foveon sensors work as our eyes; they have three sensor layers that measure the primary colors, as shown in this diagram. If these color layers are combined, it will result a digital image, formed by a mosaic of pixels of uniform colors that are so tiny that it appears uniform and smooth. The only cameras that provide Foveon sensors are Sigma SD9 and SD10 digital SLRs.

The Current Color Filter Array Sensors

The other cameras have sensors that measure only the brightness of each pixel. A color filter array is positioned on top of the sensor to capture the red, green, and blue components of light falling onto it. This way only one primary color is measured by each pixel, while the other two colors are just estimated, based on the surrounding pixels. As a result of these approximations, the image sharpness is reduced. However, as the number of pixels in current sensors increases, the sharpness reduction becomes less visible. Also, as the technology develops, many refinements are made to increase photography quality.

Active Pixel Sensors (CMOS, JFET LBCAST) versus CCD Sensors

To understand the term of digital sensors we have to imagine an array of buckets collecting rainwater. Digital sensors work the same way buy they consist of an array of pixels collecting photons, the minute energy packets of which light consists. The light sensitive photodiode converts the number of photons collected in each pixel into an electrical charge. For a camera to be able to process the values into the final digital photography, the electrical charge is converted into a voltage, amplified and then converted again to a digital value, using analog to digital converter.

In CCD (Charge-Coupled Device) sensors, the pixel measurements are processed sequentially by circuitry surrounding the sensor; while in APS (Active Pixel Sensors) the pixel measurements are processed simultaneously by circuitry within the sensor pixels and on the sensors itself. Capturing images with CCD and APS sensors is similar to image generation on CRT and LCD monitors respectively

The most common type of APS is the CMOS (Complementary Metal Oxide Semiconductor) sensor. CMOS sensors were initially used in low-end cameras but recent improvements have made them more and more popular in high-end cameras such as the Canon EOS D60 and 10D. Moreover, CMOS sensors are faster, smaller, and cheaper because they are more integrated (which makes them also more power-efficient), and are manufactured in existing computer chip plants. The earlier mentioned Foveon sensors are also based on CMOS technology. Nikon’s new JFET LBCAST sensor is an APS using JFET (Junction Field Effect Transistor) instead of CMOS transistors.

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Pixel Quality

November 1st, 2010 Andrei

In digital photography, when we hear the expression “more megapixels” we tend to believe that “more is better”. But in this case “more” doesn’t necessary mean “better”. The photography quality depends on a multitude of factors, the number of pixels being just one of them. Each pixel value has a quality that can be described in terms of geometrical accuracy, dynamic range, color accuracy , noise and artifacts. Also, the quality of each pixel value depends on the number of photodetectors that were used to determine it, the level of sophistication of the in-camera imaging processing software, the quality of the lens and sensor combination, the photography file format used to store it, the size of the photodiode, the quality of the camera components. Each sensor and camera design has its compromises.

Geometrical Accuracy

The number of pixel locations on the sensors and the ability of the lens to match the sensors resolutions determines geometrical or spatial accuracy. The resolution topic explains how this is measured at this site. Interpolation will neither improve geometrical accuracy, nor will create what hasn’t been captured.

Color Accuracy

Conventional sensors, using a color filter array, have only one photodiode per pixel location. This means that each color channel has some missing pixels which are estimated based on demosaicing algorithms and which will determine some color inaccuracies around the edges. If we increase the number of pixel locations on the sensor, the result we’ll get is the reducing the visibility of these artifacts. Foveon sensors (will be explained in a later post) have three photodetectors per pixel location which allows them to create higher color accuracy by eliminating the demosaicing artifacts. Unfortunately this kind of technology is available only on few cameras and their sensitivities are currently lower than conventional sensors.

Dynamic Range

The size of a photodiode is very important for the dynamic range. This size is determined by the size of the pixel locations and the fill factor. Higher quality sensors are more accurate and will be able to output a larger dynamic range, which can be preserved when storing the pixel values into a RAW image file. In order to increase the dynamic range, some cameras use two photodiodes per pixel location. Each photodiode has an important role: the more sensitive one measures the shadows and the less sensitive one measures the highlights.

Noise

The pixel value consists of two components:

  • what you want to see (the actual measurement of the value in the scene)
  • what you do not want to see (noise).

A pixel has a better quality if the part you want to see is larger than the one you don’t want to see. The noise depends on the quality of the sensors and the size of its pixel locations. Also, the noise can be changed by increasing sensitivity.

Conclusion

Photography quality across different types of sensors and cameras can’t be compared because there isn’t just a standard objective. For instance, a 3 megapixel Foveon type sensor uses 9 million photodetectors in 3 million pixel locations. The resulting quality is higher than a 3 megapixel, but lower than a 9 megapixel conventional image, and it also depends on the ISO level you compare it at. Likewise, a 6 megapixel Fujifilm Super CCD image is based on measurements in 3 million pixel locations. The quality is higher than a 3 megapixel image, but lower than a 6 megapixel image. A 6 megapixel digital compact image will be of lower quality than a 6 megapixel digital SLR image with larger pixels. To determine an “equivalent” resolution is tricky at best.

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Pixels

October 28th, 2010 Andrei

To understand the term of digital sensors we have to imagine an array of buckets collecting rain water. Digital sensors work the same way buy they consist of an array of pixels collecting photons, the minute energy packets of which light consists. The light sensitive photodiode converts the number of photons collected in each pixel into an electrical charge. For a camera to be able to process the values into the final digital photography, the electrical charge is converted into a voltage, amplified and then converted again to a digital value, using analogue to digital converter. Digital compact cameras have substantially smaller sensors then a digital SLR, although they might have a similar pixel count. This fact determinants a change in the pixel size (smaller for digital compact cameras), the resulting photography having a lower quality.

To understand better, we have to compare a digital photography with a spreadsheet with rows and columns which stores the pixel values generated by the sensors. Pixels in a digital photography have no size until they are displayed on a monitor or printed. For instance, on a 4″ x 6″ print, each pixel in a 5 megapixel image would only measure 0.01mm, while on an 8″ x 10″ print, it will measure 0.05mm.

Array of pixels

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Micro lenses

October 25th, 2010 Andrei

Microlensses are usually placed on top of the color filter array on certain sensors of digital cameras, their principal role being to overcome the limitation of a low fill factor. Microlenses are also used in order to funnel the photons of a larger area into a smaller area of light sensitive photodiode.

Microlenses are small lenses with diameters less than a millimeter or as small as 10 micrometers. Although there are so small, they can give a good optical quality. But there are situations when unwanted effects arise, due to optical diffraction at the small features.  In designing a microlens we have to take into consideration the substrate supports, which is thicker than the lens.  Usually a microlens is made of a single element with one plane surface and one spherical convex surface to refract the light. But there are more sophisticated lenses which are made of aspherical surfaces, or, of several layers of optical material, in order to achieve their design performance.

Microlens

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Manual focus

October 21st, 2010 Andrei

There are two ways of focusing: the automatic one (built in your camera) and the manual one (by hands). When you use manual focus the automatic focus is disabled. Usually, manual focus is used when you have to deal with low light or macro/special effects while taking a picture. With some digital cameras, manually focus can be used only to a few present distances. Other digital cameras use the normal focus ring on the attached lens to focus, like in conventional photography.

Being often implemented on a fly-by-wire basis, the manual focus inputs to focus in or out are relayed to the autofocus system, changing the focus type.

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LCD

October 18th, 2010 Andrei

The LCD is used as a viewfinder by digital compact cameras. To be able to use it you need to capture a live video feed of the scene, which is provided by the LCD. The LCD’s normally measure between 1.5″ and 2.5″ diagonally, with typical resolutions between 120,000 and 240,000 pixels. But there are LCD’s that function at a higher level of performance. For example, some LCD’s have an anti-reflective coating and/or a reflective sheet behind the LCD, to allow for viewing in bright outdoor daylight. Other LCD’s can be flipped out of the body or angled up or down to make it easier to take low angle or high angle shots. There are cases when an electronic viewfinder supplements the main LCD. A smaller LCD (0.5″) is used to simulate the effect of TTL optical viewfinder. On Digital SLRs, LCD is used only to review images or change the camera settings. It doesn’t support live previews.

If our camera has an LCD screen we have an advantage, we can play back our images immediately after shooting. However, due to the fact that only about 120,000 to 240,000 pixels are used to represent several millions of pixels in the original digital image, the image might not be sufficiently sharp. To be able to determine if the image needs reshooting, further magnification is needed. But the magnification factor can be different per each model of digital camera, or can be missing. Some cameras allow basic editing functions such as rotating, resizing images, trimming video clips, etc. In playback mode you can also select an image from the thumbnail index.

Each digital camera with LCD has a set of buttons that can be use to change the camera settings. These buttons are also used to change adjust the brightness or the color settings of the LCD itself. Sometimes, the main LCD is supplemented by one or more monochrome LCDs, located on top or at the rear of the camera. These monochrome LCDs show the most important camera and exposure settings.

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Firmware

October 13th, 2010 Andrei

Each digital camera has a set of microprocessors which control camera’s sensor, buffer, LCD, autofocus, etc. These microprocessors are also controlled by “firmware”, which is software stored in the Read Only Memory (ROM) of the camera. In order to enhance performance and/or add new features, most of the cameras allow the firmware to be upgraded. The up grader process starts with downloading an installer from the camera manufacturer’s website and finishes with running it from a computer connected to the camera via a USB cable, or by running the installer from the memory card inside the camera.

When we talk about firmware upgrades, there are two situation that we must analyze. Sometimes, the firmware upgrades are major, adding raw support or adding features from a newer to an earlier model. Other times, the upgrade isn’t necessary because it merely adds support for a language that you do not speak, or adds a feature that you do not plan on using. However, if the firmware at the same time enhances general performance it is worthwhile upgrading, e.g.

Before performing an upgrade, always read the instructions very carefully, in advance, as the instructions vary depending on the model and make. You have to make sure that you use the correct firmware upgrade version. It is also very important to use a freshly charged set of batteries, because firmware upgrades use more power. If you run out of power during the upgrade, your camera may crash and not function anymore. You may even not be able to run the firmware upgrade again.

Firmware upgrade normally means a change for the better, but this isn’t always the case. Sometimes you may not be able to restore the camera to the earlier firmware version, even you want to do so.

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Fill Factor

October 9th, 2010 Andrei

Fill factor, in the context of solar cell technology, is defined as the ratio (given as percent) of the actual maximum obtainable power, (Vmp x Jmp) to the theoretical (not actually obtainable) power.

In the context of photography and cameras, the fill factor indicates the size of the light sensitive photodiode relative to the surface of the pixel. Each pixel has around it an extra electronics. That’s why fill factor tends to be quite small, especially for Active Pixel Sensors which have more per pixel circuitry. To overcome this limitation, often an array of microlenses is placed on top of the sensor.

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EXIF

October 5th, 2010 Andrei

A digital camera doesn’t store only information about the pixels of the image, it also stores information about the date and time a picture was taken, aperture, shutterspeed, ISO and many other camera settings. All these information, also called “metadata” are stored in a header. One of the most used type of header is EXIF (Exchangeable Image File) header. Japan Electronic Industry Development Association (JEIDA) created EXIF as a standard for storing information, to encourage interoperability between imaging devices. As you use EXIF you do not need to worry about remembering the settings you used when taking the image, EXIF will do that for you.

After taking a picture and transfer it to your computer, you can use an image editing and viewing program, which allows you to display or to edit the EXIF data. To make sure you don’t loose the EXIF data while saving a photo after editing you should always preserve your original image and use “Save As” after editing it.

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Effective Pixels

October 1st, 2010 Andrei

As we look at a digital image, we must understand that the number of pixels is not the same thing with the number of sensor pixels measurements that were used to produce that image. In conventional sensors, each pixel has one photodiode which corresponds with one pixel in the image. A conventional sensor in, for instance a 5 mega pixel camera which outputs 2,560 x 1,920 images has an equal number of “effective” pixels, 4.9 million to be precise. The effective area has some additional pixels that surrounds it, which are used for demosaicing the edge pixels, to determine “what black is”, etc. Not every time all the sensor pixels are used. For example Sony’s DSC-F505V which effectively used only 2.6 mega pixel (1,856 x 1,392) out of the 3.34 mega pixel available on the sensor. This phenomena is the consequence of trying to fit the then new 3.34 sensor into the body of the previous model. The lens couldn’t cover the whole sensor because the sensor was slightly larger. So the total number of pixels on the sensor is larger than the effective number of pixels used to create the output image. Most of the times this higher number is used to specify the resolution of the camera for marketing purposes.

Interpolated Number of Sensor Pixels

Usually, to the measurement in one pixel location corresponds each pixel in the image. For instance, a 5 megapixel image is based on 5 million pixel measurements, give and take the use of some pixels surrounding the effective area. But there are times when a camera with a 3 megapixel sensor, is able to create 6 megapixel images, for example. Shooting in JPEG mode offers you an advantage the images resulting being of better quality than those performed on your computer, because it is done before JPEG compression is applied. Enlarging JPEG images on your computer also makes the undesirable JPEG compression artifacts more visible. However the quality difference isn’t that obvious, and you are dealing with a slower 3 megapixel camera which fills up your memory cards twice as fast. The very same process takes place when you use your digital zoom. Details you didn’t capture can’t be created by interpolation.

Fujifilm’s Super CCD Sensors

Although most of the sensor pixels are square, Fujifilm’s Super CCD sensors have octagonal pixels. therefore, the distance between the centers of two octagonal pixels is smaller than the distance between two conventional square pixels, resulting in larger (better) pixels. However, the information has to be converted to a digital image with square pixels. So for a 4 x 4 area of 16 square pixels, only 8 octagonal pixel measurements were used: 2 red pixels, 2 blue pixels, and 4 green pixels (1 full, 4 half, and 4 quarter green pixels). In other words, 6 megapixel Super CCD images are based on the measurement by only 3 million effective pixels, similar to the above interpolated example, but with the advantage of larger pixels. In practice the resulting image quality is equivalent to about 4 megapixel. This leads to double the file size (leading to more storage and slower processing), while the quality improvement is equivalent to only 33% more pixels.

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