Digital Camera basics - Part 3: Capturing images
November 13, 2007
Digital cameras use different methods to capture the image depending on the type of image sensors and colour filters. As the image sensors are only capable of recording the intensity of light (i.e. brightness or darkness), digital cameras use colour filters to produce a full colour image. Thus, digital cameras use the combination of sensors and filters in different ways to record the image.
Capturing the brightness of image
What happens, when the shutter opens (though in fractions of a second) and light enters through the aperture? The light falls on CCD sensor whose pixels (in fact they are light sensitive diodes) convert the light into electrically charged particles. Each pixel, depending upon the brightness of the light that falls on it, becomes charged with different magnitude. Brighter the light highly electrically charged it becomes. In other words, pixels that record the brighter portion of the image are highly charged while the pixels that record the darker areas of the image will be less electrically charged. Thus, the image is wholly converted into electrons of various magnitudes on a CCD sensor.
Capturing the colours of image
The image sensor in a digital camera records only the brightness of the image in a series of various intensities. To produce a wide range of colours, the digital camera uses filters when the sensor is exposed to the light (image). The principle behind the colours dates back to the late nineteenth century, when James Maxwell discovered that a full colour photograph can be formed by using three filters of primary colours – red, green and blue. To recreate the natural colour tones of the image, all one has to do is to capture the image using these colour filters in quick succession and put them together on a screen or a print. Digital camera uses the same principle to capture colours, however, it differs from a conventional camera by the way it captures the image.
Single exposure of image sensor to the subject
Some digital cameras use single-shot method, in other words, the sensor is exposed to the image only once. To produce colours, two mehods are used – one way is to use three individual sensors of red, green and red and another way is to use a single sensor with a Bayer filter which has all the primary colours (RGB).
The first method uses beam splitter which splits the light and makes it pass through image sensors of the primary colours (RGB). The Image captured using this method usually will have more vivid, very precise colours and some of the high-end cameras use this type of image capturing. However, these cameras usually come in bulky models and with very high price tags.
The second method is better suited to compact digital cameras. This method uses a single sensor and a colour filter which is placed over it. This colour filter (mostly Bayer filter) helps to produce all natural colours that make up the image. The Bayer filter mosaic is a colour filter array (CFA) made up of red, blue, and green filters each sitting on every pixel or photosite of the sensor. CFA will usually have more green elements as half of the array is made up of it. Red and blue will constitute 25% each. According to this Bayer filtering method, each pixel will represent only one colour of the primary colours (i.e. RGB) and all the pixels need to be interpolated to produce the complete colour details of the image. For interpolation or deducing the complete colour information of each pixel and to produce the full image, a set of demosaicing algorithms are used by a digital camera. The raw data provided by the Bayer filters produce a jpeg or Tiff file depending on the type of demosaicing algorithm used.
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While filter mosaic refers to the filter pattern (arrangement of colour filters on the sensor), demosaicing refers to the technique to deciper or interpolate or guess colour values for unknown areas on the sensor. The Bayer colour filter which is commonly used provides insufficient colour data so demosaicing algorithms are needed to produce a complete image by using a few sampling methods which involve nearby pixels and thus filling up the rest of the colour information. |
Multiple exposures of image sensor to the subject
In some digital cameras, the sensor is exposed to the subject (image) three or more times in succession. Most multi-shot cameras use a single sensor with RGB filters which move over the sensor to get the colour values of each pixel so that the image with true colours can be captured. In another variation, a CCD sensor and a Bayer colour move on the plane where the image is focused to produce sharper picture quality. Apart from these two methods, some digital methods use a hybrid of the two methods, however, the Bayer filter is generally not used.
Scanning technique
This is another method used by digital cameras to capture the image. Here, the sensor scans over the plane where the image is focused. Most sensors used in this type of cameras are either linear or tri-linear which scan a single or three arrays each representing a colour of RGB. Some digital cameras use advanced scanning technique and the camera is rotated while scanning over the focal plane. These cameras usually much more picture details (resolution) than other cameras.
Kodak Linear CCD sensor
When comparing these different techniques of image capturing, multi-shot (multiple exposure of the sensor) and scanning methods offer much better quality in terms of resolution and colour information than a simple single shot technique. Hence, most high-end cameras used for professional use adopt multi-shot and scanning technique while a single shot technique produces decent image quality for amateur use.
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