The big difference between traditional film cameras and digital cameras
is how they capture the image. Instead of film, digital cameras use a
solid-state device called an image sensor, usually a charge-couple device
(CCD). On the surface of each of these fingernail-sized silicon chips is a
grid containing hundreds of thousands or millions of photosensitive diodes
called photosites, photoelements, or pixels. Each photosite captures a
single pixel in the photograph to be.
The Exposure
When you press the shutter release button of a digital camera, a
metering cell measures the light coming through the lens and sets the
aperture and shutter speed for the correct exposure. When the shutter
opens briefly, each pixel on the image sensor records the brightness of
the light that falls on it by accumulating an electrical charge. The more
light that hits a pixel, the higher the charge it records. Pixels
capturing light from highlights in the scene will have high charges. Those
capturing light from shadows will have low charges.
When the shutter closes to end the exposure, the charge from each pixel
is measured and converted into a digital number. The series of numbers can
then be used to reconstruct the image by setting the color and brightness
of matching pixels on the screen or printed page.
It’s all black and white
after all
It may be surprising, but pixels on an image sensor can only capture
brightness, not color. They record only the gray scale-a series of 256
increasingly darker tones ranging from pure white to pure black. How the
camera creates a color image from the brightness recorded by each pixel is
an interesting story.
What is color?
When photography was first invented, it could only record black and
white images. The search for color was a long and arduous process, and a
lot of hand coloring went on in the interim (causing one photographer to
comment “so you have to know how to paint after all!”).
One major breakthrough was James Clerk Maxwell’s 1860 discovery that
color photographs could be created using black and white film and red,
blue, and green filters. He had the photographer Thomas Sutton photograph
a tartan ribbon three times, each time with a different color filter over
the lens. The three black and white images were then projected onto a
screen with three different projectors, each equipped with the same color
filter used to take the image being projected. When brought into register,
the three images formed a full color photograph. Over a century later,
image sensors work much the same way.
Colors in a photographic image are usually based on the three primary
colors red, green, and blue (RGB). This is called the additive color
system because when the three colors are combined or added in equal
quantities, they form white. This RGB system is used whenever light is
projected to form colors as it is on the display monitor (or in your eye).
From black and white to
color
Since daylight is made up of red, green, and blue light, placing red,
green, and blue filters over individual pixels on the image sensor can
create color images just as they did for Maxwell in 1860. In the popular
Bayer pattern used on many image sensors, there are twice as many green
filters as there are red or blue filters. That’s because a human eye is
more sensitive to green than it is to the other two colors so green’s
color accuracy is more important.
With the filters in place, each pixel can record only the brightness of
the light that matches its filter and passes through it while other colors
are blocked. For example, a pixel with a red filter knows only the
brightness of the red light that strikes it. To figure out what color each
pixel really is, a process called interpolation uses the colors of
neighboring pixels to calculate the two colors that the pixel didn’t
record directly. By combining these two interpolated colors with the color
measured by the site directly, the full color of the pixel can be
calculated. “I’m bright red and the green and blue pixels around me are
also bright so that must mean I’m really a white pixel.” It’s like a
painter creating a color by mixing varying amounts of other colors on his
palette. This step is computer intensive since comparisons with as many as
eight neighboring pixels is required to perform this process properly.
There’s a computer in your
camera
Each time you take a picture millions of calculations have to be made
in just a few seconds. It’s these calculations that make it possible for
the camera to preview, capture, compress, filter, store, transfer, and
display the image. All of these calculations are performed by a
microprocessor in the camera that’s similar to the one in your desktop
computer.
| How works a Digital Camera ?
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Digital cameras are very much like the still more familiar 35mm film cameras. Both contain a lens, an aperture, and a shutter. The lens brings light from the scene into focus inside the camera so it can expose an image. The aperture is a hole that can be made smaller or larger to control the amount of light entering the camera. The shutter is a device that can be opened or closed to control the length of time the light enters. |
| An image sensor sits against a background enlargement of its square pixels, each capable of capturing one pixel in the final image. Courtesy of IBM. |
| When the shutter opens, light strikes the image sensor to form the image. |
| The gray scale contains a range of 256 tones from pure white to pure black. |
| RGB uses additive colors. When all three are mixed in equal amounts, they form white. When red and green overlap, the form yellow, and so on. To see how this works, visit Konica’s interactive presentation by clicking the MoreInfo button below. |
| Colored filters cover each photosite on the image sensor so the photosites only capture the brightness of the light that passes through. The lenses on top of each pixel are used to collect light and make the sensor more sensitive. Courtesy of Fuji |
| Here the full color of a green pixel is about to be interpolated from the eight pixels that surround it. |