Before a micro image sensor can
be used, it must be charged. The
camera’s main processing chip
sends a small, positive electrical
charge to the bottom level of the
N-layer of silicon in each of the
sensor’s photodiodes. This
ensures that the charge on each
of the pixels is uniform so that the
light intensities recorded by them
will be in sync.On some image sensors, light next hits a layer called a hot mirror. It lets visible light
pass but reflects light from the hot, invisible infrared part of the spectrum—just past the
red light band—to heat sinks that draw the energy from the infrared away from the
photodiodes. The diodes are also sensitive to infrared light, but because infrared is
invisible, it would only distort readings of visible light.3
The first things light strikes after passing through the
camera’s lens, diaphragm, and open shutter are mil-
lions of tiny micro lenses, one over each of the photodi-
odes on the image sensor. These lenses don’t attempt to
focus the image. That job has already been done by the
camera’s lens. The micro lenses capture the light that
otherwise would fall into dead areas between the pho-
todiodes and direct it toward the light-sensing areas.2
1
How a Microchip Catches a Picture
Here we are, finally, at the point that separates the digital from the analog. The microchip imaging sensor that takes the place of film is
the heart of digital photography. If you’ve been reading since the beginning of the book, you already have an idea of the workings of
those microscopic devices that make up an image sensor: pixels. It’s a term that’s often used loosely but is short for picture elements.
Basically, it’s a supercharged version of the photodiode we saw at the core of light meters in Chapter 6, “How Digital Exposure Shifts,
Measures, and Slices Light.” While the basics are similar, the light meter measures only a blurred average of an image. Unlike the light
meter, the image sensor doesn’t measure simply the intensity of light. It must distinguish all the gradations of light in all the colors
that play on its surface, pass that information on in a way that lets it be translated to numbers, and then get ready to capture
the next image—all in slivers of a second.
(^102) PART 2 HOW DIGITAL CAMERAS CAPTURE IMAGES