Added some content about scanner types, accessories and CCD sensors.

how-scanners-work.html

@@ -36,12 +36,84 @@
     </p>
 
     <h2>Hardware (or: what is inside the scanner?)</h2>
-    <h3>Sensor types</h3>
+
+    <h3>General Scanner Types</h3>
+    <h4>Flatbed Scanners</h4>
+    <p>
+      Flatbed scanners provide a glass pane where the physical image is placed
+      face down. The image is scanned by a movable sensor below the glass while
+      the original does not move. The size of the physical image is limited by
+      the glass pane.
+    </p>
+    <h4>Sheetfed scanners</h4>
+    <p>
+      This type of scanner has a fixed sensor whereas the physical image (piece
+      of paper) is moved during the scan. Most scanners of this type have an
+      automatic document feeder which can carry more than one piece of paper.
+    </p>
+    <h4>Film/Slide/Negative Scanners</h4>
+    <p>
+      These scanners are used to scan transparent material like negative films
+      or slides.
+    </p>
+    <h4>Handheld scanner</h4>
+    <p>
+      Handheld scanners are moved manually on the physical image. Because of
+      practical reasons the image width is limited but the length is
+      theoretically endless. While handheld scanners were qwuite popular about
+      10 years ago they are seldomly used nowadays.
+    </p>
+    <h3>Accessories</h3>
+    <p>
+      For most scanners one or more of the following accessories exist, either
+      optionally or already included.
+    </p>
+    <h4>Transparency Adapter (TA)</h4>
+    <p>
+      A transparency adpater is used to scan transparent images (negative
+      films, slides) on a flatbed scanner. A fixed or movable lamp is mounted on
+      top of the scanner. Therefore the physical image is between the light and
+      the sensor. Most scanners turn off the internal lamp when using the
+      TA. Often the scan area is smaller then the full flabed area and the
+      images are mounted in special frames.
+    </p>
+    <h4>Automatic Document Feeder (ADF)</h4>
+    <p>
+      While sheetfed scanners always provide automatic document feeders, ADFs
+      are also available for some flatbed scanners. Usually a stack of papers
+      can be placed on the scanners. With an automatic document feeder, many
+      pieces of paper can be scanned in one batch without manual
+      intervention. The pages are either moved through the scanner at constant
+      speed and fixed sensor or scanned by a moving sensor after beeing placed
+      on the fladbed automatically.
+    </p>
+
+
+    <h3>Image Sensor types</h3>
+    <p>
+      The image sensor is that part of the scanner, which translates the physical image it
+      "sees" to analogue electrical signals. The physical image which is to be scanned is
+      illuminated by a lamp (or LEDs). For reflective scanning (e.g. papers),
+      the lamp is usually part of the sensor mounting plate or slider. For
+      negative or positive scanning, the negative film or slide is located
+      between the lamp and the sensor. While several types of image sensors are
+      used, these are the main categories:
+    </p>
     <h4>CCD</h4>
+    <p>
+      A charged-coupled device (CCD) sensor is an integrated circuit based on a
+      one-dimensional array of capacitors. The image is projected on the CCD by
+      a lens and/or mirror system from the physical image. There is one sensor
+      line with included filter for each color (RGB, sometimes infrared). Older
+      three-pass scanners had only one sensor line and a movable filter, which
+      required three passes two scann in a color image. Usually cold cathode
+      fluorescent lamps (CCFL) are used to illuminate the physical image.
+    </p>
     <h4>CIS</h4>
     <h3>Analog frontend (AFE)</h3>
     <h3>Interface chips</h3>
-    <h3>Stepper motor</h3>
+    <h3>Stepper motor and driver</h3>
+    <h3>Image buffer (RAM)</h3>
     <h3>Calibration area</h3>
 
     <h2>Software (or: how does this thing work?)</h2>