Archive for April, 2010

To scan a slide, the slide is placed in the scanner. All scanners feature a holder that keeps the slide intact and few also have the capability to scan multiple slides. The carrier can be metal or plastic, which is where the slides are kept for scanning. LED lights or fluorescent bulbs are source of light for slide scanners. The lighting will be constant throughout the scanning process irrespective of which source is used. Sensors perform the function of converting analog light passing through the scan to digital format. The slide scanner sensors vary with type of scanner. CCD (Charged Coupled Device) array is the sensor that is used often. Sensors have a bunch of diodes, being light sensitive are also responsible for converting analog light to digital signal.

Slide scanners are classified into sequential array scanner and full frame sensor. Sequential scanner divides the slide into units for scanning and comes with three rows of sensors. Red, Blue or Green light is measured by each row. Full frame sensor and sequential array have separate sensors for colors yet vary in one aspect. The slide or the sensor is not moved when capturing the image. The carrier moves the slide across the light source and sensor assembly or moves the assemble over the slide. The moving rate of a carrier is determined by, electrical stepping motor which uses a drive belt or a rack and pinion system.

With a variation in sensor also comes a variation in scanning process. Light source is on the opposite side and passes through the slide when using a sequential sensor. First step is calibration where measurements of calibrate color balance, contrast and brightness are determined. The the carrier positions itself at the portion where scanning is to be done. The image is divided into portions or “slices” for scanning in sequence. The images are then arranged in order and made ready for transferring to a computer. On the other hand, scanners that come with full frame sensor generally take the slide carrier into the carriage. The image is moved to a predefined position after which the scanner scans the entire image as a whole. Using a USB or Fire Wire the images are then transferred to a system.

Scanning is done for two reasons: digitizing images and to get a clearer picture of them. The aspect that decides the clarity of the image is DPI or “dots per inch”. 300 DPI is the ideal resolution or DPI. With the advent of a slide scanner, preserving the slides are no longer needed as they can be digitized.