I'm looking at buying a scanner with a document feeder. The feeder can feed in 30 ppm, but the scanner can only can at 12 ms/line. So how fast is the scanner scanning? Is having such a fast document feeder useless?
Well an A4 page (I live in the UK) has about 50 lines of text on it, so to do an A4 page it would take 12 x 50msec = 600msec or a little more than half a second. A 30 ppm sheet feeder would feed one every two seconds leaving you with a reasonable margin for error.
I would suggest you only use the numbers as a guideline, see if you can find a review or some real world tests. Find out what people actually think about it, Amazon.com is quite good for user reviews of consumer products.
I believe the way to calculate the time (in seconds) a scanner takes to scan a page is as follows:
(Scanner DPI) x (Length of page in inches) x (scanner speed in msec/line) / 1000 =
eg: For a scanner which scans at a speed of 12 msec/line at 1200 DPI (dots per inch) to scan a Letter size page (8.5 x 11 inches) it would take:
1200 (DPI) x 11 (inches) x 12 (msec/line) / 1000 = 158.4 seconds So approximately = 160 seconds
Notes and assumptions:
- I believe "line" in msec/line refers to a line of pixels so at 12msec/line the scanner's sensor array can acquire 1 line of pixels in 12 milliseconds where the length of the line is the length of the scanner's sensor array, which is typically the width of the largest page that can be scanned (see next point).
- It is assumed that the scanner's sensor array move along the length of the page (this is typical for most commercial flat-bed scanners). If it move along the width then use the width in the calculation above instead.
- It is assumed that the warm up time, time for data transfer to the PC and any other time associated with the process of scanning is negligible when compared to the time taken by the sensor array to acquire the picture of the physical medium. Thus time calculated is approximate at best and likely slightly less then the total time required to scan the page.
- Scanner may have different scan speeds at different DPIs and the scaling may not be linear. eg: Scanner which scans at 10 msec/line at 600 DPI may not be able to scan at 5 msec/line at 300 DPI.