Re: Scanning Aperutre Cards

John Gieske (mailto:johng@SUNRISEIMG.COM)
Fri, 4 Apr 1997 11:30:59 -0800 

Message-Id: <199704041940.MAA17130@dns.ccit.arizona.edu>
Date:         Fri, 4 Apr 1997 11:30:59 -0800
From: John Gieske <mailto:johng@SUNRISEIMG.COM>
Subject:      Re: Scanning Aperutre Cards
To: mailto:IMAGELIB@LISTSERV.ARIZONA.EDU


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Reduction ratio and resolution are tightly interrelated.  The two terms =
merely describe how you wish to interpret the image data.  If you =
multiply the reduction ratio by the resolution, you will get what we =
call the "true DPI".  Think of TDPI as the actual resolution relative to =
the film (not the document).


In your example, you have an aperture card that is actually 24X and 200 =
DPI but somebody scanned it as if it were 20X and 200 DPI.  The TDPI for =
the scanned image would be 20*200=3D4000 TDPI.  If the scanner operator =
had scanned it correctly, the TDPI would have been 24*200=3D4800 TDPI.  =
The scanned image can therefore be thought of as 20X at 200 DPI, or 24X =
at 167 DPI (20*200/24=3D166.667).


>Image Degradation?

Yes, you will see a slight loss of detail because the image has =
effectively been scanned at a lower resolution than it should have.  You =
can see the difference in areas of fine detail, such as text using =
smaller than 8 point fonts.


>Can images be rescaled?

As I said above, you can interpret the image data however you want.  In =
the case of images being scanned at too low of a TDPI, there isn't much =
you can do.  Scaling algorithms will simply replicate data, but will not =
increase legibility.  On the other hand, if the images were scanned at =
too high of a TDPI, then a scaling algorithm will throw out the extra =
data, and give you the exact image dimensions you wanted.  Most photo =
editing software packages can do this and have fairly good looking =
output.




John Gieske
----
SunRise Imaging, Inc.
(510) 657-6250 x26                                        =
mailto:johng@sunriseimg.com
(510) 770-9209 FAX                                   =
http://www.sunriseimg.com
(510) 657-6281 BBS                                    =
ftp://ftp.sunriseimg.com
-----Original Message-----
From:   Lorrin Read, CRM [SMTP:mailto:readl@OHIOEDISON.COM]
Sent:   Friday, April 04, 1997 8:25 AM
To:     mailto:IMAGELIB@LISTSERV.ARIZONA.EDU
Subject:        Scanning Aperutre Cards


If a specific size of engineering drawing is microfilmed at a certain
reduction ratio (ie. 24x) and then the resulting aperture cards are
electronically scanned at a different setting (ie. 20x or 30x) will this
result in any image degradation?


Can raster images resulting from scanning aperture cards be plotted back =
to
the same scale as the original drawing?


Thanks in advance for your responses.
Lorrin
-----------------------------------------------------------
Lorrin N Read, CRM
Ohio Edison Company
76 South Main Street
Akron Ohio 44308-1890
Phone (330)384-5648
Fax (330)384-5014
mailto:readl@ohioedison.com




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