Some document scanners have a special mode for scanning transparencies. This includes medical imagery like xrays, your old collection of 35mm slides and the negatives / filmstrips that used to come with printed photos.
If you are scanning xrays or similar large photographic sheets then 300 DPI or 600DPI is said to be sufficient (medical disclaimer: use a suitable resolution for what you are trying to identify). However, you’ll need higher resolutions for 35mm slides. One example I saw was: in order to enlarge a slide to 20×30 cm (8″ x 12″) with a resulting 300 DPI quality, you need to scan the slide with at least 2500 dpi.
Google tells me that 35mm film is, since the 1950s, 24mm x 36mm. This means enlarging to 20cm from 24mm is 8.33x (“to 30cm from 36mm” is the same). 8.33 x 300 DPI = 2500 DPI so that’d be how they got that figure. It’s scary how high the DPI figure would have to be if you wanted the digital equivalent of projecting your slides on your old projector.
CmdTwain allows you to send commands to your document scanner to activate modes like the transparency one. It is a little complicated at present but, if you need it, it can be done.
The command to turn transparency mode OFF is ICAP_LIGHTPATH, TWTY_UINT16 TWLP_REFLECTIVE (0x111e,4 0). You can send this to your document scanner by adding option “/S 4382,4 0” to the CmdTwain command line.
The command to turn transparency mode ON is ICAP_LIGHTPATH, TWTY_UINT16 TWLP_TRANSMISSIVE (0x111e,4 1). The CmdTwain option is: “/S 4382,4 1”.
Once you have it on, you can also specify whether you’re scanning a positive (eg a 35mm slide) or a strip of negatives. Scanners that understand this setting should automatically reverse a negative so that you get a normal picture. These commands are:
Set ICAP_FILMTYPE, TWTY_UINT16 TWFM_POSITIVE (0x115f,4 0). CmdTwain: “/S 4447,4 0”
Set ICAP_FILMTYPE, TWTY_UINT16 TWFM_NEGATIVE (0x115f,4 1). CmdTwain: “/S 4447,4 1”
I hope this helps those of you scanning transparencies.