A flatbed TMA (Transparent Media Adapter, sometimes called a film adapter) is basically a lamp in the lid (or a replacement lid) of a flatbed scanner, to allow it to scan film. The film is placed on the scanner glass bed, but instead of being illuminated by reflected light from below like a print, this lamp shines down through, from behind the film. Then everything else pretty much works the same, we scan film using the same basics as for prints.
When the proper TMA is detected present, the software enables more mode choices, with modes of Reflective (prints and documents), Transmissive (positive slides), or Negative mode (inverts negatives). Then the scanner firmware knows how to run which hardware to do what. Umax MagicScan 4.2 is shown.
In the film modes, the software disables the lower lamp in the scanner base unit, and uses the upper lamp above the film. The light must shine through the transparent media onto the CCD below.
The TMA on a top-end flatbed scanner is a good choice for large film. In general, you will get a lot more film capability for $400 or $1000 than for $100 (true of both flatbeds and film scanners). 6x7 cm medium film scanned at 1200 dpi can give a 2600x3200 pixel image, large enough to print 8x10 inches at 300 dpi. But 35 mm film is simply too small, and a much larger problem.
Scanning 35 mm film on a flatbed is a very popular issue, but there are many nagging details with a TMA. The best answer is a 35 mm film scanner (next chapter), but using a TMA on a good flatbed can work for some purposes. Still, it is not the best way to scan 35 mm film. Yes, it works. No, it may not be good enough. It may be fine for casual purposes, for viewing smaller images on the screen, but printing size may be an issue. Scanning film is generally better than scanning prints, but for inexpensive flatbeds, scanning prints often gives better results.
The minimum requirement to scan film on a flatbed is a TMA (the lamp), used with scanner firmware designed to do calibration from the upper lamp when in film mode, to adjust CCD gain for lamp intensity and color. Flatbeds with proper TMA do this. This implies a fixed location calibration area on the glass bed near the parked position, so that the CCD can see the upper lamp for calibration. It may be a separate glass slot so the film cannot block it, or it may just be a reserved area at the top of the regular flatbed glass. It is very important that the fixed location film calibration area must be kept clear and clean (don't block the calibration area with the film). Also the film mode must turn off the lower lamp. The simple free loose film adapter gadgets or the home-made kludges cannot do any of that.
Larger 8x10 inch TMA use a motor in the lid (expensive) to move the upper lamp carriage along in sync with the CCD carriage below. Microtek has some additional methods, a MTMA unit (Magnetic TMA) on wheels that is pulled along the bed by magnets in the CCD carriage below (typically covers about 7x8 inches). Some models place the film in a film drawer instead, which has glassless film carriers. The larger TMA is intended for much larger film sizes, 4x5 or 8x10 inch photographic film, x-rays, or other large scientific or industrial film. The TMA on inexpensive scanners are often much smaller, 35 mm size or perhaps up to 4x5 inch size, so no expensive motor is needed.
An 8x10 inch TMA may seem useful for proofing an entire roll of 35 mm film strips in one 8x10 inch image, like a contact sheet. However 8x10 TMA have become scarce except at the high end, and this will not be the same as a photographic contact sheet anyway, because the limited resolution of inkjet printers cannot show the same level of detail as a real contact sheet. If the printed results were viewed with an 8X magnifier (like a photo contact sheet), it will be disappointing, you will just see printer ink dots.