Dots Per Inch (DPI) is a widely misused term in digital imaging. DPI is used as a "catch-all" term for some very different measurements.
DPI only truly applies to printing with Ink Jet printers. It is the measurement of the number of ink drops placed on the paper.
DPI, as used in the real world, generally refers to the quality of the scan or image. A higher-resolution photo will have a higher "DPI". In scanning, when DPI is used in place of Samples Per Inch (SPI), a higher number results in a larger file size. As an example, a photo scanned at 300 DPI will result in a file size that is appropriate for sharing on the web and social media. The same photo scanned at 600 DPI will be quite large, and will need to be resized (made smaller) before sharing. For Archival prints, a higher DPI is appropriate. You can make larger prints with a higher resolution file. 300 DPI images can generally be printed at the same size they were scanned. E.G. a 4" x 6" photo scanned at 300 DPI can be printed at 4" x 6" without loss of detail or obvious pixelation. If you want to save your old photos in the best possible resolution, perhaps for later printing, you will want to have them scanned at 600 DPI (or even higher).
As a consumer, when you see DPI, PPI or SPI, you can consider them all the same thing. It's not technically correct, but it gets the job done 🙂
Transparent Media includes Slides, Negatives and Film. Transparent Media is translucent, meaning that light shines through the media. Scanning transparent media involves the use of a light source positioned behind the slide, negative or film, that shines the light onto a sensor of some type through the media. The sensor is then able to capture the image.
Reflective Media is anything that is opaque and reflects the light. In this case, a light source shines directly onto the media, and the reflected light is captured by the sensor. This applies to photographs, magazine pages, newspaper, or anything that is opaque.
In some cases, you can scan transparent media using a reflective scanner. However, the image result will be a negative of the original, and you will need to convert it to a positive in your image editing software.
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