This is known as reflective image capture, as light is reflected off of the image to the digitizing pickup. Flat panel scanners can also digitize slide and negative film if they have a transparency adapter. This is known as transmissive capture.
Light is transmitted through the image onto the pickup.
Optical resolution for these scanners is typically under 800 DPI, and the glass plattens that sandwich the image in the transparency mode scatter the light from both the source and from the image itself. This limits the effectiveness of using flat panel scanners in the transparency setting. Images often appear fuzzy when compared to the original, or scans from equipment designed especially for transparent media. The advantage of using panel scanners for transparencies is that they can commonly scan 20 or more slides at a time and are not limited to the 35mm slide format. They can scan at a rate generally about a tenth of the drum scanners, but are improving, and adequate for much of the work done in the academic environment.