What should I look for in optical/hardware resolution when buying a scanner?

I'm debating between these two scanners:

1. Canon CanoScan LiDE 110 Flatbed Colour Scanner 2400DPI 48/24BIT USB2.0


2. Epson Perfection V33 Flatbed Scanner USB2.0 9600X4800DPI

I've got some old family photos I want to scan and archive. I'd like high-quality scans.

I'm a bit confused about the differences between "Optical" and "Hardware" resolutions, and why they are giving X*Y DPIs; normally DPIs are given as a flat numbers. Does that mean they have a higher horizontal resolution than vertical resolution?

The interpolated/software DPI is meaningless if I'm not mistaken. I can blow it up in Photoshop if I really want to.

What should I buy based on these technical criteria?

Answer

The word scan-head simply means the horizontal bar that goes up-and-down along the scanner (for flatbed scanners). It is static for auto-feed scanners.

Optical resolution

This is often given as a single number say 600dpi and is related to the amount of detail the scan-head can capture horizontally -loosely speaking.

Technically, it is the number of sensors placed per inch (horizontally) on the scan-head. In this case, there are 600 sensors horizontally per inch on the scan-head.

Side note: If the maximum paper size of a scanner is legal, then there are (600 x 8.5 = 2550) sensors along the scan-head.
some scanners using a single-pass might have 3 rows of sensors on the scan-head (this doesn't improve quality of scan, it only increases the speed).

Hardware resolution

This is often given as two numbers say 600 x 1200 dpi; the first is often the same as the optical resolution and the last is related to the quality of detail that can be captured vertically - again loosely speaking.

Technically, the last number is the number of vertical step movements of the scan-head per inch. In this case, this means the scan-head moves 1/1200 inches (vertically) per sampling - allowing for more detail to be captured vertically than one with a 1/600 step.

Maximum, Interpolated, etc resolutions

These are just software/hybrid means of increasing the resolution. They are not important because they simply add (gradient) pixels horizontally/vertically to the original scan (to give a smooth larger image); they don't add any further detail to the original scan

Verdict

Pick a scanner where the optical or hardware resolution is as high as possible.

I know that the optics: lens, type of light source and sensor technology etc play a role in scan quality, but these are out of this question's context. Oh, and don't forget to up-vote if you found this useful

Regards.