I need to know the difference between a direct and indirect reading instrument. Would a caliper be a direct and a gauge block be indirect? Is a direct instrument something with a variable measurement and indirect something with just a fixed measurement?
Direct measures the physical property directly. So if you want to measure length you would measure length. Both calipers and gauge blocks do that. But say optical measurement unit would not directly measure length, it would measure something else. Such as time for bounce in case of time of flight laser rangefinder, intensity of reflection for infrared distance units or angle for triangulation with a camera.