How is the sapped of light measured?

Answer 1

Never knew that light could be 'sapped' !
But, if what you mean is 'speed, then :
The speed of light in a vacuum is denoted by the letter c for constant or the Latin celeritas (speed). The speed of light through a transparent medium (that is, not in vacuum) is less than c; the ratio of c to this speed is called the refractive index of the medium.
In metric units, c is exactly 299,792,458 metres per second or 1,079,252,848.8 kilometres per hour. Converted to imperial units, it is approximately 186,282.397 miles per second, or 670,616,629.384 miles per hour. Note that this speed is a definition, not a measurement, since the fundamental SI unit of length, the metre, has been defined since 21 October 1983 in terms of the speed of light—one metre is the distance light travels in a vacuum in 1/299,792,458 of a second.

Answer 2

Here is a very simple & clear explanation of the ways this has been done:

http://www.colorado.edu/physics/2000/waves_particles/lightspeed_evidence.html


(I'm assuming you meant "speed", not "sapped", right? LOL)