I have not come across this method before, but it seems to be similar to Fizeau's Wheel. This is in many textbooks, and is a method of measuring the speed of light from a lamp to a distant mirror and back again. The beam is interrupted on its outgoing and return journeys by a rotating toothed wheel. If the light hits a tooth on its way back then there is darkness, and we then know the time delay of the light from the speed of rotation.
In Houston's method, it is using an electronic shutter to do the same task as Fizeau's Wheel. A piece of quartz is used as the shutter. When it is fed with a high frequency, it interrupts a light beam each cycle. The frequency is adjusted until darkness occurs, and the frequency being used then tells us the time of travel of the light.
Quartz works as a shutter because it becomes a diffraction grating when driven with high frequency voltage. A diffraction grating is usually a piece of glass with closely spaced fine lines ruled across it; the action is seen in everyday life with a CD. We usually see colours in a CD, because the fine lines deflect each colour differently. It will deflect monochrome light to just give a dot. Quartz exhibits the action of ruled lines because minute mechanical waves are created in it by the applied frequency. By using the quartz device, the spacing of the lines can be adjusted by altering the frequency of the applied voltage, so in effect a bright dot can be deflected to pass through a hole or not. This creates a light valve action.
