With the attention so frequently given to optical-type detectors, I am surprised that nobody
has yet (to my knowledge) mentioned what I believe to be the most clever invention of them
all.
The great R. V. Jones, who is best known and quoted in seismology for his work with
capacitive sensors, while at Aberdeen invented a modified optical lever that uses Ronchi
rulings.  Ronchi is in this modern era largely unknown, but his contributions in the field of
figure-testing of optical components was of truly great significance.  Those who want to
study Jones' system can download the following paper:
http://iopscience.iop.org/0950-7671/38/2/301/pdf
What is most remarkable about the coarse gratings (Ronchi rulings) used for this work is that
they work with ordinary (non-monochromatic) light from an incandescent bulb.  I have for
about two decades demonstrated to students the physics of this remakable system by means of a
spherical mirror in which the white light bulb/grating combination is situated at the center
of curvature.
    To those who are impressed by observing displacements as small as a nanometer, consider
the following:  In the 1960 paper referenced above, Jones claims a resolution of 0.1 nrad!
It should be noted that this device is not a simple (standard) optical lever where one
typically is limited to tenths of a microradian;  Jones combined Ronchi rulings with the
optical lever to produce a whole new concept--much better for seismic purposes, if anybody
wants to try it.  In my case--so much of interest, limited time left to pursue them!