Is there an instrument that measures the Frequency of light directly Is there an instrument that measures the frequency of light DIRECTLY?
By "DIRECTLY" I mean without using any properties of the Wavelength.
 A: The photoelectric effect can be used to derive the frequency of the photon directly. This is because only a certain threshold frequency will allow for the emission of electrons from a metal. 
Therefore judging from the energy of the emitted electrons, the frequency can be observed.
A: You have to be clear as what you mean by "frequency" and "directly."  If what you mean is an instrument like an oscilloscope with a photo-detector, where the light is incident on the photo-detector without any other source coincident with it.  Then what you'll need is an oscilloscope and a photo-detector capable of responding to electric fields at $10^{15}$ Hz, and as such does not yet exist.
Since most photo-detectors are square-law detectors, ie. they respond not to the electric field but to the modulus squared of the field (also known as intensity), then the information is lost.  However, if you performed what's known as a heterodyne or homodyne detection method whereby a reference light field is added to the light field you'd like to measure then you can measure the beat frequency of the source, provided the beat frequency is within the bandwidth of the apparatus.
