What is the difference between psychophysics and neurophysiology? I'm an engineering student who is doing some subjective tests. 


What are the difference between psychophysics and neurophysiology? 
Is it correct that in the latter case, we have to implant to read the brain signals?
I'm glad you asked! The two fields are really quite different, but I think people get them confused since the terms both make reference to physics and something vaguely mental. 

Psychophysics is basically the measurement of subjective percepts corresponding to physical stimuli (e.g. measuring pain thresholds or visual acuity). So here the 'physics' refers to the physical reality we're trying to represent in our mind.

Research here is almost exclusively behavioral, with the classic paradigm being to try and determine the 'just noticeable difference' between two stimuli. Signal detection theory has also been hugely influential for data analysis in the field. I find it helpful to think of psychophysics as experimental cognitive psychology at its least abstract.

Neurophysiology, on the other hand, investigates the anatomical structures underlying the brain (e.g. categorizing the types of neuron, figuring out what glia are made of). Thus the 'physics' referred to here is the physical structures underlying the brain.

Here the methodology involves various sorts of neuroimaging, tissue sampling, and other ways of probing human and animal brains at various levels of intrusiveness.

Note that these definitions paint the two fields with very broad strokes, so I'd do further research if you want to differentiate say, neurophysiology from neurobiology, but I think this suffices for the purposes of your question.

Hope that helps!