Novelty has two aspects. One is rarity: some event may turn up infrequently or unexpectedly.  For example, seeing a familiar face in a crowd of unknown persons  is a rare  event.  Since you don't expect it to happen,  we might call this a surprise reaction.  The second aspect is unfamiliarity. Unfamiliarity means that a person or object is completely new for us:  there is no representation or 'template ' in our long term memory (brain)  that matches with  it.  For example, when an unfamiliar face turns up in group  of familiar persons we will most  likely register the event,  but pay no further attention to it.  An exception are unfamiliar events that  for some reason do attract our attention, because  they are biologically, or even 'evolutionary' significant. They  could signal something rewarding or pleasant (e.g. a beautiful abstract painting  or  attractive woman) or something unpleasant of even dangerous.  For example,  animals living in the wild  will generally feel disturbed and  threatened by  an unfamiliar species or sound entering their territory. 

Measuring novelty  Novelty has for many years been been a core issue in psycho-physiological  research.  Human reactions to biologically relevant, unexpected or emotional events have traditionally  been associated  with autonomic physiological responses such as the Skin Conductance Response. This has often often been referred to  as the Orienting Response (OR). More cognitively oriented psycho-physiologists  have discovered about 50 year ago that the human brain reacts to novel  events  with a large positive electrical response called P300.  P300 (sometimes called P3b; see the link below)  reflects that something relevant has turned up unexpectedly  that has attracted our conscious attention. Perhaps also a manifestation of the OR, but generated ''higher up'' in our brain.  Other  brain research has been directed to detect automatic (un- or preconscious) reactions of the brain to novel events. This brain response (called MMN or Mismatch Negativity)  however mainly concerns deviations in physical characteristics of sounds (e.g.  pitch,duration, loudness) that are automatically detected by the auditory center  in the brain. 

Polich J. & A. Kok. Cognitive and Biological Determinants of P300: an Integrative Review. Biological Psychology 41(2):103-46 · November 1995· Source: PubMed

Kok, A. Probability mismatch and template mismatch: A paradox in P300 amplitude? Behavioral and Brain sciences, Volume 11, Issue 3 September 1988, pp. 388-389

https://en.wikipedia.org/wiki/P3b

Thanks for your responses.