The question is "interested in the terms 'primary' and 'secondary' in the technical sense they would be used by a professional historian."
Wikipedia defines primary sources as original materials that have not been altered or distorted in any way. In the study of history as an academic discipline, a primary source (also called original source or evidence) is an artefact, a document, a recording, or other source of information that was created at the time under study. Generally, accounts written after the fact with the benefit of hindsight are secondary.
The UCLA Institute on Primary Resources says that 'primary resources' provide first-hand evidence of historical events, and gives several complementary definitions from other, respected academic sources:
- The Library of Congress' Learning Page: Primary sources are defined as "actual records that have survived from the past, such as letters, photographs, articles of clothing." In contrast, secondary sources are accounts of the past created by people writing about events sometime after they happened.
- The Ohio Historical Society defines primary sources as a "source created by people who actually saw or participated in an event and recorded that event or their reactions to it immediately after the event." In contrast, a secondary source is defined as a "source created by someone either not present when the event took place or removed by time from the event."
- The Teaching Library at the University of California at Berkeley says that primary sources are the evidence left behind by participants or observers, and provides a bullet list of examples, including published materials written at the time of the event.
The Institute on Primary Resources summarises by stating, "The common thread running through the examples is that primary sources of material can be in any form, and are a source of direct evidence that describes or documents an historical event from the perspective of someone who was there.
Time under study
The following definition forms part of the question to be answered:
A primary source is a document or physical object which was written or
created during the time under study
This is true to the extent that a document written during the time under study is likely to carry independent evidentiary weight, whether as written by an eyewitness or otherwise reflecting close knowledge of a contemporary event. Although it is certainly not the only reason to regard a document as a 'primary document', I will begin by dealing with this ground for regarding Luke as a primary or a secondary document.
In this case, the answer must be influenced by factors such as when we believe Luke to have been written. Someone who believes Luke was written in the 50s or early 60s of the first century might consider it (from a date point of view) to be at least somewhat primary in that it was written within one generation after the period under study (up to about 33 CE), whereas someone who believes it to have been written in the latter part of the century, and probably in the 90s, would probably not.
John Dominic Crossan says in The Birth of Christianity, page 109, the theory that the Gospel of Luke was actually based on Mark's Gospel is held today by a fairly massive consensus of contemporary critical scholarship. The consequence of this is that Luke's Gospel must have been written somewhat (say at least ten years) later than Mark, which in turn is dated by most critical scholars to approximately 70 CE. In fact, the general consensus is that Luke was written in the 90s, long after the period under study.
Dependence on prior sources
A source ought not be regarded as primary if it can be shown that it is dependent on prior sources. Ian W. Scott says, in 'Is Luke a Primary Source for the Historical Jesus?', that where Luke simply quotes Mark we might consider him a secondary source because his reliability depends entirely on the reliability of another source we have – Mark. The same should also be true when Luke quotes 'Q', the hypothetical sayings document shared by this author and the author of Matthew. This then covers the major part of Luke's Gospel. Note: I do part company with Scott, when he says that if the author of Luke might have had access to parallel sources (unknown to us) that enabled him to verify or interpret Mark, then we might call Luke a primary source.
Luke also contains material that was not copied from either Mark or Q. Should we regard Luke as a primary source for this material or as a secondary source, even if any primary source used by the author of Luke is not known to us? For example, Luke chapter 1 tells us about the birth of John the Baptist, approximately one hundred years before the Gospel was written. Either the author relied on a prior record of this event, or Luke is a primary source simply because there was no prior record of the birth of John.
Other material that now exists only in Luke's Gospel includes stories of the birth of Jesus, the empty tomb and the appearances of the risen Jesus on the day of the Resurrection. If the author of Luke was relying on prior sources, he was clearly relying on different prior sources than was the author of Matthew. For example, Archbishop Peter Carnley, former Anglican primate of Australia, writes of the empty tomb, in The Structure of Resurrection Belief:
The presence of discrepancies might be a sign of historicity if we had four clearly independent but slightly different versions of the story, if only for the reason that four witnesses are better than one. But, of course, it is now impossible to argue that what we have in the four gospel accounts of the empty tomb are four contemporaneous but independent accounts of the one event. Modern redactional studies of the traditions account for the discrepancies as literary developments at the hand of later redactors of what was originally one report of the empty tomb...
There is no suggestion that the tomb was discovered by different witnesses on four different occasions, so it is in fact impossible to argue that the discrepancies were introduced by different witnesses of the one event; rather, they can be explained as four different redactions for apologetic and kerygmatic reasons of a single story originating from one source
On this evidence, Luke's Gospel would be regarded by most historians as a secondary source for the the life of Jesus. Even secondary sources can be a valuable source of information about events of the past. Scott say (ibid) the question of how we label someone like Luke is less important than the way we actually handle the evidence.