Even though it is authenticated by a witness, real evidence is separate, distinct, and doesn't rely upon a witness' testimony. It has the weight of being additional evidence and can serve many purposes. Demonstrative evidence is evidence that illustrates or helps explain oral testimony, or recreates a tangible thing, occurrence, event, or experiment. Scientific evidence falls into this category, as when a toxicologist testifies that the victim died of lead poisoning and refers to a chart of the human body showing the circulatory pathways that the toxin traveled. Visual aids of this type are intimately tied to the credibility of the witness who testified with them, and although they are not separate pieces of evidence, like real evidence, a jury can usually view them again while it deliberates.
Demonstrative evidence is intended to be an adjunct to testimony.
The next most general rule involves the foundational requirements for demonstrative evidence. Certain preliminary steps must be followed such as authentication and accuracy. What it conveys must not alter, distort, or change the appearance or condition of something in any significant way. A computer enhanced photograph, for example, to make a crime scene area look lighter than it actually was is probably inadmissible. There are specific rules, however, that do allow computer enhancements under some circumstances.