Richard Hartley, who has a doctorate in mathematics and specialises in video-analysis research at Canberra's Australian National University, was hired by the lawyers of Arkansas death row inmate Terrick Terrell Nooner in the hope his expertise overturns Nooner's murder conviction and death sentence.

A crucial factor in the case is Nooner's height.

According to Hartley's analysis, the man seen on fuzzy video surveillance footage in the shooting death of Scot Stobaugh at a laundry in Little Rock, Arkansas, in 1993, was almost 8cm shorter than Nooner.

Nooner, 37, who was previously convicted of the murder and is being held on death row, is 175cm tall.

The mathematician told Little Rock's Federal Court he came to his conclusion the killer was shorter than Nooner after measuring from the security footage the geometry of the triangle formed by the gunman's stance.

Hartley's conclusion the shooter was 167cm tall pointed to another man, Robert Rockett, who was previously convicted of being an accomplice in the murder and is serving a life sentence, Nooner's lawyer said.

"Piece by piece, the state's case against Mr Nooner has collapsed, and the evidence now reveals the truth: Robert Rockett killed Mr Stobaugh, and Terrick Nooner is innocent,'' Nooner's lawyer, Julie Brain, told the court, according to Arkansas Democrat-Gazette newspaper.

A prosecution expert rejected Hartley's evidence, telling Judge Leon Holmes it was not possible to tell the height of the gunman on the footage with "any degree of accuracy''.

The case continues.