IN three decades of working in forensic science Tom Nelson has seen his field evolve far beyond anything he could have envisaged as a fresh-faced rookie in his native Northern Ireland.

"When I started out you needed a blood stain the size of an old 50p piece," he recalls. "From that all you could do was an ABO blood grouping and perhaps get statistics of maybe one in 200."

These days Nelson, director of Forensic Services at the Scottish Police Authority (SPA), and his team are able to gather DNA evidence using everything from saliva and seminal fluid to a lone strand of hair or the tiniest flake of skin, providing a match so accurate that it can be honed to a billion to one.

It is a technique which has proved key in solving some of Scotland's most high-profile murder cases. Their work is featured in a new BBC Scotland series, Crime Scenes Scotland: Forensics Squad, which continues on Wednesday, charting the case of Glasgow businesswoman Moira Jones who was abducted, raped and murdered in the city's Queens Park in May 2008.

The programme also re-visits the murder of Eleni Pachou, the assistant manageress of a pizzeria in the west end of Glasgow, who was found stabbed to death only two days later. Although unconnected, both would draw heavily on the work of the SPA forensics team.

For Nelson, there is no such thing as a quiet day at the office. Statistically, Scotland has Western Europe's worst murder rate per capita while Glasgow, where he is based, deals with 60-70% of the country's serious crime.

When we meet, Nelson has the reassuring air of a man who has a good head in a crisis. His delivery is precise and methodical, although he isn't adverse to a tongue-in-cheek quip delivered in his softly lilting accent. Not least when talking about a book his father, now in his eighties, bought him as a gift. "Forensics for Dummies," he smiles. "I have to admit it was a great read. I thoroughly enjoyed it."

While these days Nelson's role is more about finance and planning strategy than meticulously picking through crime scenes or hours spent hunched over a laboratory bench, his passion for the job hasn't waned.

"I love the puzzle-solving element," he said. "There was a case years ago where I had to piece together tiny fragments of glass to work out how many milk bottles had been used in a particular crime. I enjoy that sense of satisfaction and completeness.

"That said, I was never big into jigsaws as a kid. For me, a jigsaw, you finish it, break it up and put it away to start again another day. I like doing things where I can think: 'What is it going to deliver?' In forensic science, puzzles have an end result."

Nelson, 51, began his career at the Northern Ireland Forensic Science Laboratory in 1980, developing a specialist expertise in fire investigation and chemical analysis. In 1995, he moved to Scotland to take charge of the chemistry laboratory at Lothian and Borders Police.

Six years ago, Nelson was appointed director of the Forensic Services at Scottish Police Service Authority (SPSA), tasked with creating a new "crime-scene-to-court" model for forensic services, bringing together four key disciplines under one umbrella: scene examination, forensic laboratories, fingerprint units and business support. In April, with the creation of a single Scottish police force, the SPSA became the Scottish Police Authority.

The eldest of two sons growing up in Lisburn near Belfast, the seeds of Nelson's future career path first took root in childhood. "Even at a young age I was always interested in right and wrong," he said. "I think that drew me, not necessarily into forensic science because back then it was still very much in its infancy, but to see how science could help injustice."

An upbringing in a fractured Northern Ireland would indelibly sculpt this view too. "The Troubles were all around us," he said. "Forensic science was able to make a significant contribution to the whole peace process."

It is a field which, as science rapidly progressed, has gained an ever-growing public awareness, seeing a raft of television shows, including Silent Witness, Bones and the CSI franchise, capitalise on the intriguing and often grisly subject matter.

Nelson gives short shrift to such fictional depictions. "These programmes can frustrate me at times," he said. Does he watch them? "No. I've seen the odd episodes here and there, but I don't follow them. They build expectation. I remember someone once saying to me: 'Tom, I saw this programme at the weekend and they could do this particular analysis in a few minutes. Why do we have to wait a couple of hours?' I replied: 'Because you were watching TV'."

Although he does admit a soft spot for a certain legendary TV medical examiner. "I do remember watching Quincy as a child and probably didn't realise at the time how much of an impact that had on me," said Nelson.

"But when I see a programme and someone is walking into a crime scene without wearing protection, I'll be shouting at the TV: 'You can't do that, it's wrong, you've compromised the crime scene'. My wife had to stop me watching those shows."

Based in Glasgow, he and his wife Helen, 49, a health visitor, have three children studying at university. Nelson talks about fun-filled skiing holidays, his prolific youth work through the church and having a golf handicap estimated to be "around 18".

But then the shutters come down. He is reticent to be drawn any further. "I'm trying to protect my home life as much as I can," he asserts, apologetically.

It is a statement which is perhaps stark testament to his line of work. "Back when I started out in Northern Ireland we were trained to not bring a situation back into your own life," he said. "If you came across a body or something extremely gruesome, the worst thing you could do would be to make a connection between that death and your family.

"I remember once I was out at a crime scene involving a young kid. My wife was pregnant at the time and I remember I made the connection back to Helen and broke down in tears. I had to go away, control myself and then return to the crime scene later on.

"As a forensic scientist you have to make sure you don't let that happen. That's why I don't talk about work when I go home. I have a coping mechanism which I've developed over the years, but my wife and family might not have that same way of dealing with things, so I don't want to expose them to it."

Nelson is at pains to stress that, ultimately, he and his team are human beings and not machines. "As a forensic scientist you go into court to give evidence which requires a lot of preparation beforehand including the reading of case notes, papers and different scientific journals," he said. "Then the first question you are asked is your age and I don't know how many times that has stumped me.

"In that moment you freeze, thinking: 'Right, I've done all of this preparation, I know the case inside out, but what's my age?'. I've had to do a quick calculation, thinking: 'Right, I was born in 1962 ...'"

But then his tone grows serious again. Nelson acknowledges forensic science is not without its foibles. In 2011, a public inquiry into the Shirley McKie fingerprint scandal found she was a victim of "human error". The former police officer was accused of perjury after a fingerprint found at an Ayrshire murder scene in 1997 was wrongly identified as hers.

After a long battle by Ms McKie and her father Iain, she was cleared and awarded £750,000 in compensation. Following publication of the inquiry report, the Scottish Police Services Authority apologised to Ms McKie.

"If we make a mistake, we have to put our hands up and say we got that wrong," said Nelson. "Certainly in the McKie case, that's what I felt was the right thing to do. I believe it's what the public would want rather than hide in a box and say everything is perfect. If there are issues, face up to those and put a process in place to prevent it from happening again. That gives the public confidence in what we do."

Crime Scenes Scotland: Forensics Squad is on BBC One Scotland, Wednesday, 10.35pm