The acid attacker, also a teenager, has been convicted, but he will serve less than 11 months in jail.
Courtney Bunce, 18, left Stephen Kerslake with such horrific burns that the latter will have to cover his scars with factor 50 sun block (sun screen lotion) every time he steps outdoors for the next two years.
Doctors said it was only his contact lenses which protected Kerslake from going blind when he stepped in to stop a gang viciously beating up three strangers.
Landscape gardener Kerslake ran into Bunce and seven other youths he recognised from school days while returning home from a rugby club charity night in June last.
He was chatting and walking with them when Bunce showed him a medicine bottle full of acid, but did not explain why he had it, an Essex court heard.
As they walked, the group got involved in an unprovoked brawl with passers-by, and soon they were raining blows on one man.
'Mr Kerslake was concerned for the man on the ground and made his way through the group to help.'
Bunce challenged Kerslake when he tried to stick up for the injured man.
Prosecutor Michael Fraser said: 'Mr Kerslake challenged the group who were assaulting the man, saying that he hadn't done anything wrong and he couldn't defend himself against the group of men.
'He said: "If you're going to have a go at him, why not have a go at me?"
'Mr Bunce came towards him and in his hand was the bottle and as he swung his arm, the liquid came out of it.
'As he did so, Mr Kerslake was able to turn his head and shut his eyes and the liquid made contact with the left side of his face.
'He suffered pain and could feel the heat on that side of his face as the liquid trickled all over him.'
Speaking after the attack, the teenager said: 'My face felt on fire. It was a searing, burning sensation which left my skin bubbling.
'I would rather have been stabbed than that.' Kerslake has been left with facial scars, which may be permanent, and lasting damage to his left eye.
He had to take three weeks off work and still suffers from violent nightmares as a result of the attack, the court heard.
But even at that moment he demonstrated his humanity by grabbing the injured man and making for a nearby kebab van where he splashed water on his face. Subsequently both were taken to hospital.
Judge Michael Brooke QC said Bunce had caused a 'most unpleasant set of injuries' during the 3.15am attack on June 21 in South Woodham Ferrers, Essex.
He sentenced Bunce to 21 months at Basildon Crown Court after the boy admitted the charge of grievous bodily harm.
However, the teenager will serve only ten-and-a-half months in a young offenders institution before being released on licence. Bunce was also ordered to pay Kerslake Ł1,000 in compensation.
Bunce had previous convictions too, including one for causing grievous bodily harm to a 15-year-old boy in 2004, when he was just 13, the judge was told.
In that attack he punched the boy in the face, displacing the youngster's jaw, and was ordered to pay compensation.
Speaking after the sentencing Kerslake, said: 'I would have liked him to have got longer, but I'm pleased he's now inside. He's a mindless thug.
'Hopefully I can put it all behind me now and live my life. I don't regret going to help, I would do it again. I wouldn't have been able to live with myself if I had walked away.'