When Calum Macdonald arrived at the Vietnamese border, he couldn't read the administrative forms in front of him. All he could see was a blinding kaleidoscopic light.
He had just stepped off an overnight bus with his friends from the popular tourist destination of Vang Vieng in Laos.
The day before, the group had been staying at a hostel where free whisky and vodka shots were offered to guests. Calum was mixing them with soft drinks.
It was only at the border that he suspected something might be wrong with his sight - which he told his friends.
"[We agreed] it was strange but we thought it was food poisoning and the light I was seeing was some kind of sensitivity," he tells BBC Breakfast.
