Bright or ultraviolet light, in particular, could cause irreversible damage to the letter.
“What we need to do is monitor, control and, where possible, just reduce the amount of light exposure and that way we protect it for generations to come.”
The letter will be on display for three months as part of an exhibition at Perth Museum which is due to open on 23 January.
Ashleigh Hibbins, head of audiences at the museum, said it was a “huge coup”.
She said Mary had many links to the area and spent a lot of time in Perthshire.
“Most famously, she was actually imprisoned for 11 months in Loch Leven Castle, ” she said.
This was where she abdicated the throne in favour of her baby son, James VI.