The court heard that Mr Wood, a former teacher, had suffered from irreversible dementia since 1993.
In 1994, Mr Wood made a "living will" indicating he did not want to receive life-prolonging treatment in the event of serious illness. In October of last year he was diagnosed as having Parkinson's disease.
On 18 October, Mrs Wood took her husband from a care home to their house, near Totnes in Devon. She later told police she gave her husband a total of six sleeping tablets, put on his favourite music - Beethoven - undressed him and lay beside him. "I then put the pillow over his face," she said. "I told him I loved him and everything would be all right."
But then Mr Wood fell to the floor and, fearing he may be injured, Mrs Wood called the emergency services and police. The court heard that she told ambulance officers: "I know legally what I did was wrong but I know morally it was right."
Mr Wood has no memory of the incident.
Mr Justice Toulson said to Mrs Wood: "I accept without hesitation that you love him and that you believed that what you were doing was the right thing, but neither the fact that your motive was to spare him wretchedness nor your conviction that you were doing right means it was right."