Previously indecipherable letters that provide insight into the innermost thoughts of the Victorian explorer David Livingstone are being read by scientists using ground-breaking imaging technology.

In the last years of his life when he was in remote parts of Africa, Livingstone had run out of writing paper and had to scrawl his letters on used newspapers and pages from books using ink from berries. Most of the letters are now virtually unreadable and historians have had to rely on versions that were heavily censored by Livingstone's friend and biographer Horace Waller. But Livingstone's true thoughts are being revealed by scientists using techniques that enhance the ink while suppressing the background print.

The first deciphered letter, written on 5 February 1871 from a village called Bambarre in what is now the Democratic Republic of the Congo, has just been made public in the first stage of a project to reveal the contents of Livingstone's previously unpublished final diary from 1870-71.

Waller, to whom the letters were addressed, omitted (at his friend's request) some references to Livingstone's ill health and weaknesses. Livingstone wrote to him: "I am terribly knocked up but this is for your own eye only: in my second childhood [referring to his lack of teeth – several of which he extracted himself] a dreadful old fogie. Doubtful if I live to see you again."

Despite his ill health in the midst of a cholera epidemic that devastated the local population, he wrote of his determination to complete his search for the source of the Nile: "Well I am off in a few days to finish with the help of the Almighty new explorations."

The letter also includes Livingstone's thoughts on the "awful traffic" of the slave trade which he said could be "congenial only to the devil and his angels". Fiercely competitive, he was critical of the achievements of fellow explorers Samuel White Baker, Richard Burton and John Hanning Speke. The Scottish missionary also talked about the prospects for commerce and Christianity in Africa, gave details of lakes and rivers in central Africa and expressed his disgust with the British government's policies in Africa and the Middle East.

It is thought the Bambarre letter was only delivered to England in 1872 by the New York Herald reporter Henry Morton Stanley who met Livingstone in late 1871 with the words: "Dr Livingstone, I presume?"

Morton had been despatched by his paper to find Livingstone, who had been lost to the outside world for six years. Livingstone died at the age of 60 in 1873 in Zambia from malaria and internal bleeding caused by dysentery. He suffered extreme ill health in the final years of his life, including horrific tropical ulcers on his feet and legs.

Dr Debbie Harrison, a medical historian and the project's contributing editor, said: "His closing line to Waller indicates Livingstone's anxious and depressed state of mind. He did not know that in just a few months Stanley would arrive, bringing desperately needed food, medicines and the longed for news from an outside world he thought had forgotten him."

The letter disappeared for almost a century before it was bought at a Sotheby's auction in 1966 by the American photographer and diarist Peter Beard, in whose private collection it remains.

The work on restoring the letters is being done by researchers from Birkbeck College, University of London, the National Library of Scotland and the David Livingstone Centre. The letters are illuminated with successive wavelengths of light, highlighting the text – a method that has also been used on documents containing Archimedes' "Eureka" theory and parts of the US Declaration of Independence. All the letters will eventually be made public on the Livingstone Online website.