The Laghukālacakratantrarājatikā (Vimalaprabha) is an exhaustive commentary on the Kalachakra Tantra of Buddhism. There are two volumes of this text that have been recognised in the International Memory of the World Register in 2011 by UNESCO. One of these is the oldest known copy of the text, written in the late 10th century. The second is a partial copy of the same text, written in the 15th century. The Asiatic Society, Kolkata, is the custodian of these scripts. 

It is believed that the Kalachakra Tantra was explained by the Buddha himself at Dhānyakataka in South India. Manjushriyasha, a king in the line of Kalkin rulers, prepared an abridged version of the Tantra, which is known as the Laghukālacakratantra. Pundarika, his successor, composed a commentary on the abridged version, known as Vimalaprabha, in the 9th century A.D. 

The two volumes of the Vimalaprabha in the Asiatic Society collection are hand-written on palm leaves in the Sanskrit language. While the one with the complete text is in the Gauḍi Bengali script of the 10th century, the other is in Newari Script of the 15th century. The 10th-century manuscript was written in the thirty-ninth year of King Harivarman of Bengal (last half of the 10th century), which is a little over one hundred years after the commentary was composed. This makes it the oldest surviving manuscript of Vimalaprabha. 

In addition to the text of the Tantra, the commentary contains detailed discussions on philosophy, astrology, and astronomy. It has five sections called patalas. These are Lokadhatupatala, Adhyatmapatala, Abhisekapatala, Sadhanapatala, and Jnanapatala. There is also a discussion on Svarodaya and Ayurveda. The measurements of the globe given in the Lokadhatupatala are more detailed than those given in the Abhidharmakosa, a famous 4th-century text of Acharya Vasubandhu. Various techniques of the making of Yantra, a kind of technological mechanism, and devices for the building of castles are also discussed in the text. Verse 4.119 of the Vimalaprabha offers one of the earliest definitions of the concept of Hatha-Yoga. It also offers a critique of the tantric tradition of Shaivism. 

Composed and written in India, these manuscripts were taken to the neighbouring countries of Nepal and Tibet in the 12th and 13th centuries. They were discovered in the 18th century by European scholars such as William Jones and his associates, who brought them back and deposited them with the Fort William College Library, Kolkata. In 1808 the manuscripts were transferred to The Asiatic Society, Kolkata. 

Vimalaprabha is still a vital part of Buddhist life, not only in India but in all other parts of the world where Buddhism has spread, especially in Nepal and Tibet. The Asiatic Society manuscript is the oldest and the most complete commentary and is a national treasure worth preserving. Calligraphically it demonstrates fine artistic expertise and as a treatise it stands out as one which is complete in itself.