The Mantra of Light is important in the Shingon School of Buddhism in Japan, where it is known as kōmyō shingon, or 光明真言. It is associated with the Buddha Vairocana.
According to John Stevens the mantra translates as Infallible brilliance of the great mudra! Creating the radiance of the Jewel and the Lotus.
The mantra starts with oṃ at the 12 o'clock position.
oṃ amogha vairocana mahāmudra maṇipadma jvāla pravarttaya hūṃ
The last character is a full-stop, this is very useful in this case because dividing the cirle into 24 parts is relatively straight forward, whereas 23 is very difficult.
There is more information on the Vairocana page.
Shingon Refractions by Mark Unno