In Tibetan Buddhism, often the so-called 'clear-light mind' is mentioned. This is the most subtle level of mind, which we are normally not even aware of. It appears to the very advanced meditator and during the death process, but in this case, also only advanced meditators will be able to notice it. It is a non-conceptual, 'primordial' state of mind. "I don't think that in the term clear light, light should be taken literally. It is sort of metaphoric. This could have its roots in our terminology of mental will. According to Buddhism, all consciousness or all cognitive mental events are said to be in the nature of clarity and luminosity. So it is from that point of view that the choice of the term light is used. Clear light is the most subtle level of mind, which can be seen as the basis or the source from which eventual experience or realisation of Buddhahood, Buddha's wisdom might come about, therefore it is called clear light. Clear light is a state of mind which becomes fully manifest only as a consequence of certain sequences or stages of dissolution, where the mind becomes devoid of certain types of obscurations, which are again metaphorically described in terms of sun-like, moonlike and darkness. These refer to the earlier three stages of dissolution which are technically called, including the clear light stage, the four empties. At the final stage of dissolution the mind is totally free of all these factors of obscuration. Therefore it is called clear light. Sort of a light. It is also possible to understand the usage of the term clear light in terms of the nature of mind itself. Mind or consciousness is a phenomena which lacks any obstructive quality. It is non-obstructed." (The Dalai Lama)
