Here, Sam Harris uses a "jargon"-term: luminosity.
In Mahamudra/Dzogchen, it has a specific meaning. Sam Harris does not define it anywhere. Sam Harris has used the term only once in the period in which I created the Sam Harris Daily Meditation word-cloud. So here we go.
Sam Harris
This unstructured and luminous clarity of awareness
Here Sam Harris refers to a seemingly visual quality of awareness. Luminosity comes from light. How is this meant?
Is "luminosity" meant metaphorically, or does the statement refer to the quality of a visual sense impression?
I can only assume that Sam Harris speaks about the non-physical, metaphorical meaning of luminosity.
Note that another term for luminosity is "lucidity" (as in (L. T. Namgyal & Rinpoche, 2011)
The theoretical / metaphorical meaning of luminosity
Wikipedia tells us:
The luminosity of mind is of central importance in the philosophy and practice of the Buddhist tantras, Mahamudra, and Dzogchen (Wikipedia)
Wikipedia isn´t exactly easy reading here, and furthermore, the definition varies by tradition.
Tashi Namgyal, in "Moonbeams of Mahamudra" writes about this term. He advises to not take the experience too literal.
"The uneducated presume that the clarity of aspect of any type of meditative experience is what is meant by natural luminosity and think that what is called luminosity is something that illuminates, like sunlight. They are very mistaken" (Namgyal & Callahan, 2019a, p 340)
In this view, luminosity is not a physical characteristic of mind, otherwise the mind would have luminosity as characteristic (which is not the case in this philosophy of mind).
In his view, "luminosity" is more of metaphor for a somewhat mysterious hard to explain purity of mind.
And here is in "The Royal Seal of Mahamudra II" :
This abiding of the mind neither is dull, indifferent, distorted, nor does it remain in a neutral calm; rather it is the self-luminous essence of the mind. This luminosity is not due to light, or other such means, but it is without an object or anything to be identified. As there is nothing to be identified, there is no obstruction to its cognizance, which is vibrant, clear and open, raw, fully awake, and knows itself. Therefore, it is said to be self-knowing awareness.(Khamtrul Rinpoche, p 27)
This is also called "cognitive lucidity"
The defining characteristic of the mind itself as a whole, is cognitive lucidity—the capacity to know, the capacity to experience. (L. T. Namgyal & Rinpoche, 2011, p. 29)
The mind is empty, but it is not dead; it is not incapable of experience. It is not static or devoid of cognition. The mind can and does experience, can and does know. The mind’s capacity to experience and to know is what we call its cognitive lucidity.(L. T. Namgyal & Rinpoche, 2011, p. 120)
But even the Dalai Lama "cannot simply explain" luminosity in words. Instead, in order to create clarity, he gives some basic instructions on how to experience this luminosity. For example, he sais, look for the space between thoughts ( btw, something that is also a meditation error.. so I am confused..). That´s where you find the luminosity, the emptiness that is knowing awareness....
He uses an analogy that doesn´t help much either.
If you undertake such practices, such experiments, when you say ‘consciousness’, it will not be a mere word. You will be able to understand what it is. Consciousness is a phenomenon that is nonobstructive; it is nonphysical and has the quality of luminosity. It is analogous to a crystal. If a crystal is placed on a coloured surface, the real clarity of that crystal will not be seen. If it is removed from anything coloured, however, then its real form will be seen. The luminosity of the mind, the nature of clarity of the mind, is something that I cannot simply explain in words to you. But if you undertake this kind of experiment on your own, you will begin to understand,’ Ah, that’s the luminosity of the mind!’ (Dalai Lama, 2020)
In the end, the Dalai Lama implies, that luminosity must be experienced to be understood, and you can only experience it through meditation.
This is also indicated by a frequently used meditation manual for Mahamudrea:
The term "luminosity" refers to the "knowing" quality of the mind. This has been definitively clarified by Tibetan teachers. It does not refer to the "experience of light" to which some mystics refer. Rather, it is the knowing, awareness, clarity or lucidity of mind. (Barth 2007)
So, here we are! It is still not "explained".
Unless you meditate, it´s a waste of time to explain. Sorry!
Luminosity and other visual phenomena as sensory meditative experience
What about the direct physical experience of "luminosity", then? I have personally experienced a kind of "luminosity" in two experiences of mystical states. Thus, I understand why they could be confused.
In Tibetan Buddhism, moreover there are many occurences of visual phenomena through deep meditation.
I will therefore sketch four views of the topic:
Visual phenomena in Tibetan meditation
Visual phenomena in fire cassina meditation
My own experience of visual phenomena in a Mahamudra/Dzogchen retreat
My experience in a psychedelic ceremony (just to show the overlap)
Visual phenomena as meditative experience in Tibetan Buddhism
Here, I quote from the extraordinarily beautiful book "Rainbow Body" by Loel Guinness (yes the one rich guy of the Guinness dynasty - he is deep into Buddhism).
The example describes the visions are they appear in a particular stage of meditation practice. There is a sequence to the types of visual modifications corresponding to the stage of meditation the meditator has reached.
The example is from the "Waterfall or the Stage of Arising and Increasing". He quotes a 13th-century master.
There may appear white atmospheric phenomena, straight lines, or zig-zag-like phenomena like lightning, or phenomena like a mirror of crystal [note: this comes close to my own experience] or like the wings of a bee, or like golden eyes, or like smoke, or like a mirage, ....there may arise various forms, such as rainbows, or glossy silk cloth being opened, or patterns of nets and half-nets, or chessboard patterns, or patterns of triangles...or like the threads of compassion that are rays...(in Guinness 2018, p 181 f).
And this is just be beginning, as this an early stage.
Visual phenomena in the fire casina meditation
Here, I have to rely on Daniel Ingram, a modern meditation teacher. He teaches a meditation method whereby one focuses on light.
My reason to include him here is the mentioning by him of the "luminosity" phenomenon in this podcast.
He claims to be able to create "real-seeming" hallucinations, from abstract patterns as described in the 13th century (see above) to "a red tiger in the room").
But, I would recommend you listen to the podcast.
Do I believe him? Based on my own experience even as a novice meditator with some luck, and perhaps some priming of the mind through psychedelics, yes.
Luminosity in a psychedelic experience of the "mirror mind"
This is how I have described my experience of luminosity in a meditation retreat.
Suddenly my inner space somehow began to unfold as a giant luminous internal space that wrapped around the external space of the small room. It slowly acquired a blueish luminescence. I had my eyes open, and I noticed that the lit physical room darkened down somewhat while my inner space began to glow more and more intensely.
Luminosity in an Ayahuasca experience
This is how I described my experience during an Ayahuasca ceremony. This is a quote of my own description:
The visual field had changed into being energetically loaded, scintillating, brilliant luminosity as in radiating in and out of itself.
Seeing more pixels through meditation: neurological research
I recently came across recent neurological research mentioned in a podcast with Shinzen Young.
Research confirms that indeed our sensory perception becomes clearer when the ego/self-constituting processes are reduced, eg through meditative states.
We literally "see more pixels" (this is a quote) because the conceptual mind and the ego sense get out of the way.
Possibly, the experience of "seeing more pixels" translates into an experience of luminosity.
The "seeing more pixels" had already been described by Aldous Huxley in "The Doors of Perception" as a result of taking mescaline. But Huxley at the time could not explain it neurologically.
Luminosity in Plato´s world
The following is a historical excursion: Luminosity is a phenomenon that also characterized Plato's world. As described by Huxley,
"in the Phaedro Socrates speaks about a world in which everything shines.. The very stones of the road and on the mountains have the quality of precious stones... A luminous other world" (quoted in Shannon p 391f).
The quoted author, the Israeli cognitive scientist, quotes Huxley quoting Phaedro in his astonishing book "The Antipodes of the Mind". In this impressive book Shannon creates a complete phenomenology of the Ayahuasca experience. Many of these types are light / luminosity phenomena.
Resources
Barth, P. (2007). A Meditation Guide for Mahamudra [PDF]. Mahamudra Meditation Center, Petaluma, USA. (Original work published 1998)
Note: This book is not for sale in the trade.
My core mystic experiences with psychedelics and in meditation. (2022c, August 21). Till Gebel. Retrieved 7 September 2022, from https://www.till-gebel.com/post/mystic-experiences-with-psychedelics-meditation
Dalai Lama. (2020, August 22). Luminosity of the Mind, by HH Dalai Lama. Buddhism Now. Retrieved 9 September 2022, from https://buddhismnow.com/2009/11/28/luminosity-mind-dalai-lama/
Gebel, Till. (2020a, May 24). Mirror Mind in a Meditation Retreat. Till Gebel. https://www.till-gebel.com/post/mirror-mind-mahamudra-experience-daniel-p-brown
Guinness, L. (2018, May 4). Rainbow Body. Serindia Publications, Inc. https://www.amazon.de/Rainbow-Body-Loel-Guinness/dp/1932476873
Ingram, D. (n.d.). Daniel Ingram. Integrated Daniel. https://www.integrateddaniel.info/
James, S., & Ingram, D. (2022, September 9). Ep166: Fire Kasina Mystic - Daniel Ingram. YouTube. Retrieved 10 September 2022, from https://www.youtube.com/watch?v=FW884GpnDvc&feature=youtu.be
Namgyal, L. T., & Rinpoche, K. T. (2011). The Ninth Karmapa’s Ocean of Definitive Meaning (New ed). Snow Lion.
Namgyal, T. D., & Callahan, E. (2019a). Moonbeams of Mahamudra (Tsadra). In No Title. Snow Lion. https://www.amazon.de/dp/1559394803/
Wikipedia contributors. (2022, June 28). Luminous mind. Wikipedia. Retrieved 9 September 2022, from https://en.wikipedia.org/wiki/Luminous_mind