Buddhist Inner Sound and Light Meditation
The Self-Light of Dharmata
There is one thogal method that can be practiced quite simply. The instructions say to sit up straight in the Seven Postures of Vairochana and to close your eyes as tightly as you can. Sometimes you may even cover your eyes with the palms of your hands. Once your eyes are shielded from any external source of light, you simply look into the darkness with a relaxed mind. At first, there is just darkness, but if you keep looking, then various light forms begin to manifest. You may see blue, white, yellow, red, green or even black bindus. The key is to remain relaxed and to watch straightforwardly whatever appears in the space before you.
– Mind-Beyond-Death, by Dzogchen Ponlop:
https://selfdefinition.org/zen/Dzogchen-Ponlop-Rinpoche-Mind-Beyond-Death.pdf
Taking Sound as the Path
There are similar practices with sound that lead us to the experience of sound-emptiness. A simple method is given here for tuning in to the natural sound of dharmata, which is always present within our mind but is usually unobserved. The instruction, again, is to begin by assuming a correct posture. When you have settled your mind, you clench your jaws and close your ears to external sounds by plugging your ears with your fingers or pressing your hands against them. This will amplify the basic sound of dharmata and make it more perceptible. It is easiest to hear this sound when it is quiet, particularly at nighttime.
Once you have identified this sound, then you place your awareness on it without wavering. Resting your mind in the sound, you continue to listen, going further and further into the sound itself. The more precise and clear your focus is, the more vivid and sharp the sound becomes. Eventually, your experience of sound deepens to the point that you experience its emptiness, which here is known as the self-sound of the emptiness of dharmata.
– Mind-Beyond-Death, by Dzogchen Ponlop:
https://selfdefinition.org/zen/Dzogchen-Ponlop-Rinpoche-Mind-Beyond-Death.pdf
Buddhist Inner Light or Dark Retreat Meditation
https://en.wikipedia.org/wiki/Dark_retreat
Dark retreat (Wylie: mun mtshams) refers to advanced practices in the Taoism, Dzogchen lineages of the Nyingmapa, Bönpo, other schools of Tibetan Buddhism. A dark retreat is a solo retreat in a space that is completely absent of light. Because there is no optical stimulation, one can experience “prisoner’s cinema”, commonly known as the lights [phosphenes]. The time period dedicated to dark retreat varies from a few hours to decades. Dark retreat in the Himalayan tradition is a restricted practice only to be engaged by the senior spiritual practitioner under appropriate spiritual guidance. This practice is considered conducive for navigating the bardo at the time of death and for realising the rainbow body. The traditional dark retreat requires stability in the natural state and is only suitable for advanced practitioners. Ayu Khandro and Dilgo Khyentse are examples of modern, if not contemporary, practitioners of significant periods of Dark retreat sadhana.
Also see, Dawning of Clear Light: A Western Approach to Tibetan Dark Retreat Meditation