The dust holographic display (henceforth DHD) consists of a chamber enclosed by glass, and containing a few million particles of dust. The dust particles are sufficiently irregular in shape (and partially transparent) that light hitting them is scattered at random. A gentle fan keeps the dust particles moving, though not very fast. [EDIT - on reflection, moderately fast random motion would be better.]
In the base of the chamber is at least one laser unit. The unit consists of one low-power infra-red laser and one higher power visible laser, with the beams collinear with one another (this can be done). The combined beam is directed by, for instance, a MEMS mirror so that it scans the entire volume of the chamber.
Most of the time, only the low-power IR laser is active, sending out a modulated signal. If it strikes a dust particle, the scattered light is sensed and used to calculate the distance from the laser to the dust particle. Since the angle of the beam is known, this means that the exact position of the intercepting dust particle is also known.
At that instant, the visible laser is turned on for a few microseconds. The intensity of the visible laser is adjusted, to illuminate the dust particle with the appropriate brightness (or, if the particle is in a "dark" part of the 3D image, the visible laser is not fired at all).
With the right density of dust particles and a fast enough scan, it should therefore be possible to build a complete 3D image by illuminating the appropriate dust particles at any instant. Obviously, three lasers (R, G, B) would allow colour holograms; and it might be desirable or necessary to have multiple lasers to achieve higher scan speeds.
The density of dust particles also needs to be right. If they are too sparse, the image will be made up of too few points of light, and will be dim and speckly. If they're too dense, the lasers will seldom be able to hit dust particles on the far side of the box, since nearer ones will intervene.-- MaxwellBuchanan, Sep 21 2019 Audiologram [2 fries shy of a happy meal, Sep 22 2019] Nice, Even just the dust, case and pump might make a beautiful art piece with the right mineral specs.-- wjt, Sep 22 2019 Cool. One of my earlier ideas might help to give the laser light consistent dust particles to reflect from. [link]-- 2 fries shy of a happy meal, Sep 22 2019 //throw a large number of face-tracked parallax enabled video feeds // [bigsleep] That's some processing right there or would there be a set of pre-rendered angled views which can just be displayed when needed? So not a true different view for each person.-- wjt, Sep 22 2019 I think you'll find that this won't work very well, though I'd be happy to be proven wrong.
With dense enough dust to get a bright image, I'm pretty sure that your laser beams will strike multiple particles at any given time, each particle being at a different distance from the projector, due to the laser beams having nonzero width. This will cause radial blurring centered on the projector. Maybe, if your rangefinding uses something like a TSADC instead of a TDC, you can detect when multiple motes are in the beam and avoid illuminating them in that case. Either way, though, to get a radial resolution of 0.3 mm (comparable to 2D displays), you need a ToF resolution of 1 ps.
Initially, I was expecting this idea to be based on time of flight or two-beam intersection somehow. Actually, you might be able to do a hybrid of those two techniques. Being a biologist, you might be familiar with fluorescence lifetime imaging microscopy, but that's not really all that relevant to my suggestion, just something it reminded me of. Anyway, could you make fluorescent dust that is charged by one flash of light, and then has a very narrow time window in which it will emit light only if triggered by another flash of another wavelength? Then you could charge the whole volume of dust with a flash from a lamp of some sort (which might as well be a laser or LED as well, for a narrow enough pulse width), followed by triggering selected regions of the dust with a laser projector, with range selectivity achieved by the relative timing between the charge flash and the trigger flash. Therefore, the lamp and the projector would have to be mounted at different locations, so that their pulses don't just follow each other with constant time difference across the width of the volume, which would defeat the selectivity.-- notexactly, Sep 26 2019 //your laser beams will strike multiple particles at any given time, each particle being at a different distance from the projector, due to the laser beams having nonzero width//
Not so. Obviously, it will be desirable to have as thin a beam as possible. However, if the system detects multiple reflections of the IR "probe" beam (ie, if two or more particles are both in the beam at the same time, and the nearer particle does not completely occlude the further one), the visible laser will not fire. If it detects a single reflection (ie, there is only one particle in the beam, OR the nearest particle completely occludes the further particle), the visible laser will provide illumination appropriate to the position of that single (or nearest) particle.
In reality, given a finite beam width, there will have to be a compromise in the density of the dust: too little, and the image will be sparse because only the dust particles act as points of illumination to the viewer; too much, and the system will seldom have a clear shot at only one particle, and hence it won't illuminate.
What you'd probably want is a low density of particles, moving randomly and moderately fast, and a visible laser that can deliver a very powerful, very short pulse.-- MaxwellBuchanan, Sep 26 2019 I said most of that. And the last sentence suggests that the particles might need to be replaced on a continuous basis.-- notexactly, Sep 28 2019 random, halfbakery