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Accelerating demise of Video Projector DLP dead pixels.
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bw2341:

--- Quote from: tooki on May 29, 2022, 11:31:24 am ---In both cases, I wonder what the advantage is of the blue laser over using either RGB LEDs or blue LEDs and yellow phosphor. If someone knows, please share! :)

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https://training.ti.com/etendue-how-brief-introduction-etendue-projection-systems

It's the étendue of the optical system. I stumbled upon this TI training video. In even more more simplified terms, the tiny exit aperture and narrow cone of light from the laser is much easier to work with as a light source. The large surface area and broad emission angles of LEDs would require larger lenses and larger DLP chips to achieve the same optical efficiency.
ebastler:

--- Quote from: tooki on May 29, 2022, 11:31:24 am ---For those who don’t know, single-chip DLP laser projectors use two color wheels: one with RGB (or more) filters, just like in traditional DLP projectors, plus another color wheel with one transparent section for blue, and the rest with the yellow phosphor.

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Sharp/NEC have nice diagrams of the different projector types' beam paths, including animated versions for each of them: https://www.sharpnecdisplays.eu/p/laser/en/technologies.xhtml. Here's the single-DLP, laser+phosphor illuminated design:


Berni:

--- Quote from: tom66 on May 29, 2022, 04:10:05 pm ---
--- Quote from: tooki on May 29, 2022, 11:31:24 am ---I wonder what the advantage is of the blue laser over using either RGB LEDs or blue LEDs and yellow phosphor. If someone knows, please share! :)

--- End quote ---
I couldn't say for projectors, but as I understand it the advantage in car headlamps is that because the light output is extremely coherent, it is possible to design a far more precise lens.  This allows for brighter light in the allowable area of the road, whilst reducing the glare for oncoming traffic.

In fact, this is the same motivation behind segmented headlamps.  A different way to solve the same kind of problem.

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I think the benefit of those laser phosphor headlamps is that the light emitting area is very very small. The phosphor does not preserve the coherence of light. But the small spot size does make it easier for the optics to collect all the light and direct it into a tightly controlled beam. Quite a lot of complication for just a light in my opinion.

In projectors i am guessing phosphors are used to reduce the cost of the unit by requiring only one high power laser and no alignment to other lasers. There certainly are projectors that use a RGB laser too. Most of the tiny portable ones do this (needs less optics so it is smaller).

There are OLED screens that use phosphors to produce the red and green from the blue LEDs. LCDs are also moving towards using quantum dot technology to convert a blue backlight into red and green.
ebastler:
bw2341 has given the right answer above, I think: Coherence of the laser light source is not relevant here. Collimation (small divergence angle) of the laser is relevant, in combination with its small beam diameter. Actually, the product of beam cross section and divergence angle, known as étendue, is the key property.

As discussed in the short video linked by bw2341 (worth watching!), étendue is preserved in a perfect optical system, and can never get smaller in a real-world system. The core DLP element can only accept a limited étendue: Its mirror array has a limited size, and the tilt angle of the mirrors limits the angle under which you can illuminate them (while still being able to separate the reflected light from the incoming illumination).

Since the component with the smallest étendue limits the optical throughput of a light path, using a light source with a large étendue -- LEDs or lamps -- together with a DLP array with small étendue will cause light losses: At some point in the beam path one has to "throw away" some of the illumination light via optical apertures, to limit its étendue to what the DLP array can accept.

That's undesirable, of course. Lasers, and even the diverging light from a small phosphor area illuminated by a laser, have smaller étendue and are hence advantageous.
tom66:
I wonder why they don't use a red phosphor and still have a colour wheel.  Presumably the efficiency or accuracy of the red phosphor is worse.

It still seems absurdly inefficient:  generate 100% blue laser light (already quite inefficient), then strike that onto a phosphor wheel to generate green and yellow, and then filter out that combined mess to make the colour you need.  A true-RGB single-chip projector with an RGB LED is surely the better implementation, once the optics get there...
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