RGB laser projector: a reader replied

Current situation: RGB laser technology is new in that it does not use a single laser module with bluish light as the light source, as is generally the case in laser projectors, but three modules, two blue and one red. In front of one of the two blue laser modules, a lens called a 'G-refiner' changes the blue light into green light. This produces the three primary colours: red, green and blue.

About a week ago, I took apart LG's first ultra-short-throw projector with RGB laser technology: theHU85LS. The aim was to explain the technology hidden inside an RGB laser projector.

The problem: in order to get usable photos, we had to run the HU85LS ragged. So much so that we could never plug it in or turn it on. So I was able to see where the laser modules are - but I don't know which one lights up in which colour.

Reader suggestion MHenkel:

@Luca: you can try lighting the inside with an RGB torch, can't you?

It's so obvious.... I'm ashamed I didn't think of it. Thanks for your comment MHenkel.

Projector destruction, episode two

A quick clarification about "my" HU85LS projector: it's a pre-sale model. That means it's advanced enough to be used for demonstration or evaluation before market launch. But not for sale afterwards. So the HU85LS would have ended up on a shelf gathering dust if LG hadn't given it to me to explain the technology and take photos of the inside.

Let's get back to the disassembly. First of all, we need to remove the grooved aluminium cooling system. It blocks the view of one of the three lenses. But the tiny screws that hold it in place are stubborn - I can't get at them with the screwdriver.

It's time to use the force. The grille is destroyed. I need more room to get the screwdriver through. With a little imagination, the cooler now reminds me of Kylo Ren's ship Can I loosen the screws now? Still no... Meanwhile, the heads of the screws have turned into a misshapen mush. Kevin, a colleague, comes to my rescue. With his mini milling cutter, he cuts what connects the projector to the cooler. Thanks Kevin.

Now I can use the [RGB torch](/en/s1/product/led-lenser-t-qc-970cm-140lm torch-5618907) to simulate a laser module. By turning the head of the torch, I can choose the colour of the LED lamp: red, green or blue. The light cone isn't as focused as with a laser, but it should be enough to know where each of the colours is being generated. At least, I hope so.

Let's get practical.

My theory doesn't hold water

It's becoming clear that my theory deduced from the reverse-engineering article doesn't hold water. The blue laser module is where I suspected the red laser module was. The red module is where I thought the blue laser, which then turns green via the G-Refiner lens, should have been. I guess you could say I've got my facts pretty mixed up.

The light therefore follows the following path: the lens directly in front of the lamp focuses the light so that there is less scattering loss. Next comes the oblique dichroic mirror, which allows only the blue spectral components of the light to pass through. These mirrors can reflect or transmit selected spectral components - such as red only or green. As required. By the way, in the projector, the two mirrors are dichroic. So I'm dropping the term "dichroic" for the sake of fluency.

It is only on the opposite side of the housing, behind the mirror, that the blue is converted to green by the G-Refiner lens and reflected back to the mirror. From there, the green turns right towards the lens.

The location of the red laser module now seems obvious. This is because if I shine blue light from the top left, the projector's second mirror prevents the blue from passing through the lens. Instead, it is reflected back towards the flashlight.

If, on the other hand, I use red at the same position, the red spectral components pass straight through to the lens.

To be sure, I do a cross-check: if I shine blue light down to the left, the same mirror in front reflects the blue, but to the right and the lens this time, because the light is now reflected off the mirror on the other side.

If I lit with red in the bottom left instead of blue, the light would radiate directly through the mirror, because the mirror would only reflect blue so there would be no red on the lens. Therefore, the red laser module cannot be on the bottom left.

We therefore obtain the following diagram:

The position of the RGB laser modules is thus clarified.

Once again, thank you MHenkel. And well done.

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Here, now, is the icing on the cake. I'm not sure what you can do with the remains of the HU85LS. There really isn't much left. But there are obviously readers who are interested.

Hence my question: do you want the projector back as a kit? You can, on the sole condition that you come and pick it up. But first, you'll need to win the prize draw.

Prize draw: RGB laser projector

To enter the draw, click on the "Enter" button.

The competition has ended.

The draw will take place in a week's time. If you win, I'll contact you by email and we'll arrange to meet at the digitec shop in Zurich to hand over the projector.