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Laser with LCOS is Focus Free — Yes Really!
Yes, when LCOS panels are used with lasers they can be “Focus Free.” I have found that even very technical people have a hard time believing this as it goes against one’s everyday experience dealing with “normal” light and lenses. People assume that the main function of a lens is to “focus light.” After all, people are used to having to focus a camera lens or with a projector using lamps or LED light.
The optical physics of why it is focus free would take a long technical discussion, but it has to do with the laser light being effectively infinite f-number. It is analogous to stopping a camera down to a high f-number where the depth of focus become very large.
Hopefully, “seeing is believing.” I have uploaded a couple of still pictures (click on images for larger versions) and a short YouTube video demonstrating the focus free nature of the a Laseno Projector (sold in the U.S. as the AAXA L2). This projector used a SYL2010 SVGA (800×600) LCOS plane with a 5.4 micron pixel pitch and 0.21″ diagonal.
For the top picture in this article, I projected the image the ceiling and some crown molding in my house. This ceiling has lots of angle and different depths to it and with the crown molding in the way there is some obvious depth differences. Of course with the Laseno/AAXA L2 projector, there is no focusing necessary.
For the picture on the left, I projected the image at a skewed angle from the side to cause a range of depths to be displayed. The problem you have is that while the projected image is focus free, when the laser light hits the screen it looses it high f-number characteristics and thus the camera needs to focus. By projecting the image on a flat piece of paper and shooting the picture straight onto the piece of paper I was able to focus the camera while demonstrating the focus free nature of the projector.
But perhaps the best way to demonstrate the focus free nature of a laser/LCOS projector is with a video. I shot a short ~1 minute video where I mounted the projector on a little dolly and pulled it back away from the screen. There was some shaking as I moved the projector so I stopped occasionally as I moved it it back so you could see it was still in focus. I zoomed with the video camera in so you could see the detail in the video image. Note that the camera’s exposures was locked/fixed on the starting frame, so as the image gets larger, it becomes darker by the ratio of the area so as the projector pull back the video gets a little dark.
I would recommend watching the video at 720p and in full screen to see how the focus is maintained.
Focus Free Video Demonstration
Other Information on the Images
The ~2 year old Laseno projector I used for these pictures has a fixed focus lens. The image become well focused about 8″ from the projector to infinity.
The Laseno projection lens is not of high quality and you will see some serious chroma aberrations in picture as well as some spots having some blur due to the quality of the lens. Additionally the projector has “100% offset” meaning that it projects through only the top half of the projection lens so that the projector will project upward from a flat surface without having a keystone effect. Because of the offset projection, the image is best at the bottom of the image (which is from the center of the lens) at the chroma aberrations (color separation) become progressively worst toward the time.
You definitely will see laser speckle in the images. The despeckle design was low cost and done over 3 years ago and it uses frequency doubled green lasers which inherently have a high amount of speckle. Most people who have seen the AAXA L1/L2 “live and compared it to the ShowWX have said that the speckle with the Laseno/AAXA L1/L2 is less than that of Microvision’s ShowWX.
i’m not sure you can answer all of these questions. but i would be interested in any of the questions you can answer.
1. Does Syndiant need to use the same lasers as MVIS?
2. What size dia. lens (encased) does Syndiant’s displays need to cover the dia of the display? a length too would be nice. old specs are ok to start with.
3. Does Syndiant’s displays need a Vibrating Despeckle unit with DGLs?
4. Does the light coming off the Vibrating Despeckle unit need to cover the homoginizer?
5. Does the dichroic color combiner lens need to cover the surface of the Vibrating Despeckle unit?
6. What are the dimensions of Syndiant’s PCB? the new one or the old one to have a starting point.
7. Does Syndiant plan on building the Light Engine?
8. Is Syndiant depending on some one else to make the Light Engine and Syndiant only sell the display panel?
9. which VGA display panel is best suited for the cell phone market?
10. how big is Syndiant Controller? i’m thinking one size fits all.
11. What is the lumen coming from off Syndiant’s displays from the lasers coming out that MVIS will use?
12. what is the power to run the PCB and the light engine on a Syndiant’s Displays? (SVGA and WVGA)
Hi,
Sorry about this question, I’m sure you get asked this one most often, but I was wondering, how is it possible to be focus free if one uses a Projection lens because even the Laser will have a Rayleigh Range which is inversely proportional to the divergence angle. A normal laser beam will have low divergence and consequently a large Rayleigh range, but with a lens, wont the angle increase significantly (thus reducing the range which can be called ‘in focus’)?
Or is there no Projection Lens needed at all? If so, how does one get the image on the LCOS onto the screen (i.e if I simply expanded the beam reflecting off of a LCOS, and onto a screen, will I see a in-focus image at almost all distances)?
In other words, does the LCOS (which I dont really understand completely, but I suppose its OK for this discussion to simply say its a reflective LCD) not need to be imaged by a lens system (with magnification if need be)? And if one is imaging the LCOS using a lens, then the collimation of the laser beam is destroyed no which would imply I think, that it cannot be in focus for infinity?
Thanks you in advance. This is very interesting and exciting, hope to see this technology more proliferated soon!!
Actually, you are the first person to ask this question. Frankly, I am not an “optics expert” and I am not intimately familiar with the Rayleight Range (I have heard of it before but that is about it).
First, there is definitely a projection lens in an LCOS Laser projector. The lens is there to shape/spread the image on the panel and does focus the image; but the depth of focus is very large. In the case of the AAXA L1/L2, it becomes acceptably in-focus from about 1 foot away to infinity (in camera terms it is “hyperfocal” from about 1 foot away).
A LCOS+laser projector is leveraging the very high f-number and near zero etendue of the laser light. The process of spreading the light to cover the panel, homogenization (mixing the red, green, and blue light), and reducing speckle the f-number is reduced but still very high. In the case of LED LCOS pico projectors, the f-number of the lens in on the order of f/1.4 to f/1.8 whereas with a laser projector the lens is greater than f/4 (the lens f-number really doesn’t matter because the illumination f-number effectively limits the effective aperture. Because the illuminated light is such a high f-number the depth of focus is very large. In the case of the AAXA L1/L2 when they built the projector, the lens is focused and then glued in placed. The optimum focus point would be like you are doing hyperfocal focusing with a camera (you focus at point beyond the minimum focus point). Similar to the way smaller sensors in cameras, a smaller panel also helps with depth of field for the same image size. Generally pico projectors have very short throw and thus very wide angle lenses which need to be even wider (shorter focal length) due to small panels (focal lengths are on the order of 6mm). As with cameras, the very wide angle lens results in a wide depth of field.
We’re building our laser pico projectors with a diffuser followed by an imaging fly-eye pair, I’ll report back on the focus range with Syndiant 720p panels.
My understanding is that a narrow beam for a MEMS mirror with small exit aperture or small pixels for LCOS help keep the high F-number because more diverging rays drop out of the optical path of the system.
Also, if one focusses one’s eyes to an object significantly on front of the display will the display still be in focus?
What about DLP? Should also work, right?
Yes, DLP illuminated by lasers can be focus free as well.
Interesting. There are quite a number of powerful laser diodes these days in the 1W range. I’ve personally driven a mitsubishi 700mW red diode at 1000mW with sufficient active cooling (aluminum housing, fins and PC fan), also Oclaro green diode at 1000mW and Oclaro 450nm blue diode at 3500mW. When laser diodes are fed to an optical fiber the speckle is greatly reduced. I think an optical engine using these would be perfect for the not quite pico but portable projectors by being decently priced, not suffering from laser speckle yet being “focus free”. Such focus free projectors would also be great for 3d mapping which I’ve also done profesionally.
how could you maintain focus free by coupling to a fiber? i assume you mean coupling to a multimode fiber would reduce speckle but then would focus free not be destroyed by an increase of etendue?
A 50 micron diameter fiber would have the same “emitter” diameter as a multimode diode laser dye where it appears >80% of the power is concentrated on the central 50 microns of the emitter.
Why not focus a pico projector LED to a 50-100 micron fiber? Sure there will be losses but small yet affordable bright lasers are already falling behind LEDs in brightness and the tip on the other side of the fiber will be the diameter of a typical alser diode emitter this way and can be used as such without having to worry about speckle.