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304 North Cardinal St.
Dorchester Center, MA 02124
Today I’m going to give a bit of a guided tour through the Celluon optical path. This optical engine was developed by Sony probably based on Microvision’s earlier work and using Microvision’s scanning mirror. I’m going to give a “tour” of the optics and then give some comment on what I see in terms of efficiency (light loss) and cost.
Referring to the picture above and starting with the lasers at the bottom, there a 5 of them (two each of red and green and one blue) that are in a metal chassis (and not visible in the picture). Each laser goes to it own beam spreading and alignment lens set. These lenses enlarge the diameter of each laser beam and they are glued in place after alignment. Note that the beams at this point are spread wider than the size of the scanning mirror and will be converged/focus back later in the optics.
Side Note: One reason for spreading the laser beams bigger than the scanning mirror is to reduce precision required of the optical components (making very small high precision optics with no/extremely-small defects becomes exponentially expensive). But a better explanation is that it supports the despeckling process. With the wider beam they can pass the light through more different paths before focusing it back. There is a downside to this as seen in the Celluon output, namely is still too big when exiting the projector and thus the images are out of focus at short projection distances.
After the beam spreading lenses there is glass plate at a 45 degree angle that splits a part of the light from the lasers down to a light sensors for each laser. The light sensors are used to give feedback on the output of each laser and adjust to adjust them based on how they change with temperature and aging.
Side Note: Laser heating and the changing of the laser output is a big issue with laser scanning. The lasers very quickly change in temperature/output. In tests I have done, you can see the effect of bright objects on one side of the screen affecting the color on the other side of the screen in spite of the optical feedback.
Most of the light from the sensor deflector continues to a complex structure of about 15 different pieces of optically coated solid glass elements glued together into a complex many faceted structure. There are about 3 times as many surfaces/components as would be required for simply combining 3 laser beams. This structure is being used to combine the various colors into a single beam and has some speckle reducing structures. As will be discussed later, having the light go through so many elements, each with their optical losses (and cost) results in loosing over half the light.
For reference compare this to the optical structure shown in the Lenovo video for their prototype laser projector in a smartphone at left (which uses an STMicro engine see). There are just 3 lenses, 1 mirror (for red) and two dichroic plate combiners to combine the green and blue and a flat window. The Celluon/Sony/Microvision engine by comparison is using many more elements and instead of simple plate combiners they are using prisms which while having better optical performance, are considerably more expensive. The Lenovo/STM engine does not show/have the speckle reduction elements nor the distortion correction elements (its two mirror scanning process inherently has less distortion) of the Celluon/Sony design.
Starting with the far left red laser light path, it goes to a “Half Mirror and 2nd Mirror” pair. This two mirror assembly likely being done for speckle reduction. Speckle is caused by light interfering with itself and by having the light follow different path lengths (the light off the 2nd mirror will follow a slightly longer path) it will reduce the speckle. The next element is a red-pass/green-reflect dichroic mirror that combines left red and green lasers followed by a red&green-pass/blue-reflect dichroic combiner.
Then working from the right, there is another speckle reduction half-mirror/2nd-mirror pair for the right hand green laser followed by a green-pass/red-reflect dichroic mirror to combine the right side green and red lasers. A polarizing combiner is (almost certainly) used to combine the 3 lasers on the left with the two lasers on the right into a single beam.
After the polarizing combiner there is a mirror that directs the combined light through a filter encased between two glass plates. Most likely this filter either depolarizes or circularly polarizes the light because on exiting this section into the open air the previously polarized laser light has little if any linear polarization. Next the light goes through a 3rd set of despeckling mirror pairs. The light reflects off another mirror and exits into a short air gap.
Following the air gap there is a “Turning Block” that is likely part of the despeckling. The material in the block probably has some light scattering properties to vary slightly the light path length and thus reduce speckle and thus the reason for the size/thickness of the block. There is a curved light entry surface that will have a lens effect.
Light exiting the Turning Block goes through a lens that focuses the spread light back to a smaller beam that will reflect off the beam scanning mirror. This lens set the way the beam diverges after it exits the projector.
After the converging lens the light reflects off a mirror that sends the light into the beam scanning mirror assembly. The beam scanning mirror assembly, designed by Microvision, is it own complex structure and among other things has some strong magnets in it (supporting the magnetic mirror deflection).
Side Note: The STM/bTendo design in the Lenovo projector uses two simpler mirrors that move in only one axis rather than a single complex mirror that has to move in two axes. The STM mirrors both likely uses a simple electrostatic only design whereas Microvision’s dual axis uses electrostatic for one direction and electromagnetic for the other.
Finally, the light exits the projector via a Scanning Correction Lens that is made of plastic. It appears to be the only plastic optical element as all the other elements that could be easily accessed. Yes, even though this is a laser scanning projector, it still has a correction lens, in this case to correct the otherwise “bow-tie” distorted scanning process.
In addition to the obvious cost of the lasers (and needing 5 of them rather than just 3) and the Scanning Mirror Assembly, there are a large number of optically coated glass elements. Addtionally, instead of using lower cost plate elements, the Celluon/Sony/Microvision engine use much more expensive solid prisms for the combiner and despeckling elements. Each of these has to be precisely made, coated, and glued together. The cost of each element is a function of the quality/optical efficiency and which can vary significantly, but I would think there would be at least $20 to $30 of raw cost in just the glass elements even at moderately high volumes (and it could be considerably more).
Then there is a lot to assemble with precise alignment of all the various optics. Finally, all of the lasers must be individually aligned after the unit with all the other elements has been assemble.
The light in the optical engine passes through and/or reflects off a large number of optical interfaces and there are light losses at each of these interfaces. It is the “death by a thousand cuts” because while each element might have a 1% to 10% or more lose, the effects are multiplicative. The use of solid rather than plate optics reduces the losses but as at added cost. You can see in the picture of the walls of the chassis spots of colored light that has “escaped” the optical path and is lost. You can also see the light glowing off optical elements including the lens; all of this is lost light. The light that goes to the light sensors is also lost.
Some percentage of the light that is spread will not be converged back onto the mirror. Additionally, there are scattering losses in the Correction Lens and Turning block and in the rest of the optics.
When it is multiplied out, more than 50% of the laser light is lost in the optics.
This 50% light loss percentage agrees with the package labeling (see picture on the left) that says the laser light output for Green is 50mW even thought they are using two green lasers each of which likely outputs 50mW or more.
The Celluon system consumes ~2.6 Watts to put up a “black” image and ~6.1 Watts to put up a 32-lumen white image. The delta between white and black being about 3.5 Watts or about 9 lumens per delta Watt from back to white. For reference, the newer DLP projectors using LEDs can produce about double the delta lumens per Watt. Next time, I plan on drilling down in the power consumption numbers.