Some Insight Regarding LaserVue White Spot Problem
The purpose of this note is to share my experience with and analysis of the LaseerVue DMD white spot problem. I have owned a first generation Laservue A90 set since the summer of 2008. My set was calibrated in January 2009 by one of the most well known calibrators active on this site at that time.
White spots first began to appear on my TV about one month ago, almost eight yhears to the date. Using the Service Menu code(2-4-7-0) at the timme of first appearance, noted that I had logged 7500 hours of use on the set. This amounts to a usage duty cycle of ~ 11% --that is about 2.6 hours per day over those 8 years. Affected users should note the facct that had I been a heavier user, say 8 hours per day, this particular set would have turned up the white spot problem, proportionately earlier in years clocked, (2.6/8)*8 years = 2.56 years.
Against this backdrop, what I want to focus on in this note are the implications and causality of this white spot failure mode specifically for A90 owners. First off, let me point out, that, perhaps with the exception of computer microprocessor chips, the DMD chip is one of the most thoroughly characterized large scale intergrated chips ever invente. The Texas Instruments Co has documnted in quite a bit of detail, the lifetime statistics from accelerated life studies over the years. Also much field experience has been gained from the millions of chips sold in to the industrial and consumer markets over those years. What seems to have been underestimated and misunderstood by consumer OEMs, was the necessity of very thourough thermal design in the application of these chips. In the case of the A90, the OEM problem was made worse by the ridiculous suggestion by Mitsubishi, that an entire optical engine needed to be replaced to fix this simple problem. Had consumers known early on, that they swap out DMDs as easy as changing lamps, lifetime performance even at this modest level would have much more palatable. Let me just list in passing some key advantages to just swapping out the DMD:
1. Much lower cost to procure and install by a factor ten than replacing the entire optical engine
2. Does not require the disconnection of the laser sources from their fiber optical bundles.
3. The fact that the laser couping is preserved intact implies that all set u p features including set calibration is unchanged.
Item three is a huge benefit. It dervies from the fact that these external cavity VCEL based solid state lasers do not degrade/change over time. The DMD is just a set of monochromaticc mirrors, so when you swap it out, you have no deleterious effect on the relative RGB amplitudes etc. AS an example of this, way back in 2009, the calbrator set my Natural Mode luminance at 90.5 lumens with this expensive instruments. In the wake of his calibration, I measured the set luminance with my Sekonic L758DR, and "calibrated my meter reading to exact agreement at that time. Immediately after installing a new DMD I repeated this luminance measurement with identical results after eight years of use. I am not surprised by this but it is truly exciting to prove experimentally.
Now to some implication/causality discussion. Here it is important to note that Texas Instruments has clearly shown that the DMD chips follow an Arrhenius type ratecurve versus mirror temperature, similar to that observed for chemical reactions in the field of chemical kinetics. AS such it is clear that the observed lifetime of my set(7500 hr) implies a specific temperature at the mirrors. I have fitted TI data for DMD chips similar to mine and obtained the following equation:
Ln(L/11700) = 9045(1/T -1/338)
Working backwards, setting L to be my observed lifetime of 7500 hrs, one can easily calculate the average mirror temperature of my chip to be 343.7 deg K or 70.7 deg C
Now this is interesting ideed! The question now is why are my mirror temeratures so high. It was expected by TI that consumer based DMDs would be operated below, say, 65 deg C. In any case, following the folks at TI, we note that chips like this dissipate ~2.9W of electrical power and ~ 3*10^-3 W absorbed optical power per Screen Lumen. if we worst case at 400 Lumens at the screen in the brightesst mode, we can get a total power dissipation at the mirrors of ~4 Watts. This tells us indirectly, the overall value of the thermal resistance from the mirrors to the, say 25 deg C ambient:
We have Tmirror = Tamb + Power * Rth , or Rth =( 70.7 - 25) /4 = 11.4 deg C per Watt.
It almost jumps out the page, that the only way to get a thermal resistance this large, for this arrangement with finned heatsink and cooling fan, is for there to be an air gap between the heat sink and the DMD. Indeed, a 0.1 mm air gap 0.6" by 1" gives rise toa thermal resistance of ~9 deg C per Watt. Add to this the Thermal resistance from mirrors to ceramic carrier and heatsink to ambient and ergo.
How might this arise? In my case, the entire interface between the heatsink and DMD, occurs down inside a blind hole with good contact depending solely on sufficient pressure from two spring loaded screws. As far as I can tell there were no heatsink pads present or thermal paste, just a requirement for metal to metal contact with sufficent pressure. Possible shortfalls arise because there is no easy way to determine whether you have made good contact or how much pressure you have applied by proper torquing. We do know that TI recommends that this force be 40 Newtons, 20 each from the two screws.
To sum up: I do think it is possible to get enough force on the heat sink to DMD interface for good thermal contact. But I have not been able to figure out a way to be certain. I would suppose that the springs have been chosen so they might be fully compressed without over stressing the underlying PCB or DMD. This would certainly give best case thermal resistance. I have also thought that a copper or aluminum shim might be inserted to good effect. I can say this much, it ought to be easy to get that contact thermal resistance down to less than one deg C per Watt. That would lower the over Rth down to~ 3 deg C per Watt. At this level the mirrors would be only 12 degres bove ambient, ~37 deg C, with huge increase in expected lifetime!
A significant take away of all of this is that you should consider yourself fortunate if you still are in possession of one of these TVs. It is completely repairable to like new condition by DMD install. It is important in such installation, the the Laser to fiber optic coupling is not disturbed. Care in this, along with general cleanliness, will allow you to reproduce your entire optical performance intact. Meanwhile the lasers, themselves, can be expected last more than 20,000 hours and to drop dead suddenly when they do fail. Because of possible future scarcity, I would go ahead and buy a replacement DMD now. These can be bought for less than $200 these days. One last point, you might want to consider attaching a monitor thermistor onto the heat sink. It won't measure the mirror temperature directly but variations in this thermistor's resistance will reflect proportional variations in the mi\DMD lifetime. We have, in both cases,
Ln(R/R0) = A*(1/T -1/T0) and Ln(L/L0) = B*(/T- 1/T0) and so forth.