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Thread: Holy crap! mirrors on DT40 pros vs CT6215, wow!

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    Lightbulb Holy crap! mirrors on DT40 pros vs CT6215, wow!

    After some of the recent discussion on the percentage of reflectivity on the mirrors, I did a quick survey of the 2 dozen scanheads we currently use. I did not have the old DT-40 (non pro) model here as one of our guys took it home to play with it, but they were just classic aluminized mirrors. Great reflectivity but no power handling capabilities. Here I am comparing the CT6215 with stock mirrors as shipped ($75 mirrors), CT6800 with expensive aftermarket high power mirrors ($150 mirrors) and the DT-40 Pro with stock mirrors as shipped. All three use dielectric mirrors, in fact I was rather surprised when I had first discovered that the new DT's used dielectric mirrors!

    The line up, the each pair is psyching themselves up before the battle!


    I'm sure the newcomer DT-40 pro is nervous about competing with such heavyweights.


    The contestants greet each other.

    (The interesting thing to note here is the mirror thicknesses. The thickness is part of how you get away with >30k scanning. The DT is obviously not suitable for 60k scanning.)

    So there I was rigging up a table and playing around with power meters when I found a shocking discovery... so crystal clear that I didn't even need to use a power meter! Pictures will tell the story! No photoshopping tomfoolery is involved, it's actually 10x even more dramatic in real life!

    First up is the CT6800HP, a solid galvo which is still the baseline of the industry.



    Because we regularly use higher power (>10W) lasers in shows we have upgraded most of ours to high power dielectrics. In terms of specifications these are nominally rated >98% reflectivity, though as with most "visible range" dielectric optics the deep blues and violets are not fully reflected. These are $150 mirrors.

    From a distance straight on it's clear that it meets or beats the 98% claim,


    There is only the barest light leakage even up close.


    At nominal 45 degrees (the angle the coatings are designed to run optimally at) it is as perfect as 99.9% aluminized mirrors.


    Next up is the DT-40 Pro, a new entry into the market it certainly claims a list of features that could make it a serious player in the market.



    This unit is bone stock, and is completely "as shipped". As you can see above, the mirrors are very thin, this reduces rotational inertia but they're definitely fragile. Jian rates these at >95% reflectivity.

    From a distance they definitely let more light leak than the 6800s. It's hard to see in the photo, but in person you can see a hair more purplish from the light than the 6800s.


    Even visually you can tell that it's far better than 95% even dead-on
    even in the lossiest zone of deep blue and UV.


    At the optimal 45 degree angle they are almost perfect, but you can see
    slight losses in the deep blue and UV range.


    Obviously in terms of reflectivity they are not perfect. I would definitely like to see aftermarket mirrors available for these as I would not put more than 6 or so watts through these.

    Last up, the CT6215 with stock mirrors as shipped from cambridge.


    This unit is the surprise for the day, with mirrors nominally rated at >95% reflectivity, the CT6215 is quickly pulling the lead as the defacto show scanner. These are the same standard mirrors you order for $75. These can nominally take up to 10W of power, an improvement on the stock 6800's
    5W.

    Here you can start to see some interesting behaviour. This effect is even more dramatic in person!


    The mirrors are translucent! The camera can't even see itself in the reflection!


    At the optimal 45 degrees, you can guess how CT computed >95%...


    You can also guess why they can take 10W of power!

    Synopsis: I don't want to hear about "cheap chinese mirrors, buy Cambridge instead"...

    It was so hard to believe I put an LED flashlight up behind the CT6215!





    Thoughts? different testing mechanisms? problems with my droid?
    You can get the highres versions of these photos from their gallery.
    http://photonlexicon.com/gallery/album64?page=1
    Last edited by yaddatrance; 03-31-2007 at 03:11.

  2. #2
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    Damn! Pretty good work there of you yaddatrance

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    Default Thankyou

    Yadda
    Many thanks for the efforts and time you have put into this report , some fantastic images to show clearly and backup what your saying.

    However I think you were expecting people to "read between the lines" in some of your text ,but with guys like me slowly building knowledge you really need to spell it out or spoonfeed :-)

    I kinda gather your saying the DT40s are amazing value for money , their mirrors are performing wondefully for the money ? I have a set and dont need convincing , if it wasnt for products like these i couldt afford to be in this hobby ..............its always fascinating to see the "emerging" players stand side by side and be tested against the well known "Established" players and I think what your saying is the Ct6215 mirrors are dissappointing considering they are 3/4 times the price? ..can you just clarify that for me ?

    Further the price/value performance is always factor is always facinating for me ... i think one of the simplest most critical ways to test scanners is simply to write some text and project it .......for many years working my ways up the galvo ladder i have spent many years looking at severly flickering text ! I found a supplier last year that seems to b offering decent 30k scanners for around half the price of jians ......but i would IMHO say only 10% drop in performance from jians units .........although when it comes to text I would say that 10% make all the difference !!!!

    Now i have a baseline for DT40pros .i understand and can compare their real world limitations and uses i would love your opinion on how much an improvemnt the cambridge ranges offer particulary on reducing the flicker when projecting text .

    PAUL
    In the beginning there was none. Then came the light - #1 UKLEM - 2007
    BUY UK LEGAL LASER POINTER :: NEW - Blue 460nm Laser Pointers

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    Hi Yadda

    a good detailed test and great photos ... mate

    would it be possible to repeat the test with a laser reflected and measure the beam prior and after the mirror with a power meter... just for comparison .


    all the best ... KARL

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    x2



    Dave..
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    https://www.facebook.com/kvantaus/

    Lasershowparts- Laser Parts at great prices
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    Hi Yadda,

    Interesting results. I haven't seen the same results here at Pangolin. Would you do us a favor and do a higher quality study by using a multi-wavelength laser that you can use to characterize the mirrors at various angles and at various wavelengths? Holding a mirror up to the light might not reveal the whole story. Just as a point of comparison, if you hold a high-reflector (supposed to be 99.9% reflective) up to the light, or shined your LED flashlight through it, I bet you would see some light through that too

    As far as I know, for the longest time, the standard mirrors on Cambridge 6215 scanners were "Dielectric over silver". We have seen that "Pure dielectric" mirrors are not really all that great for scanning, because the reflectivity of certain wavelengths change, depending on the scan angle. If you use "Pure dielectric" mirrors for scanning, you will have what we call a "green dropout band" where the image goes from white, to magenta(ish), to white. The reflectivity of the dielectric portion drops at certain angles and certain wavelengths, but the reflectivity of silver is (if memory serves) 93%. So the point of "Dielectric over silver" is that, when the green dropout band happens with the dielectric portion of the coating, the reflectivity of the mirror drops to a maximum of 93%, which is not visible to the human eye.

    Over the last few years we have purchased probably 200 pairs of 6215 scanners for use with our SMS systems. Some of these are used with multi-watt lasers. Also, we are close friends with laserists all around the world who also use Cambridge 6215 scanners based on our recommendations. One is Doug McCullough who uses these scanners with high power lasers (Sabre, etc.) for outdoor shows. Another is Hugo Bunk who uses these scanners to put on shows the likes of the MTV2 gig that everyone was talking about a month or so back. These are a few of the top guys in the laser field, and I have not heard them complain about the same kind of problem you reported there. It could be that you have a bad mirror, but I would still like to hear the results of some better testing just to confirm it, one way or another...

    I must say I have seen some inconsistency from the mirrors on Cambridge scanners, but nothing to the degree that you are showing there. First, in looking at your pictures, it is obvious that these are "Pure dielectric" mirrors. So there are a few possibilities here:

    a) Cambridge misunderstood when you ordered, and purposely supplied the scanners with the wrong mirrors, possibly from some other application.
    b) Cambridge understood your order, but accidentally placed "Pure dielectric" mirrors on your scanners before shipping
    c) Cambridge mirror supplier supplied them with mirrors that were grossly out of spec
    d) All Cambridge scanners come with crap mirrors...

    Note that, having been involved with Cambridge since 1992, I don't believe "d" for a second, and would place "b" as a higher likelihood or "c" as the highest likelihood...

    One thing that everyone in the world must understand is -- Cambridge doesn't make mirrors. They buy them from a supplier -- just like Jian does, and just like the $150 mirrors Yadda. Mirrors change from lot to lot and, while I will be a little surprised Cambridge didn't verify the reflectivity of the mirrors before shipping, I could see this happening. There is a tendency to assume that your supplier will do a good job for you, but I have seen even Newport Thinfilms deliver mirrors that were a bit off, a time or two... The coating of mirrors is sort of like cooking food. How many of us have gone into our favorite restaurant on an off day?

    Yadda, you stated that you too had acquired many 6215s in the past. Perhaps another good experiment for you to do, would be to characterize more than just a single pair. Likewise, it would be worthwhile to characterize more than just a single set of your $150 mirrors and more than one pair of the DT40s? My belief (hope?) is that you would find that the particular set of Cambridge scanners was having an "off day" and perhaps even you found a particularly good pair of DT40s? You of all people should know that a single data point can not provide valuable statistics...

    And one final note. The thickness of mirrors is of huge importance for a couple of reasons. One of them is dynamic stiffness (something hardly anyone in the lightshow industry knows about, but its the first number that real scanning people -- particularly resonant scanning people -- enter when doing calculations on mirror size for scan rate), and the other is flatness. At Pangolin, we use a simple rule of thumb that the thickness must be at least 1/12 the longest dimension of the mirror.

    There is a intuitive myth that using thinner mirrors will make the scanners faster, because thinner = less mass = faster scanning. But believe me, Jian and all other companies are not doing you any favors by using thin mirrors!!! Even when using thick mirrors as those used on the Cambridge 6215, the mirror is only 1/10 the inertia of the rotor. Using thinner mirrors will NOT make the scanners faster and in fact, because of parasitic resonance effects, it makes the scanners slower.

    Best regards,

    William Benner
    Last edited by Pangolin; 04-02-2007 at 22:50.

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    Quote Originally Posted by Pangolin View Post
    The thickness of mirrors is of huge importance for a couple of reasons. One of them is dynamic stiffness (something hardly anyone in the lightshow industry knows about, but its the first number that real scanning people -- particularly resonant scanning people -- enter when doing calculations on mirror size for scan rate), and the other is flatness. At Pangolin, we use a simple rule of thumb that the thickness must be at least 1/12 the longest dimension of the mirror.

    There is a intuitive myth that using thinner mirrors will make the scanners faster, because thinner = less mass = faster scanning. But believe me, Jian and all other companies are not doing you any favors by using thin mirrors!!! Even when using thick mirrors as those used on the Cambridge 6215, the mirror is only 1/10 the inertia of the rotor. Using thinner mirrors will NOT make the scanners faster and in fact, because of parasitic resonance effects, it makes the scanners slower.

    Best regards,

    William Benner
    I agree, the WideMoves have VERY thin axles and 0.5 mm thick mirrors 12 mm long, and the parasitic resonance shows, badly:

    http://www.photonlexicon.com/forums/...0862#post20862

    I'm still hoping that someone will run that test wave I made on some other scanners. I'd do it myself and post the result, if I had access to any. I don't even have access, let alone the priviledge of ownership, so it would be great to see that test done by someone who has. It will help them prove their scanners. If there IS significant parasitic resonance, running that test at various speeds and scan angles will certainly show it clearly.

    Yadda, I felt the urge to say 'cut to the chase' but that's a good report. I think even with several things to be cautious about (expecting total extinguishing of light through any mirror, relying on anything but a before/after power measurement of RGB wavelengths at extremes of intended scan angle), it's clear from what you've shown that the DT40's are worth taking seriously. When I can afford some, I'll buy some, BUT, ONLY if they pass that test I set. (Along with any other test that they should be expected to pass). I refuse to buy, then see them fail in some dramatic way, then be told it is my responsibility. I don't think they will fail as badly as the WideMoves, because the axles seem stouter and shorter, the mirrors seem better mounted, but I want to see before I buy!

    I wouldn't place too much emphasis on weaknesses in the CT scanners unless they are found and verified by others who have what's needed to confirm or deny, impartially, but anything that can confirm good value of new makes of scanners at cheaper prices is cool. As far as the small mirrors are concerned, mounting might be a challenge, but getting them is surely not. I remember before I found this forum, or Usenet, seeing a site selling small 1/4" square dielectric mirrors that could take 20 watts! They were cheap too ($12 or so each), especially if you bought one big enough to cut your own from. With the right small power tool, grinding the corners to reduce mass would be easy and cheap. I don't know if they were dielectric over silver, I suspect they were pure dielectric, maybe broadband to accept a little extra loss and gain better tolerance to changes in incident angle.
    Last edited by The_Doctor; 03-31-2007 at 12:00.

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    Dear Bill, don't you dare patronize me. The point of this post was not to bash cambridge, but to show that the DT's aren't as bad as some people claimed. I'm confident that those 6215 optics meet/beat the 95% mark at 45 degrees.

    But... you opened another can of worms.

    Enough with your "laser industry" "standard" excuses about specifications. I have bought so many things from the "professionals" based on their specifications in the past only to hear "oh you poor thing, I'm sorry your brand new $15,000 whitelight laser you bought from me died because you ran it at 2W, when I said 4000 hours and 2 Watts, I didn't mean at the same time..."

    So much for youthful innocence.

    Of course it's silly in retrospect, but when when all I asked "how powerful is it and how long will it last"... I expected a warning if it isn't going to last...

    Now I know that every laserist goes through various versions of this, but when I started there was no internet laser forum to help me and I had to rebuild everything over and over again in the process of learning. Now what really pisses me off is that after I joined the "old boy's club" I found out that everyone really did know what I needed to know, but everyone is living hand-fucking-mouth and needs to earn a buck. I have always worked in a professional field and I had always jokingly caulked my losses up as "well I guess we needed to feed that laserist" but I never suspected it was true.

    But I am especially sick and tired of the status quo of "professional" laserists belittling others because amateurs don't understand the rather janky way we specify things and how we have our own grading of products designed to cause people to buy inferior engineering. Thats why I'm on this forum and thats why I build my software and hardware, so "oldtimers" can't go "'that's impossible" or "well, that's a good idea, but not how I do it" and why I post things like this to educate people... "that's a 5W green laser, with 4W in IR and only 1W in green"....

    I know that the thickness of the mirrors is how you increase torsional rigidity and I state as much in the post above.

    Quote Originally Posted by yaddatrance
    (The interesting thing to note here is the mirror thicknesses. The thickness is part of how you get away with >30k scanning. The DT is obviously not suitable for 60k scanning.)
    I also know that for 2 grand there is alot of optics and coatings you can buy. Did you know that just last week, I am finalizing an order of $25,000 of custom narrow bandpass (25nm) 635/660nm mirrors on fused silica. I know it's difficult, 100 layers of coatings, but I would still be a bit miffed if they didn't meet the agreed upon specifications and would send the failures back for replacement. I actually do know my optics, somewhat more than you I suspect, or maybe you just simplified your explanations too much. I understand the process of making optics throughly, what process and order you need to coat to pass both p and s polarizations, I originally bought a vacuum sputtercoat machine at my old facility! You can educate me on laser shows, but do not even try to teach me about optics.

    Quote Originally Posted by Pangolin View Post
    Yadda,
    Do you have equipment to do a higher quality study -- such as, at the very least, a multi-wavelength laser that you can use to characterize the mirrors at various angles and at various wavelengths? Sub question -- have you ever held a high-reflector (supposed to be 99.9% reflective) up to the light, or shined your LED flashlight through it? I bet you would see some light through that too
    All laserists know that because all gas lasers shoot beams out the back! Stop quoting amateur information and pretending that it's somehow prescient.


    A Coherent I90 being tuned up.

    All the scanner mirrors leak, we know that, thats why have specs... 99.9% is reflective but not VERY reflective. It's just conveniently cheap to get 99.9.

    I never stated I was unsatisfied with the standard mirrors on the CT6215...
    As you see I already explained that it probably meets the 95% at 45 degrees...

    This scanhead was one that was ordered as their "laserist special" with the missing pots that you claim is an improvement. BTW I went back through the test points with an oscilloscope and I definitely see problems on the "slaved" tuning which you claim as "improvement"...

    As far as I know, the STANDARD mirrors on Cambridge 6215 scanners are supposed to be "Dielectric over silver". "Pure dielectric" mirrors are not really all that great for scanning, because the reflectivity of certain wavelengths change, depending on the scan angle. If you use "Pure dielectric" mirrors for scanning, you will have what we call a "green dropout band" where the image goes from white, to magenta(ish), to white. The reflectivity of the dielectric portion drops at certain angles and certain wavelengths, but the reflectivity of silver is (if memory serves) 93%. So the point of "Dielectric over silver" is that, when the green dropout band happens with the dielectric portion of the coating, the reflectivity of the mirror drops to a maximum of 93%, which is not visible to the human eye.
    Ah... "please be reviewing" camtech's own site http://www.camtech.com/products/mirrors.html

    Their "true" dielectric mirrors are supposed to be >97% and angle or not, it's better than 93% (or 90% in the blue/violet range)

    That's bullcrap, 7% of light is visible to the human eye... You eye has the capability of detecting a single photon. You can't look through a 99.9% high reflector and also claim than 93% isn't noticable.

    When I buy optics from low-end dealers like edmunds, I would definitely
    be miffed if they didn't meet specs or publish the areas where they don't meet
    specs before I bought them... I do not see those number on cambridge's site.
    That is my bitch.

    <blah blah blah>

    Over the last few years we have purchased probably 200 pairs of 6215 scanners for use with our SMS systems. Some of these are used with multi-watt lasers. Also, we are close friends with laserists all around the world who also use Cambridge 6215 scanners based on our recommendations. One is Doug McCullough who uses these scanners with high power lasers (Sabre, etc.) for outdoor shows. Another is Hugo Bunk who uses these scanners to put on shows the likes of the MTV2 gig that everyone was talking about a month or so back. These are a few of the top guys in the laser field, and they would be complaining loudly if they detected a problem as severe as what you are showing there...

    I must say I have seen some inconsistency from the mirrors on Cambridge scanners, but nothing to the degree that you are showing there. First, in looking at your pictures, it is obvious that these are "Pure dielectric" mirrors. So there are a few possibilities here:

    a) Cambridge misunderstood when you ordered, and purposely supplied the scanners with "Pure dielectric" mirrors, possibly from some other application.
    b) Cambridge understood your order, but accidentally placed "Pure dielectric" mirrors on your scanners before shipping
    c) Cambridge mirror supplier supplied them with mirrors that were grossly out of spec
    d) All Cambridge scanners come with crap mirrors...

    Note that, having been involved with Cambridge since 1992, I don't believe "d" for a second, and would place "b" as a higher likelihood or "c" as the highest likelihood...
    There is nothing wrong with the fucking mirrors!!!!! CT just cheated and used dirt cheap angle sensitive coatings! Laserists are pessimists so light leaking everywhere is simply expected and accounted for and covered with blackout tape if it goes the wrong place. The mirrors look like normal mirrors if you're not aiming looking at a light through them and they work just fine, I'll be reassembling that unit and happily using them for laser shows... but cheap is cheap, and crap is crap.

    One thing that everyone in the world must understand is -- Cambridge doesn't make mirrors. They buy them from a supplier -- just like Jian does, and just like the $150 mirrors Yadda. Mirrors change from lot to lot and, while I am a little surprised Cambridge didn't verify the reflectivity of the mirrors before shipping, I could see this happening. There is a tendency to assume that your supplier will do a good job for you, but I have seen even Newport Thinfilms deliver mirrors that were a bit off, a time or two... The coating of mirrors is sort of like cooking food. How many of us have gone into our favorite restaurant on an off day?
    Yes yes, I just ran around and while there are differences, they are all like that... for curiousity I went out of my way and the stock CT6800HP mirrors
    don't have that property! Maybe they changed suppliers for the worse?

    Yadda, you stated that you too had acquired many 6215s in the past. Perhaps another good experiment for you to do, would be to characterize more than just a single pair. Likewise, it would be worthwhile to characterize more than just a single set of your $150 mirrors and more than one pair of the DT40s? My belief (hope?) is that you would find that the particular set of Cambridge scanners was having an "off day" and perhaps even you found a particularly good pair of DT40s? You of all people should know that a single data point can not provide valuable statistics...
    Sigh, this isn't scientific, I made this post more as jest mainly because I was so surprised when I lifted the scanners up and discovered that I could see the room and colors through them... This could be a bad mirror from a bad batch, entirely possible, we do buy to take advantage of price breaks as much as possible.

    And one final note. The thickness of mirrors is of huge importance for a couple of reasons. One of them is dynamic stiffness (something hardly anyone in the lightshow industry knows about, but its the first number that real scanning people -- particularly resonant scanning people -- enter when doing calculations on mirror size for scan rate), and the other is flatness. At Pangolin, we use a simple rule of thumb that the thickness must be at least 1/12 the longest dimension of the mirror.

    There is a intuitive myth that using thinner mirrors will make the scanners faster, because thinner = less mass = faster scanning. But believe me, Jian and all other companies are not doing you any favors by using thin mirrors!!! Even when using thick mirrors as those used on the Cambridge 6215, the mirror is only 1/10 the inertia of the rotor. Using thinner mirrors will NOT make the scanners faster and in fact, because of parasitic resonance effects, it makes the scanners slower.

    Best regards,

    William Benner
    Actually if camtech bothered updating their notch filter design with a dsp you could actively subue the problems with resonance, we did that for trackers.

    Sorry, if I raged too much, your post touched a very old nerve.

    [edited for spelling]
    Last edited by yaddatrance; 03-31-2007 at 14:38.

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    I just reread your post it was more my nerve that got twitched, I apologize for taking your comments personally. I have 20 years of laser resentment against some 'professionals' (most of whom have in the last decade been chased out of the field for lying, stealing and cheating...) I appreciate that you take the time to explain... but I dislike patronization, especially for such obvious things.

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    Dam!!!!!!!!
    "Gravity its not just a good idea its the law"

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