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Thread: Do A/Rs technically transmit more light?

  1. #1
    Old Optician to New OD Aarlan's Avatar
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    Do A/Rs technically transmit more light?

    Just something I've been wondering for about 10 years

    The principles behind an A/R coat are primarily reflection and destructive interference. It has been demonstrated that the phase change associated with the proper thickness and index coating can effectively 'cancel' the reflections from an ophthalmic lens by having a creating a sinusoidal wave exactly counterbalanced to the reflected light wave (obviously some portions of the colour spectrum are less affected due to differing wavelength).

    Since the reflection is destructively interfered or cancelled, my question is this:
    WHEN THE REFLECTION IS ELIMINATED/REDUCED, DOES THAT NECESSARILY HAVE ANY BEARING (increase) ON THE LIGHT TRANSMITTED, OR DOES THE LIGHT TRANSMITTED REMAIN CONSTANT?

    It would seem instinctive that by reducing the reflection from the refractive medium one would assume that the light transmitted is thus increased. But under further examination, the amount of light transmitted doesn't seem to increase, rather the light transmitted instead seems to remain constant, but the light reflected is 'canceled'...which is not the same as increasing the % of Transmitted light.

    Am I incorrect in this supposition?

    AA

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    Hasn't anyone ever measured these things before making such claims?

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    Master OptiBoarder Darryl Meister's Avatar
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    Yes. Traditional AR coatings should transmit whatever they don't reflect, less a tiny bit of light lost to absorption by the AR coating. Reflections are eliminated via destructive interference, while transmittance is increased via constructive interference.
    Darryl J. Meister, ABOM

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    Rising Star mauroventura's Avatar
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    You can try to check the transmittance , absorption and reflectance capability of AR coating , using a spectrofotometer.

    Normally the absorption of a coating is :

    ABS% = 100 -( Transmittance%+reflectance%).

    For the measurement is better to use a plano lens (CC=CX=0 diopter or near 0).

    Be careful about ripples due to a Non index matched hardcoat.


    Bye

  5. #5
    Old Optician to New OD Aarlan's Avatar
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    Quote Originally Posted by Darryl Meister
    . Reflections are eliminated via destructive interference, while transmittance is increased via constructive interference.
    But if the reflected light is destructively interfered, isn't that a result of the two different reflections both CANCELLING one another in terms of visible spectrum? If when a light wave is reflected off of a surface (lens) destructively interferes with one another light that is reflected off of a surface (coating), it seems that neither light wave has ever entered the lens. How is more light being transmitted?




    Orig total light -light reflected =Light transmitted
    or 100%- 8%= 92% (using 8% as a theoretical value for light reflected)

    What the A/R claim is that since the reflections are eliminated, the Light transmitted must be closer to the Original Total Light, or closer to 100%.

    But it seems to me that it should instead be viewed from the point of:

    The light transmitted .......................................................92%
    Light reflected ............................................................ ...8%
    Light reflected cancelled via destructive Interference.approx..-8%

    New % Light Transmisison...............................STILL 92%?


    AA

  6. #6
    Master OptiBoarder Darryl Meister's Avatar
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    The AR coating layer essentially renders the backward-moving reflections (away from the wearer) out-of-phase, so that they cancel each other out, while rendering some of the forward-moving (toward the wearer) reflections in-phase, so that they amplify each other. Remember, the light has to go somewhere, since energy cannot be destroyed.

    For comparison, here is a spectral transmittance chart of uncoated hard resin:

    Spectral Transmittance Uncoated Hard Resin

    Now, here is a spectral transmittance chart of an AR-coated hard resin:

    Spectral Transmittance AR Coated Hard Resin
    Darryl J. Meister, ABOM

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    One eye sees, the other feels OptiBoard Silver Supporter
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  8. #8
    Old Optician to New OD Aarlan's Avatar
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    Quote Originally Posted by Darryl Meister
    . Remember, the light has to go somewhere, since energy cannot be destroyed.
    If you shone two rays of light from the same point source that were the exact same wavelength, but were a half wavelength out of phase, the would 'cancel' each other out...why would it be forced to 'go somewhere'? ...Is that 'destroying' energy?

    AA

  9. #9
    Old Optician to New OD Aarlan's Avatar
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    Quote Originally Posted by Darryl Meister
    while rendering some of the forward-moving (toward the wearer) reflections in-phase, so that they amplify each other. .
    So how would that look in a ray diagram?

    Quote Originally Posted by Darryl Meister
    For comparison, here is a spectral transmittance chart of uncoated hard resin:

    Spectral Transmittance Uncoated Hard Resin

    Now, here is a spectral transmittance chart of an AR-coated hard resin:

    Spectral Transmittance AR Coated Hard Resin
    I like the tranmittance charts!! Thanx a million.

    AA

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    Master OptiBoarder Darryl Meister's Avatar
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    So how would that look in a ray diagram?
    It would look something like this:



    Note that the number of internal reflections from for an incident ray is actually infinite, though each subsequent internal reflection gets smaller and smaller. If you start with the initial ray of light, it strikes the interface between the AR-coating and air and part of it is reflected, forming Ray 1. Most of the ray is transmitted through the coating, forming Ray 1' as it passes through into the lens.

    However, at the interface between the coating and the lens, some of the light is again reflected. This light travels back through the coating to the air-coating interface, where most of it is transmitted, forming Ray 2, while some of it is reflected back. Most of the light reflected back is again transmitted into the lens, forming Ray 2', though a percentage of light striking the coating-lens interface will be reflected back into the coating again. This process repeats itself indefinitely.

    Now, Ray 2 is out-of-phase with Ray 1, and effectively "cancels" it out, which eliminates both reflections. Moreover, Ray 1' and Ray 2', the transmitted rays, are in-phase with each other, and effectively amplify each other. Consequently, reflected light is eliminated while transmitted light is increased by the same amount, which is consistent with the law of the conservation of energy.
    Last edited by Darryl Meister; 06-30-2005 at 06:14 PM.
    Darryl J. Meister, ABOM

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    Old Optician to New OD Aarlan's Avatar
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    Darryl,

    Thank you. I think I may learn more in the Ophthalmic Optics section of Optiboard than I have in 15+ years in the field, 2.5 years of Opticianry school, and countless science classes at UConn. I just wish I stumbled upon this site sooner. I appreciate your help tremendously.

    Sincerely,

    Aarlan A

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    Master OptiBoarder Darryl Meister's Avatar
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    Glad to help.
    Darryl J. Meister, ABOM

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    Bad address email on file bser's Avatar
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    ? does a mirror coated lens block u.v on a cr-39 lens?that has not been treated for u.v? compare A.R AND MIRRROR lens ,

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    Quote Originally Posted by bser
    ? does a mirror coated lens block u.v on a cr-39 lens?that has not been treated for u.v? compare A.R AND MIRRROR lens ,
    It depends on the mirror coating. Some mirror coatings incorporate a thin layer of metal, these coatings are very effective UV blockers. Many mirrors only use dielectric (aka ceramic, glassy, etc.) layers, these do not absorb in the UV. With very, very, very few exceptions AR coatings do not absorb meaningful amounts of UV.

    For consisten UV protection one must either use UV treated lenses or polycarbonate (which has a hard UV cutoff for wavelengths below ~380 nm).

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    Master OptiBoarder rinselberg's Avatar
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    Trivex lenses have inherent UV protection

    Quote Originally Posted by coda
    It depends on the mirror coating. Some mirror coatings incorporate a thin layer of metal, these coatings are very effective UV blockers. Many mirrors only use dielectric (aka ceramic, glassy, etc.) layers, these do not absorb in the UV. With very, very, very few exceptions AR coatings do not absorb meaningful amounts of UV.

    For consisten UV protection one must either use UV treated lenses or polycarbonate (which has a hard UV cutoff for wavelengths below ~380 nm).
    Trivex lenses block UV -- even clear Trivex lenses. They don't need to be "UV treated". I got this directly from PPG, which makes Trivex

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    Quote Originally Posted by rinselberg
    Trivex lenses block UV -- even clear Trivex lenses. They don't need to be "UV treated". I got this directly from PPG, which makes Trivex
    Hey Rinselberg, haven't seen you in a while, welcome back.

    Yup, I didn't think it through enough when I commented above. A number of mid and high index lenses as well as Trivex and polycarb block some or all of the UV depending on the product.

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    Unless I am miss-led all lenses block U.V. The very least of which blocks 85%.


    How much does the extra 10-15% we charge or use the property to sell with actually benefit the patient? No one knows. No one has ever done a study to even compare non-lens wearing emetropes with UV and Non-UV protected lens wearers to evaluate the effects of U.V. on the unprotected. Until someone does all claims are hype.
    Don't know how many lectures I have attended that the speaker claimed that he had traveled and studied in those parts of the globe with high U.V. He claimed that he found no higher incidence of Catarac or Macular Edema as compared to other regions. He was not sponsored by the great UV dye or UV resistant material company.
    I also have attended even more lectures (speaker sponsored by UV resistant material or dye) claiming to the contrary.

    Chip

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    Master OptiBoarder Darryl Meister's Avatar
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    Plastics generally block a substantial amount of UV, but crown glass, for instance, blocks much less.

    Material Data

    Best regards,
    Darryl
    Darryl J. Meister, ABOM

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    Herr Meister:

    Note: Even for crown glass this is an 80% blockage. Surely this must mean that wearers of same would be only 20% as likely to sustain dammage as non-wearing emetropes.

    Further question: If untreated CR-39 blocks 100% (I don't really beleive anything blocks 100%) how are we getting away with charging $20 for sheep dip to block U.V.?
    Last edited by chip anderson; 07-01-2005 at 03:55 PM. Reason: Further Comment

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    Master OptiBoarder Jedi's Avatar
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    Chip, here is an article by Chris discussing UV treatments.
    UV Treatments
    "It's not impossible. I used to bull's-eye womp rats in my T-16 back home."


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    Master OptiBoarder Darryl Meister's Avatar
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    Chip, hard resin (CR-39) blocks around 90% UVA and 100% UVB, depending upon the supplier. As for paying the extra cash for UV treatment, I guess you're doing it for the confidence of knowing that you're blocking 100% of both. Also, while various studies have debated the importance of UVA in the formation of cataract, other studies have suggested a link between shorter blue wavelengths and macular degeneration, so adequate protection against both is still a good idea. As for crown glass, it only blocks 80% of UVB and 20% of UVA.
    Darryl J. Meister, ABOM

  22. #22
    Old Optician to New OD Aarlan's Avatar
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    Darryl,

    Here's another question about A/R. When you add one of those 'Super-Slick' top coats (ex Hoya's View Protect that they are marketing the heck out of), would that alter the performance of the A/R? Do they have to alter the composition/% of coatings to compensate for this coating that is applied over the A/R, or can you arbitrarily add this topcoat feature over any A/R

    AA

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    Master OptiBoarder Darryl Meister's Avatar
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    Top coatings have only a small effect on AR performance, but one of the coating guys (like Steve or coda) could probably answer this in more detail.
    Darryl J. Meister, ABOM

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    Forever Liz's Dad Steve Machol's Avatar
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    Simply put, the AR stack is designed to take the thickness and index of the hydropobic layer into consideration.


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  25. #25
    Old Optician to New OD Aarlan's Avatar
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    Is there a reliable top coat you can put on in-office? Or would that interfere with the coating as it was designed? I would love to get a version of the Hoya View Protect to apply in house...That stuff is ridiculous...I have NEVER had a problem edging A/R lenses (Crizal Alize, Carat, etc), but a new lens with Hoya's top coat spun like a top in my edger (even with 3M pads, A/R Plastic 'Sticker'). I would have to assume they add the topcoat post edging, because that stuff is deadly...I didn't try the old Hair-Net trick yet.

    AA

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