LED light experimentation

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MrAl
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Post by MrAl » Mon Dec 29, 2008 12:33 am

You'll have to use a light meter. They make them for cheap these days.

I have a home brewed version but it's a 'relative' light meter, meaning
that it compares one source of light to another, so it doesnt take
absolute readings like an ordinary light meter does.
LEDs vs Bulbs, LEDs are winning.

rshayes
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Post by rshayes » Mon Dec 29, 2008 4:29 am

You might be able to use a silicon photodiode or solar cell to get an estimate of the light output. A silicon photodiode usually has a scale factor of .4 to .6 amperes per watt at its peak wavelength (usually near 900 nanometers). The graph of spectral response can be used to correct this for the LED wavelength. Since the light is close to monochromatic, the weighting curve for conversion to lumens will probably be reasonably accurate without doing a numerical integration over the visual spectrum. The conversion factor will be low, since the response of the human eye is low in the blue region.

Actually, you might be better off thinking in terms of watts per square centimeter rather than lumens, since this will allow comparison with measurements of the effectiveness of ultraviolet light. Lumens are defined in terms of the visual spectrum and are undefined for ultraviolet light.

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Bob Scott
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Post by Bob Scott » Mon Dec 29, 2008 6:19 am

I see future product development where future versions use less power than the initial model.

I think that you might consider initially using LEDs which use higher power than you'd like. In this case you may consider liquid cooling the LED down to body temperature using a non-toxic working fluid. Very small liquid coolant supply and return lines could be made of very small teflon tubes housed inside your main plastic or silicon rubber tube, cooling the small metal housing of the LED.

I know of a company with expertise in custom liquid cooling electronics if you are interested, I can provide a contact.

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Forrest Mims III
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Post by Forrest Mims III » Mon Dec 29, 2008 7:54 am

mklepper, the action spectrum for DNA damage leading to cancer peaks well below the 360-380 nm range of UV-A LEDs. In outdoor sunlight, the peak is about 305 nm. See Setlow's classic paper "The Wavelengths in Sunlight Effective in Producing Skin Cancer: A Theoretical Analysis" (abstract is at http://www.citeulike.org/user/harish/article/3477733). Many other papers cover the DNA action spectrum.

In short, the wavelenghts that suppress bacteria can also lead to cancer.

Of interest is that the peak of the vitamin D action spectrum in unfiltered sunlight is close to the peak of the DNA action spectrum.

This is not directly related, but here in South Texas, virtually all E. coli suspended in clear water are inactivated in less than 20 minutes of sunlight exposure on a clear September day.

Forrest

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Engineer1138
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Post by Engineer1138 » Mon Dec 29, 2008 8:20 am

mklepper:
My suggestion would be that you look into the lighting used in laparoscopic surgery. I think you'll find that they all leave the emitters outside the body and use light pipes to bring the light in.

On a larger issue however, while I admire your experimental spirit, for a device that will require FDA approval it may be a faster path to contact a firm that does this on a daily basis. In my day job (when I'm not a caped crusader fighting crime :-) )I develop software for medical devices - in vitro diagnostic instruments. I'm not a Regulatory expert and I haven't worked with implantable or in vivo devices, but I do have experience working in an FDA regulated environment. It would be a shame for you to spend hundreds of hours trying various solutions only to discover that what you settle on would never meet regulatory approval. In the long run, it can be a time & money saver to hire an expert.

I don't have any firms that I can recommend (the only outside development outfit I ever worked with delivered sub-par results), but I do have a few links to device blogs where you may be able to ask for recommendations if you are interested.

-Lyndon

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MrAl
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Post by MrAl » Mon Dec 29, 2008 11:18 am

Engineer1138 wrote:mklepper:
My suggestion would be that you look into the lighting used in laparoscopic surgery. I think you'll find that they all leave the emitters outside the body and use light pipes to bring the light in.

On a larger issue however, while I admire your experimental spirit, for a device that will require FDA approval it may be a faster path to contact a firm that does this on a daily basis. In my day job (when I'm not a caped crusader fighting crime :-) )I develop software for medical devices - in vitro diagnostic instruments. I'm not a Regulatory expert and I haven't worked with implantable or in vivo devices, but I do have experience working in an FDA regulated environment. It would be a shame for you to spend hundreds of hours trying various solutions only to discover that what you settle on would never meet regulatory approval. In the long run, it can be a time & money saver to hire an expert.

I don't have any firms that I can recommend (the only outside development outfit I ever worked with delivered sub-par results), but I do have a few links to device blogs where you may be able to ask for recommendations if you are interested.

-Lyndon
Hi again,

That was one of my main concerns from the start of this thread...
the heat that would be generated from the LED if it were to be
placed inside the body.
We know that the body has ways of dealing with excess internal
heat, but it certainly does not expect it to be a point source in
a location such as the neck.
The thing is, even a relatively small amount of power dissipation
if confined will build the temperature up to a very high level
if there is no means to get rid of that heat. Of course many
real life applications will see the temperature level off at some
terminal level, but that level could be far above what the inside
of the throat could withstand.
Of course this still depends a lot on what the study on the blue
light (or UV light) intensity vs bacteria outcome is, which i guess
you still havent done yet (i would think this would come FIRST,
as that will tell you how much light you need and that would
then tell you how much heat you will have to deal with, that's
why i suggested it earlier in this thread).
LEDs vs Bulbs, LEDs are winning.

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haklesup
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Post by haklesup » Mon Dec 29, 2008 3:05 pm

Seems the other posters have some good ideas on heat and wavelength issues. I suspect that a fiberoptic system with engineered diffuser at the end is what you want. Sure you may need a brighter source than if used internally but the heat (IR light) can be pre-filtered giving relatively cool blue light where you want it. (I would imagine that blood vessel tissue inside the body would be an excellent heat sink and without knowing the properties of that, its hard to estimate the temp rise for a given small volume even if you do know the amount of energy you are sending in)

As to the original question, how to put LEDs at the end of a catheter, I would abandon packaged devices entirely (even SMT) and look at bare dice mounted on flex substrate. Note that a bare LED die does not emit light evenly and the manufacturer can provide exact details on its diffusion pattern and even design lenses and diffusers to go with the dice. Many LED devices have bare die options on the datasheet, just look closely.

Assembly is a chore at this level, it cannot be done by hand. To develop a 4 conductor flex PCB ($500-$2000) (but you probably get like 25 for that price) mount 2-4 sampled LED die to it ($200) and slide it into a silicone catheter (low$). If its a little fat, a silicone overmold can encapsulate it and attach it to tubing as required. AFAIK, silicone is body compatible while most epoxies are not. Silicone can be mixed with silicon-dioxide (glass) spheres to make a diffuser.

LED manufacturers can obviously provide you with the best data on devices. Look around, its likely since you own a patent (valuable IP) that you can partner with a LED manufacturer to pay some of the NRE (non recoverable engineering) costs. They may be scared off by the medical aspect of it and all the extra regulations and liabilities. A medical device manufacturer is the best partner but harder to sell the idea to.

A flex PCB manufacturer who has design capabilities can also help you choose the best substrate and mounting/die bonding method.

Furthermore, there is a boom in LED control devices for pushing more light out of the existing units and offering more flexibility in dimming and power level setting through PWM control.

The concept of integrating a light sensor allows you to make a closed loop system which is safer because you can verify actual output as opposed to doing that indirectly by monitoring current for example.

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MrAl
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Post by MrAl » Mon Dec 29, 2008 4:11 pm

Hi again,

Very interesting hacklesup.

I still feel that we are getting ahead of ourselves just a little here (still)
because we dont know the bacteriological model yet. This might be
similar to the predator/prey model used in many textbooks as
a sort of introduction to differential equations, or reduced to a
simple algebraic formula. Without that, we dont know how much intensity
we really need yet.

For example, do we need 1 LED running at 1ma, or 100000 LEDs running
at 100ma each...we have no idea yet, and we really need this info
or we are stabbing in the dark.
LEDs vs Bulbs, LEDs are winning.

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