Obtaining 700nm light wavelength

[sophiecentaur]

Is this the same thing as frequency? I'm probably looking for 10hz and 40hz

So if I need 80mW/cm² in the bandwidth of 810nm to 850nm, with a 10% filter I need 800mW/cm²

Is that the bandwidth you need? That would be 5% and so a 10% filter would need twice that (160mW.cm²) with a flat spectrum. But you have confused me. Where did the 810 - 850 nm come from? The requirements are now a total mystery to me.
I was just pointing out that the narrower the filter, the less power is admitted. The power scales with the filter bandwidth if the spectrum happens to be flat. If the bandwidth (i.e. range of) wavelengths is narrow then you would need to start with a more powerful source. (pro-rata). This is why it's so important that the details of the required spectrum for the illumination are important.

PS The frequency you refer to would perhaps be the rate of flickering of the illumination (PhotoBioModulation)? (You can have a band of frequencies or a band of wavelengths and both seem to be relevant here)

 

[davenn]

If I had a white light bulb and covered it with the right colour of film (red-ish) will the resulting wavelength be about 700nm?

So if I need 80mW/cm2 in the bandwidth of 810nm to 850nm, with a 10% filter I need 800mW/cm2

so which do you really need ? something centred on 700nm or something centred on 830nm ?

 

[FredFlintstone]

Is that the bandwidth you need? That would be 5% and so a 10% filter would need twice that (160mW.cm²) with a flat spectrum. But you have confused me. Where did the 810 - 850 nm come from? The requirements are now a total mystery to me.
I was just pointing out that the narrower the filter, the less power is admitted. The power scales with the filter bandwidth if the spectrum happens to be flat. If the bandwidth (i.e. range of) wavelengths is narrow then you would need to start with a more powerful source. (pro-rata). This is why it's so important that the details of the required spectrum for the illumination are important.

PS The frequency you refer to would perhaps be the rate of flickering of the illumination (PhotoBioModulation)? (You can have a band of frequencies or a band of wavelengths and both seem to be relevant here)

Regarding the wavelengths - this is explained in posts 17 and 20 of this thread.

so which do you really need ? something centred on 700nm or something centred on 830nm ?

Please see post 20 of this thread.

 

[davenn]

Please see post 20 of this thread.

so why are you stating 810 - 850nm then ?
hence my Q, which do you really want ?

the clearer you are, the better answers you will get
else you just get everyone confused on what you are really after

 

[FredFlintstone]

so why are you stating 810 - 850nm then ?
hence my Q, which do you really want ?

the clearer you are, the better answers you will get
else you just get everyone confused on what you are really after

I believe I have clearly said that I am looking at the feasibility of producing any wavelength in the near infra red. Not everyone is confused - there has been plenty of good points and suggestions in this thread that have been enlightening and helpful - at least to me. So I'm satisfied, and thanks to all.

 

[Integral]

http://www.seabird.com/products/fluorometers
Are you aware of this line of optical oceanographic instrumentation. Seems that they measure water properties in a manor similar to what you seem to be investigating.

 

[sophiecentaur]

I believe I have clearly said that I am looking at the feasibility of producing any wavelength in the near infra red

I know you have said that but it has no meaning as it stands. There are an infinite number of ways that statement could be taken, You could mean anything from a hot filament with a peak in its spectrum at around 810 - 850nm (a very broad black body spectrum) or a near- monochromatic source that can be tuned to a very narrow band, somewhere between your specified wavelengths. Those two requirements involve totally different solutions.
Just because no one else has pointed this out does not mean it's not relevant. If you could actually tell us what you mean by a source of "any wavelength" then I am sure it can be done. This is why I asked you for a reference to a similar experiment in which, hopefully, the answer would appear. PF threads can be very wide ranging and they are likely to diverge from what the OP wanted. They are not 'chaired' by someone to keep them on track and they often turn into a pleasant chat of members amongst themselves. You want a physical working solution to this that actually does the job you want. So tell us the job and that needs to be more than a shorthand description for a light source. I know you want a 'result' here and I am trying to make sure you get it.

 

[FredFlintstone]

You want a physical working solution to this that actually does the job you want. So tell us the job and that needs to be more than a shorthand description for a light source. I know you want a 'result' here and I am trying to make sure you get it.

Sorry if my initial question caused confusion but I have already clarified these points. My intended application is photobiomodulation (PBM) - which is a branch of medicine that has to do with therapy through light, particularly infrared. Most PBM medical devices use LEDs for this purpose. I was hoping that filtered white light could be used so that multiple wavelengths could be used from a singe device. My purpose for this thread was to gain enough understanding of the different practical issues needed to produce any particular wavelength - or bandwidth - within the near IR spectrum to know which approach is feasible within my budget and timeframe.

I haven't studied Physics and did not have even a rudimentary understanding of light. So, the important take-aways for me so far have been:
1) colour is not the same as wavelength
2) you can't mix two different wavelengths to make another pure wavelength
3) LEDs can be purchased that will produce specific wavelengths, with a given power output
4) certain sources of white light that contain a desired wavelength, if powerful enough, can be filtered using optical filters to obtain a desired wavelength and power output at a desired wavelength

So I now have the knowledge to make some direction decisions. Is there more I could know? Yes, and I'm still interested in hearing creative solutions or corrections to my understanding.

 

[FredFlintstone]

http://www.seabird.com/products/fluorometers
Are you aware of this line of optical oceanographic instrumentation. Seems that they measure water properties in a manor similar to what you seem to be investigating.

Thanks for the link. Interesting gear. However they seem limited to 700nm range unless I'm missing something. Cheers.

 

[sophiecentaur]

Most PBM medical devices use LEDs for this purpose. I was hoping that filtered white light could be used so that multiple wavelengths could be used from a singe device.

Now we're talking. Your list of things you have learned is great and you may now be in a better position to make a good final decision as to which way to go with this.
Advice for the future: provide any forum with a good reference or two in your first post and save a lot of time going round the houses.
So you are interested in replacing LED sources with conventional alternatives? That's a novel thought. They have spent several years in the health business, developing high power LED arrays to replace halogen and other filament lamps. LEDs have many advantages (although, personally I haven't found long life one of them) including cool running of operating theatre lamps.
It could be worth your while approaching some manufactures with your actual requirement and see if they can offer you anything that's near to what you want or need.

 

[Merlin3189]

I wonder why no one suggests a prism?

 

[sophiecentaur]

I wonder why no one suggests a prism?

I wonder about the actual implementation of that idea over a usefully large target area. An interference filter would do the job just as well and the passband characteristic could select the precise band of wavelengths that are required.

 

[Merlin3189]

Yeah, getting enough light into a narrow beam would be difficult. It was what I first thought of - I'd imagined a bunch of cells or even molecules in a test tube.

It did seem to address the cheapness and variable wavelength issues.

It also, very early on, seemed like a good way to find out what was really wanted - colour or wavelength and degree of purity.

I suppose perspective comes into it. people being paid to do research naturally think of narrow band filters or tuneable lasers or anything with an impressive price, but people paying their own bills need to think what they can pick up from the FE for a few $.

 

[sophiecentaur]

I suppose perspective comes into it. people being paid to do research naturally think of narrow band filters or tuneable lasers or anything with an impressive price, but people paying their own bills need to think what they can pick up from the FE for a few $.

From what the OP has told us lately, we are not talking in terms of high accuracy or narrow band spectrum requirements.
Cheaper than all that are the vast number of stage gels that are available but, from what I can find in searches, the pass bands are not published. You would need to have a suitable spectrometer to select which is needed. It would be a tall order for a non- Physicist to implement a DIY spectrometer. It could be worth while approaching the major producers for any information they might have. (I wouldn't mind betting they do have it.
"Variable wavelength" can be achieved with a handful of (cheap) different filters.
I looked up Dichroic Filters and found this link which would probably have what's needed (at a price). Others are probably available.