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yiwu
Posted on Wednesday, May 26, 2004 - 8:33 am:   

Hi all,

I try to install an optical interference filter in my NIR instrument to reflect the low wavelength region, but i am not sure I really understand how it works. After installing, are there more lights or less lights getting to the detector? What does it affect the detector? Can I change a more powerful lamp?

Thanks a lot!
yiwu
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hlmark
Posted on Wednesday, May 26, 2004 - 12:35 pm:   

Yiwu - there are many kinds of filters that can be put into an optical system. Interference filters are normally designed to allow a fairly narrow range of wavelengths to pass through and block all the others. One of the key design parameters of an interference filter is the passband (the range it allows through). In fact, almost the only parameter that could be more important is the center wavelength of the filter - the nominal value specifying the center of the passband, which is generally considered "the wavelength" of the filter.

Are you sure it's an INTERFERENCE filter you're using and not some other kind? If you want to empahsize the entire low-wavelength end of the spectrum, you probably want a different kind of filter, not an interference filter

In general, however, unless you're a pretty good optical designer yourself, it's not a good idea to fool around with the innards of such a complicated device as an FTIR spectrometer. It takes some pretty sophisticated engineering to design an instrument like that to work optimally, and have non-ideal characteristics of one part be compensated by proper design of another part, etc. Changing or adding an optical component such as a filter, can have ramifications throughout the whole instrument. As an analogy, imgaine what would happen if you decided, say, to just add another gear to the transmisssion in your car!

Howard

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yiwu
Posted on Thursday, May 27, 2004 - 2:31 am:   

Hi Howard,

A million thanks first!

The filter what I will use is a longwavepass filter, which blocks the short wavelengths, and the wavelength that longer than 2000 nm can pass through. From your description, maybe it doest belong to interference filters.

I know it is not a piece of cake to change any part of the instrument, but I have to try, since it may enhance the sensitivity of the spectrometer. What confuse me are that how the filter affect the detector, is the detector not so easy to saturated with it? and if we can use a more powerful lamp than without filter. Does it really can improve the sensitivity of the instrument?

BR
Yiwu
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hlmark
Posted on Thursday, May 27, 2004 - 10:13 pm:   

Yiwu - if you put this filter in, then yes, it will certainly block out some of the wavelength range and the detector will not see it. Then you will probably be able to raise the source intensity, before you have to worry about saturating the detector, or another part of the signal-handling electronics.

BUT - - - there are some problems you will encounter.

The first, and minor problem, is that blackbody radiation increases more slowly at the longer waveelngths than at shorter wavelengths, as you raise the temperature. Since sources have limited ability to run at higher temperatures, you may not be able to increase the source intensity enough to make a difference, before you burn the source out from overheating. This is especially true since sources are normally operated at an intensity close to their temeprature limit in the first place, to get as much energy as possible.

The much larger and potentially more serious problem is a possible software problem. FTIR software uses the centerburst of the interferogram to determine the exact point of zero retardation, and several other paramaters it needs to perform the FT, to convert the interferogram into a spectrum. By cutting out all the short-wave wavelengths, the centerburst of the interferogram will not be as sharp and well-defined as it normally is, but rather will be broader and more rounded. Therefore, depending on how the software is written, there is a possibility that cutting out short wavelengths will make the computer be unable to determine all the key parameters that it needs and will therefore detrimentally affect the computer's ability to correctly convert the interferogram to a spectrum.

Of course, you may also be lucky and none of these problems will come to pass.

Howard

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