Lasers VS LEDs

A lot depends on the wavelength involved. You may end up using something you would not have thought of at first. For example, consider that different substances emit different spectra of light when exposed to energy, as observed in mass spectroscopy.

LASER used as source at 1300/1550 nm wavewlength communication systems

LED used as source at 850 nanoimeter wavelength communication systems

read optical fibre based communication systems for further information.

Electrone,

I agree with your first statement. Any recommendation would have to be made based on the wavelength that is of interest. Unfortunately, you have picked a bad example. Mass Spectrometry has nothing to do with emitting light. It is based on the separation of ions based on mass, traveling through a magnetic or electrostatic field.

Palooka-What wavelengths are you working with? Typically, fluorescence of dyes requires excitation in the UV, and a narrow band of wavelengths are not needed. LEDs are not available for deep UV, but there are a bunch of LEDs available at wavelengths that allow fluorescence of dyes.

If I have missed the mark, you may want to look at tunable diode lasers, which are diodes that emit laser light.

Tad

I don't know what your budget looks like. From the information in your question I would suggest that you use a vertical external-cavity surface-emitting laser diode. These devices are available in parts of the the spectrum that will facilitate florescence and it is fairly easy to control the output power (between mere illumination to cutting) from these devices by driving them with a current source. Caution; take care with the design of the current source to build protection in not to exceed the diode's limitations. Also ensure that you have adequate heat-sinking.

LED's typically have longer lifetimes and are smaller so installation has more options. LED's can give a patterned illumination which only matters if you are imaging an area. If all you are interested in is the total fluorescence and not the imaged area the patterning will not matter. The patterning can be due to interference fringes and can look similar to a fingerprint but is only visible in magnified images. LED's can also have wider band widths than lasers which only matters if the imaging / capture method picks up both the excitation (LED) and emission spectra, although this can be prevented by using a band pass filter to block the excitation spectra from reaching the capture source.

Lasers generally are more expensive with shorter overall lifetimes but are available at a wide range of precise wavelengths, generally ±1 nm, with wider power ranges and generally are provided ready collimated. Be careful not to over illuminate as you can bleach photosensitive dyes and thereby inactivate them. Laser pointers are relatively cheap with power restricted to 5mW USA and 1mW EU and are available in red (650nm) green (532nm) and blue (473nm), these might be worth investigating if the power limitation is acceptable - their bigger powered brothers are also available but are not classified as laser pointers.

You need to check the excitation and emission spectra for the dyes you are using, it is not generally necessary to excite precisely at the maxima, some dyes emit effectively with excitation wavelengths as low as 10% of the excitation maxima. Examples of excitation / emission spectra are readily available, see the following link: http://www.polysciences.com/shop/assets/datasheets/745.pdf

See also the following article on superluminescent LEDs, this may provoke further thought: http://www.rp-photonics.com/superluminescent_diodes.html

Hey thanks for all the input. The weavelength being looked at is in the visible range, soemwehre between 545nm and 560nm. Also the only LEDs Ive found that can emit the large amount of output I'm looking for are 10 and 15 watt LEDs, and i was wondering specifically if there is any difference between the light I will get from LEDs and the light i will get from the lasers. right now im looking into using phosphors to luminess but they dnt have very good conversion energies. so im wondering if lasers can be used to similiarly activate the phosphors to create the wavelength im looking for rather than LEDs because alsers seem to put more output than LEDs... i think.

The Wikipedia entry under "Phosphor" has some useful links, see also: http://www.ornl.gov/sci/phosphors/Pdfs/isa_Paper_473.pdf