I am using microspheres to create optical crystal patterns on a glass substrate. The source is halogen lamp and detector is a CCD camera both on opposite side of the substrate. I see a broad absorption (-ln(P/P0)). The excitation spectra of the particles also look close but I believe it is usually taken by adding filters in the light path (epifluorescence microscopy etc.)
How can I be sure that the absorption I see is not due to the excitation spectrum? Do we expect to see the absorption due to excitation in broad spectrum as well? (380 nm - 750 nm)
Could you clarify what you are measuring and how?
An excitation spectrum usually refers the the spectrum of excitation of a photoluminescent emission.
This emission is usually measured at longer wavelength that the excitation wavelength. The excitation spectrum is recorded by varying (with a monochrometer) the wavelength of the excitation, and a filter (between the sample and the detector) is usually used to cutoff the excitation, but transmit the longer wavelength emission.
As excitation is due to absorption, the excitation spectrum normally resembles the absorption spectrum.
It is difficult to answer a question that it not clear. If I understood properly, you named absorption to the power reduction when light is going through your sample. But you don’t mention that it is wavelength dependent.
Absorption and excitation are wavelength dependent. For absorption you need monochromatic light just before the sample or, if you use white light, just before the CCD to analyse the attenuated light as a function of wavelength. The main difference between absorption and excitation is that the later is a selective technique, whereas absorption spectrum integrates all possible sources of light attenuation, excitation refers only to the optically active molecules, ions, etc. that induces luminescence at a certain detection wavelength. So, it is possible to separate contributions from different emitting species.
In the simplest case both can be the same.
With the modern fluorometer, the filters in the emission path are used to avoid detection of second order light. For instance, if you detect luminescence at 600 nm, excitation at 300 nm will produce a peak due to second order light through the monochromators. Additionally you should take care because some filters present luminescence itself producing false signals.
if this is not fully answering your question, please reformulate the question and maybe someone could help you.
In Electrooptical materials with applied and without voltage ,
The hugh extinction ratio and absorption need before any thing the best the alignment this can be checked by Malta cross patterns
it can calculate the absorption by known transmission without voltage
the extinction ratio by measure the maximum transmission without voltage and close it by voltage
Thank you everyone for the answers Dr Raad Sh. Alnayli , Dr. Rafael Valiente , Dr. Hugh J Byrne . Here's the addition:
1. The absorption I see is wavelength dependent. I see a 75 nm wide peak which is something what I expect.
2. I am measuring with a halogen source (Broadband) on one end, the light travels through a focusing mirror. The sample is placed after the mirror and then there is a mirror and grating assembly to divide light to VIBGYOR and finally a CCD camera to read the intensity of light.
3. The microspheres that I used have an excitation spectra (provided by the company) which is very close to my absorption results (similar wavelengths +- 10 nm).
I guess, the question is, I am not sure if the absorption I see is due the excitation or is it the absorption due to the optical crystal (which I expect to see).
In what I can make out, I guess the magnitude of absorbance at excitation wavelength of light should be very less compared to the absorbance due to optical crystal. Also, I am not using any filters to particular focus on the wavelengths. In my understanding, with this setup and all the broad light coming in, it will be tough to see an absorption due to excitation.
With a set up of:
light, sample, grating, CCD
You measure the absorption (or change in transmission) due to your sample.
An Excitation spectrum is a term usually used when you are measuring photoluminescence. For this you should have:
light, grating, slit (to monochromate the light), sample, filter (to isolate the luminescence range), detector.
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