Nathan, Would this procedure require a database of known spectra which included the relative (or absolute) strength of each emission peak or just the wavelength of each peak? I don't think anyone has calibrated a Public Lab spectrometer so that peak height can be compared in a very meaningful way. (The problem is that the cameras record data in three color channels which vary in sensitivity across their wavelength range and also overlap with each other.) If only the wavelength of each peak is required, this approach could be tested with fluorescent bulbs which have clear emission peaks from three elements (mercury, terbium, europium) with well known peaks in the visible and near IR range. Good databases of emission peaks exist, but it might be easier to make a test database by scraping the data associated with this image: https://en.wikipedia.org/wiki/File:Fluorescent_lighting_spectrum_peaks_labelled.svg

Speaking of fluorescent peaks, I just tried your latest peak detection macro on a spectrum of a fluorescent lamp, then tried your new download macro and popped the peak trace into Excel, and Wham:

That is the peaks (using all defaults) of my Snowy Sky CFL spectrum. It seems to be an excellent record of the spectrum. Very cool.

@cfastie It is possible to use binary ICA if only the peaks are known. However, in binary ICA all smooth continuous spectra would look nearly the same, it throws away a lot of information so I would expect the results to be much less accurate. Calibrating the camera's image sensor so the the intensities are at least proportionally correct seems feasible, but I think it would require being able to photograph something with known intensities, which might not be easy to find. Maybe this could be accomplished by including standardized glow in the dark rainbow strips with the public lab spectrometers? Thanks for the fluorescent bulb suggestion. Are the emission peak databases you mentioned behind a pay-wall? And do they only include the peak locations? I'm glad you're happy with the macro :)

There are plenty of open sources of spectral data. Some are listed at this Public Lab wiki. I think the NIST database is a good standard with reasonable filtering and downloading capability. It has peak location and intensity, but includes lots of minor peaks that Public Lab spectrometers probably won't detect.

If you take a spectrum of the sky, you can see the inconsistent effect of the three color channels. Assuming that the intensity should be constant from 400 to 900 nm, you can take the non-constant spectrum and derive the function to correct it. That will be valid only for that camera at that white balance setting. But it is probably worth doing if you want to compare peak intensity.

No reason to assume that intensity is constant from 400 to 900 nm, because we apparently know that it is not:

So use a curve like this and correct a sky light spectum from your spectrometer to match it. Then use that correction on all spectra from that spectrometer. Make sure you keep white balance the same (no auto WB!) and don't over expose any peaks in any channels. I'm not really sure how to best take a spectrum of sunlight.

I think bouncing a light off the surface of a substance is not going to produce atomic or molecular emission. You have to excite the atoms or molecules. Without that excitation, you mostly learn about the reflective properties of a surface and usually not much about its elemental makeup. Raman spectroscopy uses a laser to interact with the molecules in a substance and then detect shifts in some (scattered?) wavelengths. The Public Lab spectrometer is not there yet.

Some elements will produce emission lines when heated up a little (Na or Cu in a flame) and some will fluoresce at characteristic wavelengths when hit with higher energy light (a UV laser). An obstacle to detecting the signature wavelengths is that other emission in the gas flame or fluorescing substance is noise.

When calibrating against the sun it seems like the spectrum would vary significantly depending on atmospheric conditions (e.g. in a smoggy city, during a sunset or on a overcast day).

I think you've ruled out using emission spectroscopy for analyzing material composition with a simple experimental setup. But perhaps it's possible to use absorption spectroscopy to determine an objects composition from reflected light-bulb light.

I guess you're right that taking a spectrum of the sun is not going to allow people in Santa Fe and Beijing to compare their peak heights.

Simple experimental setups that could allow elemental identification:

• gas discharge tubes
• flame spectroscopy (compare flame with flame+substance)
• fluorescence (compare UV laser in medium with UV laser in medium+substance)
• absorption spectroscopy (compare light transmission through liquid with liquid+substance)
• gas absorption spectroscopy (compare light transmission through gas with gas+another gas)
• others?

Hi @Cfastie, Do you have a reference for the sunlight spectrum you posted in your comment. for my research, I need a reliable reference for the complete spectrum of daylight. It should include the UVB, UVA, visible, and IR that exist in daylight. Any help?

I found that image in Googleland, I'm not sure where. Wikipedia has a complete article with lots of references: http://en.wikipedia.org/wiki/Sunlight