Home science optical absorption test for approximate caffeine quantity in coffee? I started with How can you measure the caffeine content of a liquid at home? in the cooking stackexchange, but the test strips were only qualitative (yes/no) and don't seem to exist anymore, and the part about rubbing dichloromethane on ones gums frightened me off.

The 2014 NY Daily News article Scientists working on at-home caffeine detection test links to Validation of Caffeine Dehydrogenase from Pseudomonas sp. Strain CBB1 as a Suitable Enzyme for a Rapid Caffeine Detection and Potential Diagnostic Test (Mohanty, S. K., Yu, C. L., Gopishetty, S. and Subramanian, M., J. Agric. Food Chem., 2014, 62 (31), pp 7939–7946) but that's not for home use. I found this nice experiment but that is still serious analytical chemistry. 

However the article does mention the use of UV spectrophotometry, and that's something I could imagine playing with at home personally, using UV LEDs or other light sources, a Raspberry Pi camera or some photodiodes, a piece of diffraction grating film, basic physics, duct tape, etc. I'm OK with the optics and electronics, it's the chemistry I need the most help with.

The 1994 paper A Simple Method for Determination of Caffein Content in Tea Samples shows a correlation of caffeine assay from various types of brewed tea with absorption at 440 nm. Strong coffee would contain perhaps 5X more caffein, but being coffee, would probably be 100X or more higher in absorption in blue or UV.

This and this discuss UV absorption of caffeine in a solvent, but I am not sure if there are any solvents that I can find or use at home. Further, while I might be able to find a 275nm LED, it probably limits me to glass optics and expensive detectors, most plastics probably absorb too much there, and most silicon detectors have thin inactive layers that absorb this wavelength of UV; backside-thinned detectors are usually necessary. However, possibly the longer wavelength tail in the direction of 300nm might be useful.

My Chemistry Question: Are there any ways to separate the caffeine in coffee from the organics that make it so dark, using "household chemistry", so that I could try to check  blue or UV optical absorption? 

I'm only looking for a way to tell if brewed coffee has a big or a small caffeine "kick" to it, so even something as bad as +/-20% repeatability might do in a pinch. I don't need an absolute measurement - just something that roughly tracks the caffeine content. 

Possibly helpful reddit, What wavelength should I measure the absorbance of my coffee to get an idea of how strong it is?, which links to an article I can't read right now, as I am drinking coffee on the wrong side of the paywall: Measurement of caffeine in coffee beans with UV/vis spectrometer.



above: An old Certificate of Analysis from a Caffeine Standards Kit, from Agilent.





above x2: Caffeine absorbance in water, from here (top) and here (bottom).
Home science optical absorption test for approximate caffeine quantity in coffee?

Yes..., except that for &quot;...in coffee?&quot; it's No.
AAI Solutions' Measuring Caffeine Concentration; Applied Analytics Application Note No. AN-019 front artwork certainly looks hopeful...
 
(click for larger)
After all an Optical Multichannel Analyzer (OMA) in this case, to a home scientist, looks like some light source, a grating, three photodiodes and some tedious calibration, this is really for extracted caffeine in supercritical CO2.
However, from Background in 

As different literatures indicated spectrophotometric determination of caffeine is also reported as preferred method of determination such as UV–Vis spectrophotometry because of its relatively low cost, rapidity, high accuracy and reproducibility. But UV–Vis spectrophotometric method cannot be used directly for determination of caffeine in coffee beans extracted with water owing to the matrix effect of UV–Vis absorbing substances in the sample matrix [9]. In aqueous solution of coffee beans it was observed that there is spectral interference from caffeine and chlorogenic acid in the wavelength regions of 200–500 nm. Yet this method requires the extraction of caffeine from the aqueous solution of coffee beans using dichloromethane for the spectroscopic determination. This is necessary since the caffeine spectrum is overlapped with other compounds found in coffee. Hence, the use of dichloromethane limits the wider application of UV–Vis method.
[9] Genetic diversity and correlation of bean caffeine content with cup quality and green bean physical characteristics in coffee (Coffea arabica L.)

In simple language: Coffee is full of all kinds of molecules besides caffein, so there will likely be many broad spectral features in the UV interfering with any simple way to measure one particular molecule's concentration.
You're going to have to resort to some kind of specific extraction. If not supercritical CO2 mentioned above, then something like this:
SPECTROPHOTOMETERS: How can I measure the caffeine content in drinks? with 16 items of apparatus, two reagents including somewhat dangerous to &quot;home scientists&quot; Dichloromethane
or like this: Jenway's 73 SERIES SPECTROPHOTOMETER Application note: A09-010A; The quantitative determination of caffeine in beverages and soft drinks using UV wavelength spectroscopy again with the dichloromethane!
Bottom line
Try not to drink so much coffee that you need to worry about all of this. Listen to &quot;Sir Professor&quot; Martyn Poliakoff's and switch to tea. (note, I'm also the OP)