Home science optical absorption test for approximate caffeine quantity in coffee?
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)