Public Lab Research note

DIY spectrophotometer?

by Mekis | April 16, 2017 10:54 16 Apr 10:54 | #14115 | #14115

Trying to build my own spectrophotometer by hand, and now I have two questions about it:

1. I will use a lens and a diffraction grating to create the different wave lengths. I found the math to calculate the strength of the lens (diopter); D = 1/f, and the place of the focal point; 1/f = 1/a + 1/b. But Im note sure I get it. Just want to check if Im right. This is what I think: If I have a diopter of 20, the focal point will be 5 cm from the lens? (1 divided by the diopter is the focal length).

2. The detector will be a mono meter or something like that. So the answer i will get out of me spectrophotometer will be in voltage. How do I calculate this into absorbance?




I am not familiar with the optics equations. My approach would be to enable precise adjustments of the components and find the best resolution (crispest diffraction pattern) by trial and error.

The voltages recorded for each cell of your sensor array will be a function of brightness. You will have to determine whether this function is linear. Then you will have the ability to measure the intensity of each wavelength in the spectral range of your device.

Absorbance (of a particular liquid) could be one of the applications of this device. You would compare the brightness of some wavelength band(s) with and without the liquid in the optical path.


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Absorbance is a chemical definition of how well a dissolved substance attenuates light at a particular wavelength as light passes through a solution of the substance dissolved in water. I assume you will plot voltage or some function of it versus wavelength and find the maximum at which your analysis will be the highest or most sensitive. To determine absorbance you must derive a ratio of the ordinate versus wavelength for water and the substance being evaluated and then do a log function of the ratio at the maximum wavelength. This calculated value is absorbance and the reason it is used in the lab is that it is proportional to concentration (Beer’s Law !!) and a quick measurement in a spec can give concentration. See

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