The optical fiber colorimeter described previously also doubles up as a Planck constant experiment. By removing the cuvettes in the cuvette holder, it is possible to directly illuminate the optical fiber with different LEDs. The Planck constant can be obtained by measuring the minimum voltage, *V*_{min}, required to power the LEDs. This minimum voltage is determined from the photodiode response which corresponds to the lowest possible signal that can be measured at the fiber tip. Using the relation for conservation of energy

*eV*_{min}=*hf*

where *e* is the electron charge and *f* is the optical frequency of the LED at its peak intensity, which can be measured with a spectrometer. The Planck constant can the be calculated as

*h=eV*_{min}/*f .*