Abstract: A solar energy flux integrator includes a semiconducting solar cell positioned to receive solar radiation, and a micro-coulombmeter connected to receive current from the solar cell. The micro-coulombmeter comprises a glass capillary filled with a mercury salt electrolyte having an indicating bubble and two mercury electrodes. A series resistor is provided which has a resistance value to slow the movement of the bubble when low levels of solar energy are incident on the cell so that movement of the bubble is directly proportional to the incident solar flux. This series resistor compensates for the spectral shift towards blue of the indirectly received solar radiation, which spectral shift otherwise causes the measured integrated value of solar flux to be too high due to the greater sensitivity of the solar cell to light of shorter wavelengths.