biased subcells in this example subcells 102 and 104 thus a small signal analysis is appropriate and the difference between the eqe measured at a test subcell 106 and the true eqe of the test subcell 106 as the current i o at the bottom subcell 106 may be influenced by the leakage current i lk m b output by the overlying subcell 104 comes from the difference in the output current i o measured from the triple junction solar cells and the photocurrent i ph generated in a subcell in other words measurement artifacts i e the difference between the true eqe and measured eqe result from the subcells e g 102 and 104 which are dc biased but which also contribute because they absorb photons weakly in the test wavelength range to the photocurrent i ph thus dc biasing the subcells under test results in an imperfect or somewhat inaccurate measurement of the eqe of the test subcell 106 because the dc biased subcells 102 and 104 contribute small signal outputs to the total output photocurrent continuing with this example in the ge wavelength range which in various embodiments may comprise the wavelength range to which test subcell 106 is tuned the i v relation of the ge subcell 106 under the chopped monochromatic light may be written as follows with reference to the notation used in equation 8 which adds an output term i o to account for the photocurrent in subcell 106 i ph b and v b may represent the photocurrent and voltage.