For multijunction cells, the spectral response allows to judge the carrier generation at various wavelengths/light colours.
But also, the SR can be used as a diagnostic measure for surface or volume recombination, diffusion lengths, carrier transport meachanisms and other device/material properties.

Due to the series connection of the individual subcells, coloured bias light is necessary to allow current flow in the series connection. If the top cell is illuminated with blue light, for example, it will absorb all the light. As a consequence, middle and bottom cell(s) would operate in the 'dark' and could not carry any current generated by the top cell.
To solve this dilemma in a measurement, middle cells and bottom cell are illuminated with colured bias light. This generates electron/holes pairs in these subcells, which then can carry the current that was geberated by the top cell. Special liht sources are used for this purpose, it can be filtered tungsten lamps or near narrowband coloured light sources. The more subcells are part of a multijunction cell, the more stringend are conditions for the coloured bias light.
Special care is necessary in setting the intensity of the coloured bias light. If the intensity is chosen too high, many carriers will be generated in the subcell. If the active cell (in this example the top cell) delivers only a small monochromatic current, the other subcells can esily be operating near their pen circuit condition. Now, they will voltage bias the active cell, if no voltage compensation is used in addition to the coloured bias light.

Spectral Response System for tandem/triple/multijunction solar cells
Cell types: all multijunction solar cells e.g. a-Si/a-Si, a-Si/u-Si, OPV, III/V semiconductor MJ cells, concentrator MJ cells, new technology thin film cells

Spectral range: any range with 250 nm to 3000 nm.
Bias light: white bias light for single junction cells. 1-4 coloured light sources to flood inactive subcells.
Bias light intensity level: up to 700 suns for concentrator - MJ cells
Cell area: sub - mm to medium area - special sizes on request
Monochromator: focal length 300mm, others on request
Input lamps: Xenon or Xenon/tungsten combinations. Lamp power between 75W and 1000W, depending on application.
Measurement procedure: AC with dual phase lockin amplifier, DC, down to < nA upon request.
Sample holder: using vacuum suction to hold cell, temperature control 18°C to 50°C (option 10°C to 80°C).

For information on measurement technoligy in general, please compare the section on single junction cells 

for more information, especially for measurement of tandem and multijunction solar cells, please see pdf:  SR for tandem/multijunction cells