Fois 1 fiber optic integrating sphere

Leaf optical measurements were conducted using a double integrating sphere system. An ISP-REF integrating sphere (Ocean Optics, Inc., Dunedin, Florida, USA) with a built-in collimated light source and a gloss-trap combined with a FOIS-1 fiber optic integrating sphere (Ocean Optics) were used. Both spheres were connected via fiber optic bundles to a S2000 UV-VIS-shortwave NIR spectroradiometer (Ocean Optics) with a resolution of ~0.5 nm. They were aligned using a rack-and-pinion slide (NT61–285, Edmund Industrial Optics, Barrington, New Jersey, USA) with a platform for each sphere. By clamping a leaf between the two integrating spheres, simultaneous measurements of leaf spectral reflectance (R) and transmittance (T) were possible. Absorptance (A) was calculated as A = 1 − (R+T).

The I–V characteristics of μ-ILEDs were measured with a probe station (Keysight B1500A Semiconductor Analyzer) to evaluate the power consumption.
Spectroscopic measurements with an integrating sphere (FOIS-1 Fiber Optic Integrating Sphere, Ocean Optics) and a spectrometer (FLAME-S-UV-VIS Miniature Spectrometer, Ocean Optics) yielded the emission spectra (absolute irradiance). Integration of the irradiance across relevant ranges of wavelengths generated the optical power at various currents (Keithley 6221 DC and AC current source, ATektronic Company). Recording the output voltage of the μ-IPDs yielded calibration curves for the intensity of red and NIR light separately.
An NTC thermistor (Murata Electronics), directly attached on top of the μ-ILEDs allowed measurements of changes in temperature. A digital multimeter (USB-4065, National Instruments), captured the changes in resistance during operation, to define the change in temperature according to $\mathrm{R}={\mathrm{R}}_0\exp \mathrm{B}\left(\frac{1}{\mathrm{T}}-\frac{1}{{\mathrm{T}}_0}\right)$ , ${\mathrm{R}}_0=10\mathrm{k}$ , $\mathrm{B}=3380$ .