The Ultra-Low DC Current source (ULCS) is a small current generating tool developed at Tubitak/UME. This provides a small reference current for the calibration of measuring instruments at very low currents. This instrument has recently been integrated into usage in the TUBITAK UME Optical calibration lab.

This ULCS is based on ‘capacitor ramp’ methods (a linear voltage ramp applied to a capacitor results in a constant current of I= CdV/dt. Traceability is provided via quantum voltage and resistance standards as for other reference sources. There are some advantages in the very low current range due to the absence of any Johnson noise (capacitance standards can also be relatively stable compared to high value resistors). The ULCS is used as a controlled reference current, in this case used to deduce the calibration factors of the current measuring instrument used in the optical detector.

Use of the ULCS in the TUBITAK UME OPTICS lab

Optical table in the UME calibration lab

The TUBITAK UME Optics Laboratories covers radiometry, photometry, spectrophotometry and fiber optic measurements. 

The ULCS is used to calibrate the sensor readout for the low level light detection measurements. Here the current range is from 3 pA to 30 fA. For electro-optical characterization of optical sensors, such as spectral nonlinearity, temperature coefficient of the responsivity from 300 nm to 1100 nm, polarization dependence, etc. the calibration range is 30 fA to 100 pA.

The ULCS calibration is used in the establishment of the low level spectral irradiance (from 3×10^-6 W/m² to 8×10^-9 W/m²) and illuminance scales (from 1×10^-1 fc to 2×10^-3 fc) of TUBITAK UME.

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