OSRAM is a large multinational lighting manufacturer and lighting solution-provider.

Historically producing incandescent light bulbs – their name is a portmanteau of filament materials Osmium and Wolfram – they are now one of the largest manufacturers of optoelectronic semiconductors in the world.

Supported by e-SI-Amp partners PTB, the OSRAM reference laboratory are now using the ULCA (Ultrastable Low-Noise Current Amplifier) to support calibrated optical measurements.

Integrating sphere and control equipment

The Central Laboratory for Light Measurements, Augsburg is the DAkkS-accredited reference laboratory responsible for providing traceable photometric and radiometric light measurements for OSRAM. They use integrating spheres (up to 3 m in diameter), gonio-photometers and optical benches (up to 35 m of measuring distance) with optical sensors, spectrometers and other equipment necessary to test light-generating devices in standard or custom configurations.

Mirror goniophotometer

In addition to other lighting and radiation measurement services, an accredited luminous flux measurement capability in the range 50-50,000 lumen was added in 2010. This uses calibrated photocurrent sensors and amplifiers traceable to SI standards at National Measurement Institutes. Electrical current measuring instruments play a key role in the calibration chain for traceable optical measurements. For instance, the robotic goniophotometer measures the angular distribution of light using mechanical movement of up to four individual photometer heads, each calibrated and matched to the spectral response function of the human eye.

The readout signal is an electrical current in the range 1 pA to 1 mA, typically measured with an integration time of only 10 ms. Sufficiently sensitive readout instruments are available (e.g. Keithley 6485), but traceable calibrations in this range using a voltage-across-shunt-resistor method have been limited to more than 1000 part per million (ppm) accuracy.

Ultra-stable Low Current Amplifier

OSRAM are now taking advantage of the ULCA as an instrument for precise current measurement and generation, reducing calibration uncertainties by up to two orders of magnitude. The ULCA is calibrated at PTB with a 14-bit cryogenic current comparator (CCC) with 0.03 ppm accuracy (corresponding to expanded uncertainty, coverage factor 2) and then used to calibrate multiple readout instruments in the optical lab. The excellent robustness of the ULCA provides temporal gain stability of typically 2 ppm/year even in this industrial calibration environment. Support and training have been provided by PTB to make best use of the instrument. Feedback from OSRAM has been positive:

 “By its ease of use, low uncertainty (even at measuring times below 1 hour), and long recalibration interval (3 years, i.e. reduced cost of ownership) it has improved our calibration and measurement capabilities significantly in the ranges 1 pA – 1 mA, and 1 GOhm, delivering better values (likewise for cost and uncertainty) to our customers in the international calibration market.”      Head of Internal Calibration Laboratory, OSRAM