CIE presenting its members:
A New Calibration Standard Based on High Powered LEDs
The rapid development of high-power LEDs in recent years has lead to a big demand for their correct measurement by use of an appropriate calibration standard. For this reason Instrument Systems saw the necessity to create a new calibration device that meets modern requirements.
A calibration LED’s radiant power must be reproducible and remain stable, which requires excellent thermal management. The conventional approach to stabilizing standard LEDs is to keep the LED chip’s temperature above the ambient temperature using a heating element controlled by the forward voltage of the LED. Unvarying forward voltage ensures the emitted light intensity remains constant. However, the recommended substitution method of the CIE requires that a reference standard is used to calibrate an instrument, whose characteristics match those of the actual measurement object as closely as possible. This means, the reference standard should be a high-power LED and not a conventional standard LED if high-power LEDs are to be measured.
Therefore, the new calibration LEDs from Instrument Systems feature high-power LEDs that, on the other hand, generate considerable heat. To prevent temperatures from rising, a Peltier element transports heat away from the LED. This cooling process has the added advantage that it effectively reduces degradation of the LED. An electronic circuit comprising a TEC controller and a temperature sensor sited near the LED chip is used to stabilize the temperature. The LED and the above mentioned components are assembled in a small housing with very low thermal conductivity. This immunizes the LED against external temperature fluctuations and achieves excellent long-term stability and reproducibility of 0,1 %, both of which are indispensable to taking reliable measurements.
The Calibration LEDs come in the four key colours blue, green, red, and white to cover the entire visible spectrum. Averaged LED intensity typically ranges from 0,5 cd to 2 cd, and luminous flux ranges from 1 lm to 3 lm depending on colour. For every LED type, the LED control software sends a discrete set of PID control parameters to the TEC controller. This reduces initial transient time to less than two minutes, after which the forward voltage (UF) and temperature remain constant. This means that the calibration LED is ready for use after just a few minutes, saving valuable time in the lab.
A diffuser located in front of the LED generates a nearly perfect Lambertian radiation pattern that is desired for calibration purposes. The diffuser’s narrow diameter of just 7 mm ensures the unit’s light emission is ideal for calibrating averaged LED intensity. What’s more, the white LED achieves constant colour temperature regardless of emission angle.
Instrument Systems’ ISO 17025-certified test laboratory can measure calibration LEDs’ averaged LED intensity (ILED-A and ILED-B) and luminous flux according to CIE127:2007. A test certificate documents traceability to the standard LEDs of the German national laboratory PTB (Physikalisch- Technischen Bundesanstalt), as well as associated measurement uncertainties.