Tutorial:
NIST Traceable Spectral Responsivity
Calibration of Photodiode Detectors

All Newport optical detectors are recommended for a 12 month recalibration interval. Newport maintains an advanced calibration facility to meet all of your National Institute of Science and Technology (NIST) traceability needs. In case of the 818 and 918D series photodiode detectors, computer-automated testing allows us to include a complete calibration report with every detector and matching attenuator, if included in the product. In-house reference standards are directly recertified at every wavelength to the NIST. Our comprehensive statistical testing, tight control of the measurement environment, and direct traceability gives you the highest-accuracy calibrations with results you can trust at every 10 nm step. This white paper provides an overview of the calibration system and procedure for calibrating photodiode detectors.

NIST Traceability

To keep the products in compliance with NIST traceability, each new low-power detector and re-calibration service comes with a full calibration report. The report lists the measured responsivity at 10 nm intervals across the spectral range specified for the particular detector. If an attenuator is included, calibration certification tables are provided for the detector with and without the attenuator. To ensure quality, Newport always inspects your detector for damage or artifacts that could affect the accuracy of your recalibration.

Newport maintains an advanced, fully computer-automated, calibration facility that has been developed with over 40 years of experience. It is built around a monochromator-based system that measures the spectral power responsivity of photodiodes in the 200 nm to 1800 nm spectral region. Newport’s 818 and 918D series detectors are calibrated to ANSI/NCSL Z540 standards using a NIST calibrated detector (hereafter referred to as the “NIST standard”) of the same, or similar, type. Newport maintains several NIST standards for various product types.

Taking into account both NIST’s and Newport’s standard uncertainties and standard deviations resulting from the calibration process, Newport specifies the accuracy of its detectors with a 95% confidence level

Detector Calibration System Overview

The NIST standards are used to calibrate Newport’s working standard detectors every three months. The working standard detectors are used to perform daily calibrations of both production units and re-calibrations of customer’s detectors. After a calibration, the detector is accompanied by a hard copy of measurements in the form of spectral responsivity (A/W) in 10 nm increments, over the entire spectral range for that specific detector.

The essential components of a calibration system include a light source, a tunable monochromator, a working standard detector, a power meter, and, finally, the detector undergoing calibration (usually referred to as a DUT, or Device Under Test). Figure 1 shows the basic block diagram of a calibration system.

Detector Calibration System Block Diagram
Figure 1: Detector calibration system block diagram.

Calibration is performed in an environmentally controlled room where the temperature is maintained at 22°C ± 0.1°C and the humidity is held to a maximum of 40%.

The calibration process is as follows. The appropriate working standard detector is put into the calibration system, as depicted in Figure 1. An automated program is initiated whereby optical power of the broadband light source is measured at 10 nm intervals. Since this detector is a fully calibrated detector, the data is accurate to within the accuracy specified by NIST, plus the error associated with transferring the calibration to the working standard. After the power data has been collected, the DUT is immediately put in the system in the place of the working standard. This time the data collected, again in 10 nm intervals, is current (in Amps) coming from the DUT. The current is divided by the corresponding power at that wavelength, as was measured by the working standard, and the detector’s responsivity is obtained (the units of responsivity is Amps/Watt).

The NIST traceability chart is given below. In the chart the term “Production Detector” refers to either a new detector going into stock or a customer’s detector undergoing re-calibration

Detector NIST Traceability Chart
Figure 2: Newport Photodiode Detector NIST Traceability Chart

Ga(u)ge Repeatability and Reproducibility (GR&R) Overview

The Gage Repeatability and Reproducibility (GR&R) is a statistical measure of the capability of a gauge (or gage) to obtain the same measurement reading every time the measurement process is undertaken for the same characteristic or parameter. In other words, GR&R is an indication of a measuring system’s consistency and stability. It is clear then that high GR&R numbers indicate instability and are thus undesirable. Lower GR&R numbers are indicative of lower detector uncertainty. Newport Corporation maintains high standards on our detector calibration systems in order to minimize the Gage Repeatability and Reproducibility (GR&R) numbers.

The GR&R has two major components, namely, repeatability and reproducibility. Repeatability is the ability of the same gauge (detector, in this case) to give consistent measurement readings no matter how many times the same operator of the gauge repeats the measurement process. Reproducibility, on the other hand, is the ability of the same gauge to give consistent measurement readings regardless of who performs the measurements. The evaluation of a gauge's reproducibility, therefore, requires measurement readings to be acquired by different operators under the same conditions.

There are several factors affecting a detector calibration system, including:

  1. Measuring instruments
  2. Operators
  3. Environment
  4. Detector standard
  5. Test Method

All of these factors affect the measurement readings acquired during each measurement cycle, although to varying degrees. Measurement errors, therefore, can only be minimized if the errors or variations contributed individually by each of these factors can also be minimized.

Newport’s detector uncertainty is calculated based on the GR&R study and the NIST standard

Calibration Modules

The calibration information is saved in a memory chip called the EEPROM, which typically packaged in the connector, permanently fixed or detachable, or in the housing of the detector. Important information such as the responsivity data, calibration date, model number, serial number, and any other company proprietary data is included. One should not forget to return the calibration module along with the detector for recalibration, if the detector comes with a detachable calibration module (Figure 3). The calibration module will be reprogrammed with the new calibration data. Also note that only Newport can update the calibration module manufactured by Newport Corporation, no other third party calibration house

If you ordered an 818 Series detector and forgot to specify the calibration module or want to obtain a calibration module without recalibrating the detector, you can simply order a new cal module by using part number 818-SCAL-OPT . When you contact Newport, a Customer Service Representative will ask you for your detector model, power meter model and detector serial number. Please have this information ready when you call us.

818-Cal-Mod
Figure 3: The /DB Calibration Module (right) and the /CM Calibration Module are detachable from the detector cable.

Newport detectors are calibrated to internal standards derived from a NIST calibrated detector of the same type with a tight process and environment control. Taking into account both NIST’s and Newport’s standard uncertainties and standard deviations resulting from the calibration process, we specify the calibration uncertainties of our photodiode detectors surpassing competitors' specifications.