FAQs for spectral responsivity calibrations

A list of frequently asked questions is provided below, grouped according to the following categories:

Requesting calibrations
Traceability and quality system
Technical questions

Requesting calibrations

1. What's the difference between calibration (fixed) services and special services?
   Calibration or fixed services (Service ID numbers ending in the letter C) have fixed measurement conditions and NIST issues a calibration report to the customer.
   
   Special tests (Service ID numbers ending in the letter S) have no fixed measurement conditions. These calibrations are for unique customer-supplied test items. Prior to submitting a formal request, you must contact jeanne.houston [at] nist.gov (Jeanne Houston).
    
2. How do I submit a formal request?
   A formal request involves two parts: 
   First, a purchase order must be placed through NIST Technology Services. For ordering instructions for domestic and international customers, see Calibration Services. 
   
   Second, all test detectors should be shipped to the following address:
   Jeanne Houston
   NIST
   Building 221, Room A216
   100 Bureau Drive, Stop 8441
   Gaithersburg, MD 20899-8441
    
3. How do I request a quote?
   Please contact jeanne.houston [at] nist.gov (Jeanne Houston) to request a quote.
    
4. How quickly will my calibration be completed?
   Our goal is to return all calibrations to the customer within 90 days from the receipt of the purchase order and the calibration artifact(s). For foreign customers, it is 90 days from receipt of the prepayment and the calibration artifact(s).
    
5. Can I pay a fee to expedite my calibration?
   No. Calibrations are handled "first come, first serve."
    
6. Where can I find the NIST Calibration Services Policies?
   See Policies for an explanation of the policies for NIST calibration services.
    
7. Where can I find more information about ordering NIST calibration services?
   For US customers, see Ordering instructions for domestic customers.
    
8. I am outside of the US, what special conditions apply to me?
   The most significant condition for foreign customers is that NIST requires prepayment before any measurements can start. See Foreign inquiries for more information.

Traceability and quality system

1. What is NIST traceability?
   The definition of traceability in the International Vocabulary of Basic and General Terms in Metrology (VIM; 1993) is "the property of the result of a measurement or the value of a standard whereby it can be related to stated references, usually national or international standards, through an unbroken chain of comparisons, all having stated uncertainties."
   
   It is important to note that traceability is the property of the result of a measurement, not of an instrument or calibration report or laboratory. It is not achieved by following any one particular procedure or using special equipment. Merely having an instrument calibrated, even by NIST, is not enough to make the measurement result obtained from that instrument traceable to realizations of the appropriate SI unit or other stated references. The measurement system by which values are transferred must be clearly understood and under control.
   
   For more information on traceability, see the NIST Traceability website.
    
2. How do I get NIST traceability? (Do I have to send my detector to NIST to be NIST traceable?)
   See the website listed in the answer above.
    
3. Can you tell me if my supplier, vendor, or commercial calibration lab is NIST traceable?
   No. "NIST does not define, specify, assure, or certify traceability of the results of measurements or values of standards except those that NIST itself provides." For more information, see the NIST Policy on Traceability.
    
4. Why will NIST not sign our PO acceptance form or other contractual statements like our other vendors?
   As an agency of the United States Federal Government, Department of Commerce, the National Institute of Standards and Technology attests solely to the provisions described by our Calibration Services Policies. Receipt of orders by NIST does not imply acceptance of any provisions set forth in the order that are contrary to the policy, practice, or regulations of NIST or the U.S. Government. In general, NIST will not sign any affidavits, acknowledgment forms, or other documents that may be required by any domestic or foreign entity for policy governing procurement of goods and services.
    

Technical questions

1. Over what period of time is the calibration valid?
   We do not give a 'valid' calibration interval because it depends on the customer's requirements, how they use the device, and their quality control system. We do recommend 'periodic' recalibration, but the 'period' is determined by the customer. Most customers send their diodes back every year or two.
    
2. Must the light beam I use with the photodiode have similar characteristics as the one NIST uses to calibrate the photodiode? (i.e., a diameter of about 1.5 mm, a power less than 20 microwatts, and a bandwidth less than 4 nm?)
   No. But, consider how varying from the stated measurement conditions will affect the results of your measurements. For example, the uniformity of the spectral power responsivity can vary greatly over the active area of the photodiode and with wavelength. If your beam is significantly different in size, you need to consider how the non-uniformity affects your results.
    
3. What is the minimum recommended beam power level?
   There is none. In practice, the minimum, power level that can be measured is a function of the photodiode and the electronics used to read the signal from the photodiode. In other words, the quality of your equipment determines your ability to make low level measurements. However, if you are using your photodiodes for photon counting, please provide us with more details about your measurement and the actual levels you are trying to measure. Photon counting is an entirely different subject.
    
4. What is the maximum recommended beam power level?
   For the typical measurements, the maximum power level occurs when the detector responsivity becomes nonlinear. This point is determined by the detector and the electronics used with the detector. For detailed information on detectors and electronics see the NIST Technical Notes listed in the next FAQ answer.
    
5. Does the photodiode come with a measuring device (voltmeter)? If not, what measuring device is recommended?
   NIST does not provide a measuring device. Most customers use an current-to-voltage convertor (op-amp), either home-built or commercial. You will want to measure the current from the photodiode, and there are several vendors who can supply equipment to do this. However, do NOT use a DMM to measure the current from a photodiode. Because most DMMs measure current by measuring the voltage drop across a resistor, you will not be measuring the current from the photodiode correctly.
   
   For information about building electronics for your detector, see the following publications by george.eppeldauer [at] nist.gov (George Eppeldauer).
   
   NIST Technical Note 1438 - Optical Radiation Measurement with Selected Detectors and Matched Electronic Circuits Between 200 nm and 20 m (13.9 MB)
   NIST Technical Note 1621 - Optical Radiation Measurements Based on Detector Standards (8.0 MB)
   
   Commercial devices vary by price and quality. Make sure to consider stability. An amp meter may have good specifications, such as low noise, high dynamic range, etc., but fluctuate daily in absolute measurements.
    
6. Can NIST provide a list of acceptable vendors for photodiodes, amplifiers, etc.?
   NIST is a US Government laboratory and as part of our policy NIST does not recommend or endorse commercial equipment, instruments, or materials. Any products mentioned in NIST publications or on the NIST web pages are identified to foster understanding. The materials or equipment identified are not necessarily the best available for the purpose.
    
7. I am intending to calibrate a radiometer detector with a detector having a diameter of 1 cm. Do you have any recommended procedure for such a device?
   No. However, you can read how we do our calibrations in the following NIST publication, NIST SP 250-41-2008 (pdf).