The Key to an Absolute Spectral Radiant Flux Measurement

The Key to an Absolute Spectral Radiant Flux Measurement

In this week’s article I’m going to continue my discussion on light measurement using an integrating sphere. As we learned in my first article, there are two basic light measurement instruments used to determine the luminous flux of a test lamp, photometers and spectroradiometers. In last week’s article I discussed the four steps necessary to make an absolute spectral radiant flux measurement using an integrating sphere. Recall that when using an integrating sphere spectroradiometer for light measurement, we are using the substitution method by comparing the output of the spectroradiometer with a test lamp in the sphere to the output of the spectroradiometer with a lamp of known spectral flux in the sphere. A key component to the substitution method is a calibrated spectral radiant flux standard lamp.

National metrology institutes (NMI) are responsible for establishing a scale of total luminous flux. In the United States, this is maintained by the National Institute of Standards and Technology (NIST). Incandescent lamps are generally used for lamp standards because of their stability. The relative expanded uncertainty in the assigned values of total luminous flux for an incandescent lamp calibrated at NIST is 0.5% (Zong et al., 2018).

A master lamp standard is a spectral radiant flux standard lamp calibrated by a third party and used only as a reference to create working standards. Master lamp standards should be obtained from an NMI, such as NIST, or other accredited intermediate calibration laboratories, such as Labsphere. A working standard is prepared by calibrating the working lamp relative to the master lamp standard. Essentially, the calibration from the master lamp standard is transferred to the working lamp standard. The working lamp standard is intended for general use such as calibrating your integrating sphere spectroradiometer for DUT measurements.

Prior to calibration, suitable lamp standard candidates are seasoned and screened for output flux stability. Once the lamps are screened and selected the lamp standards are calibrated at a specific operating current measured using a precision DC ammeter shunt. The shunt is a calibrated four terminal resistor. It is imperative that the calibration lamp standard be operated at the specific operating current used to calibrate the lamp. The current applied to a standard lamp should never exceed the specified operating value.

All Labsphere’s lamp standards are first seasoned for 1% of their rated life and then screened for stability and repeatable performance before they are selected for calibration. The selected lamps are then calibrated directly referencing the National Institute of Standards and Technology (NIST) lumen for reliable calibration results.

Spectral radiant flux Standard lamps are often maintained in a group of at least three lamps. This makes it easy to detect any changes to the output of an individual lamp by intercomparison. For example, NIST maintains working standards in groups of six.

Spectral radiant flux standard lamps need to be handled with extreme care. Precautions include keeping the bulb and base clean and avoiding mechanical shock. Large lamps are generally burned base-up to relieve mechanical stress on the hot filament. Miniature lamps are burned base down. Standard lamps should never be handled or moved while the filament is still warm. Lamp Standards are generally used for fifty (50) hours of operation.

All our integrating sphere spectroradiometers include a spectral radiant flux standard lamp calibrated in our accredited calibration laboratory. To learn more about light measurement using an integrating sphere spectroradiometer check out our light measurement video series where we demonstrate the procedure using our illumiaPlus2 light measurement system.

Labsphere is the industry leader in light measurement and integrating sphere design. We look forward to learning more about your application challenges.

References:

Labsphere Inc. (2017). Integrating Sphere Radiometry and Photometry (Labsphere Inc. North Sutton, NH), Technical Guide, https://www.labsphere.com/site/assets/files/2550/radiometry_and_photometry_tech_guide-1.pdf

Zong, Y., Nadal, M.E., Tsai, B.K., & Miller, C. C. (2018). NIST Measurement Services Photometric Calibrations (National Institute of Standards and Technology, Gaithersburg, MD), NIST Special Publication (SP) 250-95, https://doi.org/10.6028/NIST.SP.250-95