With the recent boost in LED-based replacement lamps for fluorescent lighting, accurate testing of these devices is becoming extremely important to ensure consistency both in color and in brightness. To accurately test these sources for both color and total luminous flux, an integrating sphere is often used. We’ll explain the requirements for performing optical testing using an integrating sphere, as well as walk you through the process of calibration.
Characteristics of a Properly Designed Integrating Sphere
The integrating sphere chosen should have a high reflectivity coating, preferably >98% to ensure a good response. The sphere should also be properly fitted with baffles to prevent direct rays from impinging upon the detector port. A NIST traceable total spectral flux standard should always be used in conjunction with an auxiliary lamp to correct for any self-absorption of the test sample.
The exit port of the sphere should include a diffuser to guarantee the detector is able to “see” the entire sphere and not just the backside of the baffle.
4 Step Calibration/Measurement Process for Total Luminous Flux
To obtain an accurate total luminous flux measurement, the following steps of calibration/measurement process should be used.
- Install the NIST traceable standard of total radiant flux at the center of the sphere and provide power at the specified current setting. Using the OL 770 software, you can then take a reading of lamp output.
- Shut down the standard lamp, but do not remove it from the sphere and then power the auxiliary lamp and allow it to stabilize. Measure the auxiliary lamp and save the file for future reference. This is simply a transfer calibration, making the auxiliary lamp a temporary calibration lamp.
- While keeping the auxiliary lamp powered, remove the NIST traceable standard lamp and install your test fixture into the sphere. Open the calibration software and measure the auxiliary lamp again, this time with your test sample installed, but not yet powered. You’ve now created a total spectral flux system response calibration.
- With this new calibration loaded into the software, you can power your test lamp and allow it to stabilize prior to measuring.
Learn More About Gooch & Housego’s Integrating Spheres
Gooch & Housego offers a variety of integrating spheres for your optical testing needs. For more information on our products, don’t hesitate to contact our sales team today toll-free at 800-899-3171.