Basic Light Measurement System Components
The quality of a light measurement system depends primarily on the quality of the measurement engine, the accuracy of the system calibration and the overall stability of the system.
- Measurement Engine: spectroradiometer, monochromator, photodector (including electronics)
- Optical Collection System: lens, integrating sphere, goniometer, CIE 127 condition B fixture
- Software, Power Supply for LEDS and SSLs (solid state lighting)
- System Calibration
Factors Affecting Spectroradiometer Accuracy and Performance
1. Integrated Selectable ND Filters
This allows the spectroradiometer to measure a very wide range of intensity levels form mlm to klm.
2. Bandpass Filters
Smaller filters equal better color accuracy. Smaller filters require high sensitivity detectors. Besides stray light, color accuracy of a system is determined by the size of the Bandpass and the number of pixels in the CCD. Smaller filters combined with an optimum number of pixels is recommended. Too many pixels can increase noise levels and lower sensitivity of the system.
3. Calibration Standards and Techniques
Check to ensure that the spectroradiometer is being calibrated by an accredited laboratory that maintains high standards for accuracy. Can the spectroradiometer be self-calibrated by the user, or does it need to be returned to the manufacturer for calibration? If it can be self-calibrated, how long will the calibration take and what equipment is required?
4. Dynamic Range
Dynamic range varies with different grades and types of CCD detectors. Low dynamic range means higher noise levels, which can lead to possible errors in color accuracy (especially at lower light levels).
5. CCD, Grating and Optical System Stray Light
High stray light can lead to large errors in color measurement. Quality of the CCD, grating and optical design is a major consideration. Error can measure as high as 0.05 for systems with low grade optics.
6. Temperature Stabilization and Cooling of the CCD
Temperature stabilization eliminates drift. Cooling of the CCD reduces dark current.
7. Mechanical Shutter
The mechanical shutter allows the system to measure subtract any reamining dark current and environmental noise immediately prior to an actual measurement. Mechanical shutters are useful for R&D and application that require high levels of accuracy.
Gamma Scientific Spectroradiometer Links and Resources
As the designers of the first commercial spectroradiometer for measurement of red LEDs in the early 1970’s (purchased by HP), Gamma Scientific has forty years of experience and knowldege manufacturing high-performance spectroradiometers.