Technical Tip from our Radiometric Calibration Lab
Written by Yishung Juang, Electro Optics Engineer
We frequently get questions about measuring a light source using a bare fiber, particularly when light levels are low or a reduced field of view is desired. This method requires a few extra considerations, however, which we will discuss in this tech note. To learn more about your options for radiance measurements, visit our Irradiance Measurement Techniques page.
Radiance versus Irradiance
Radiance is a measure of the radiant flux emitted or incident on a unit surface area, per unit solid viewing angle, in the units of W/cm2/steradian. This is different from irradiance, which measures the flux density passing through a plane, in units of W/cm2. We’ll begin by discussing radiance measurements, which are somewhat simpler to perform with a bare fiber than irradiance measurements.
Bare Fiber Field of View
Measurement with a bare fiber gives access to a 25° full-angle field of view (FOV). The spot size sampled will have a diameter that is roughly half of the distance to the object or plane of measurement. A bare fiber thus works well for looking at the light coming from a general area, such as the sky, or when working at a close distance to a light source so that it fills the field of view. For samples smaller than the FOV of the fiber probe, an Ocean Optics Gershun Tube Kit (GER-KIT) can be added to reduce the FOV of the bare fiber to as little as 1° to match that of the sample (although this must be accounted for in the calibration).
However, bare fibers are much more sensitive to the optical and measurement geometry relative to probes affixed with cosine correctors or other optics. To produce a consistent measurement using a bare fiber, care must be taken to ensure that the size of the sample to be measured is larger than the field of view of the fiber. A key benefit of bare fibers is that, unlike cosine correctors, they do not attenuate the signal, making it easier to measure low light levels.
Calibration of a Bare Fiber
A NIST-certified spectral radiance light source is required to calibrate a bare fiber for radiance measurements. At the Ocean Optics metrology lab, an emissive NIST-certified sphere source is used with a 2” light port with the fiber end positioned 4” from the port, as shown below.
The known spectral radiance (in μW/cm2/nm/steradian) of the NIST source is compared to the corresponding spectrometer readout (in counts/sec) measured using the fiber, allowing the ratio of the known spectral radiance over the raw spectrometer readout (scope mode) to be calculated to generate a calibration coefficient per unit angle. This yields a calibration correction in μJoule/cm2/steradian/count. The probe-to-source distance, though set at 4”, is not as critical as the FOV of the probe, which must fall completely inside the 2” port of the integrating sphere. Ocean Optics can provide either a factory calibration of the probe combined with the spectrometer or alternatively, provide a calibration light source that you may use to perform your own calibrations.
Measurement with a Bare Fiber
The ideal application of a bare fiber probe is the measurement of a large area source with diffuse emission such as an LCD screen. To ensure the accuracy of measurement, we recommend that bare fibers be used to measure objects that fit this characteristic.
When used to measure point-like objects (e.g., LEDs, lasers or fiber sources), the probe can over-estimate the optical output of the objects. In one example, when a bare fiber probe is calibrated with a NIST certified sphere source (i.e., an extended and diffused source) and used to measure a NIST certified FEL lamp (i.e., a point-like source), the probe overestimates the FEL lamp irradiance by 2-3x. In addition, the measurement is prone to inconsistency due to the probe’s sensitivity to the alignment between probe and object and to the setup perturbation that can result in a read-to-read variation of 30-40%.
For a customer who needs to measure point-like objects with a bare fiber, a NIST certified FEL lamp can be used to calibrate the probe to assure that the probe is calibrated with objects with similar characteristics as in the measurement. This helps reduce the measurement inaccuracy (e.g., the 2-3x described above), but produces no improvement of the 30%-40% read-to-read imprecision that is added twice: once during calibration and once during measurement. The calibration of the probe with a NIST certified FEL lamp is being phased out as a standard service and is now considered on a case-by-case basis. Nonetheless, by working with customers closely, Ocean Optics can typically devise an alternative calibration method with the measurement accuracy and precision that fits the customer’s needs.
Converting from Radiance to Irradiance with a Bare Fiber
Measurement with a bare fiber is typically in the unit of radiance (μW/cm2/nm/steradian). In applications where the measurement must be presented in a unit of irradiance (i.e., μW/cm2/nm), a conversion between these two units of measurement is required. In theory, the conversion is as below:
where Ls is the measured radiance (in μW/cm2/nm/steradian) and Ω???? is the solid angle (in steradian) of the probe, which is ???? ×(NA)2 where NA is the numerical aperture of the fiber (typically 0.22 for all fibers made by Ocean Optics).
Presently, calibration results with bare fiber, including the Ocean Optics calibration lamps, are presented in the unit of irradiance that is historically the default unit used in Ocean Optics acquisition software (e.g., OceanView and SpectraSuite). One must be mindful when working with a bare fiber at the irradiance space that the bare fiber calibration assumes a fiber NA of 0.22. For irradiance measurement using a bare fiber with different NA (e.g., 0.35), please consult Ocean Optics. In most cases, the solution is as simple as the application of a scale factor to the measured result.
Bare fiber probes work well for making radiance and irradiance measurements of weak sources as they have no optics to attenuate the signal. This makes them useful tools for low-light applications where optics such as a cosine corrector can be limiting. However, be mindful that fiber measurements are sensitive to illumination conditions and that errors can be introduced when conditions are different between the calibration and measurement. The standard calibration protocol now in place ensures good accuracy only when measuring an extended and diffused target. For other applications with bare fibers, please consult Ocean Optics.
Contact us to learn more about the best sampling optic for your radiance or irradiance measurements and calibrations.