What sampling optics can I calibrate for absolute irradiance?
Fewer options are available for sampling optics when making absolute irradiance measurements, but fortunately they address the most common applications and needs. It is critical to check that the calibration source is properly matched to the sampling optic to ensure that the field of view is filled during calibration, and that the correct calibration data is used. For example, power output from a calibration light source will be different when a bare fiber is attached than when a cosine corrector is inserted.
Bare fiber: gives a 25° full angle field of view, sampling a spot size with a diameter equal to ½ the distance to the object or plane of measurement. It works well for looking at the light coming from a general area, such as the sky.
Cosine corrector: has a 180° field of view, with a Lambertian response. The amount of light collected will vary as the cosine of the angle of incidence on the cosine corrector. It samples any light incident on the diameter of its white diffusing surface, and specifically in that plane. It works well for measuring the power incident on a surface, in which case it should be placed flush with that surface for measurement.
Integrating sphere (ISP-50-8-I or ISP-80-8-I): has a 180° field of view at its sample port, and measures all light that passes through its opening. In this configuration, it measures the average radiant flux of a light source at a specific distance, in the plane of the sample port. It is particularly well suited to measuring the total light output of any source that can be completely enclosed by the sphere, like an LED. Similarly, it can measure a light source if its full power can be directed into the port (as in the case of a beam, for example).