What sampling optics can I use for relative irradiance?
Thanks to easy calibration, any sampling optic can be used for a relative irradiance measurement. The choice of optic will depend on the field of view desired, and the goal of the measurement.
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.
Flame loop or other probe: gives the same 25° full angle field of view as a fiber.
Collimating lens: if properly aligned for collimation, it samples a spot size equal to its diameter. Collimation can be checked by routing a light source in reverse through the fiber/lens pair. It works well for sampling light from a specific location at a distance.
Gershun tube kit: allows control over the field of view from 1° to 28° using various apertures and configurations. It works well for sampling light from a specific location at a distance, or with a particular field of view.
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 looking at how much light is incident on a surface, since ambient lighting or sunlight tends to illuminate surfaces from a wide range of angles.
Integrating sphere: has a 180° field of view at the location and in the plane of its sample port. There is no weighting by angle (unlike a cosine corrector), as all the light that passes through the sample port opening gets sampled by the sphere equally. It works well for looking at how much light per unit area is passing through a specific plane or incident on a surface. 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.