How do I measure reflectance with an ISP-REF?
The ISP-REF integrating sphere is very convenient for general-purpose measurements of reflectance in the lab and in the field thanks to the built-in tungsten light source. Not only is it compact, portable, low cost, but it works with a preconfigured UV-VIS or VIS-NIR spectrometer (adding an L2 or L4 collection lens boosts signal). It collects light that is reflected from the sample in all directions, giving it a full 180° field of view.
Light from the built-in light source enters the sphere from a small opening nearly opposite to the sample port, illuminating the sample evenly from inside the sphere. The reflected light gets scattered around the sphere and is sampled by a fiber at the connector marked “S” (sample).
The ISP-REF also has a reference port (marked “R”) for monitoring the light source directly with a second spectrometer channel. The reference port opens directly into the sphere, and is countersunk instead of baffled. It can make a direct measurement of the illumination light within the sphere. It can also be used to sample light collected through the sphere opening when looking at radiant objects.
So what is the switch on the side of the ISP-REF? It’s the gloss trap, a light trap inside the sphere designed to remove the specular portion of the reflection from the measurement. Flipping the switch opens and closes a shutter on the gloss trap, excluding or including the specular reflection component. The gloss factor of a surface can be calculated by comparing data taken with and without the specular reflection included.
Even with its many advantages, the ISP-REF does have some limitations, the biggest one being that you can only use the integrated tungsten light source for illumination. It also has the highest port fraction of the Ocean Optics integrating spheres, giving it somewhat higher error. Generally, port openings should be less than 5% of an integrating sphere’s internal surface area (it’s 2% for the ISP-REF).