The optical properties of an irradiance collector changes when the refractive index of the media in which it is used changes. All radiometers that we calibrate, whether they measure PAR, scalar or cosine irradiance, radiance, or spectral irradiance, are calibrated in air. In the aquatic or biological sciences, measurement of light sometimes needs to be conducted in another media, with water being the most common. Knowledge of how the properties change when immersed is critical.
At Biospherical Instruments, all sensors are supplied with calibration factors appropriate for the media in which they are used. In the case of sensors which are optimized for use in water, although calibrated in air, experiments are conducted to determine the change in instrument response caused by immersion in purified water. This change is called the "immersion coefficient" and the experiment is conducted by mounting a constant light source above a specially designed tank holding the sensor and its irradiance collector. The distance between lamp and collector is fixed, and the tank is baffled to suppress reflections and other stray light. During the analysis both attenuation by water and refractive effects are accounted for. In the case of spectral instruments, the immersion coefficient is determined at each wavelength; these wavelength-specific immersion coefficients are included in the instrument calibration factors. Figure 1 shows how the irradiance collector is set up, and Figure 2 shows the test results.
Figure 1. The 1.3 cm AMOUR irradiance collector is shown immersed in water in a tank to determine the immersion coefficient. The small cylinder at the left is a sight glass to determine the water level. A light-limiting aperture at the top of the tank has been removed for clarity. Figure 2. Test results. The irradiance collector starts fully immersed in water and the water level is gradually reduced without changing the distance between the lamp and diffuser.
A reference detailing how this is done is found in Hooker and Zibordi 2005, Advanced Methods for Characterizing the Immersion Factor of Irradiance Sensors, J. Atmos. Ocean. Technol., 22,757–770.