Water
=====

The water reflectance measurement protocol using a handheld system follows recommendation in [Mobley (1999)][1]. Three measured radiances are recorded as part of a measurement set:

- radiance transmitted through the air-water surface (Lt, [sr^-1])
- radiance from sky (Ls, [sr^-1])
- radiance from a lambertian reference with known reflectance properties (Lr, [sr^-1])

Each radiance quantity is recorded at an angle of 40 degrees from vertical, either downwards towards the water or reference card, or upwards to the sky. All measurements are recorded at an azimuth as close as possible to 135 degrees from the solar azimuth. Measurements are obtained in suitable locations, away from surrounding structures that could reflect on or shade the water surface, and over optically deep water.

![2]

The iSPEX app assists the user in obtaining the highest possible quality image, including positioning away from the sun azimuth. Users should be trained to recognise issues with the correct positioning of iSPEX 2 on their phone, as image misalignment will result in erroneous observation.

![3]

Remote-sensing Reflectance (Rrs, [sr^-1]) is estimated or modelled as the ratio of water-leaving radiance (Lw, [sr^-1]) over downwelling irradiance. 

Lw is defined as Lt corrected for a fraction (rho_s) of Ls, which is either assumed fixed to the theoretical Fresnel reflectance of a flat surface, modelled as a function of surface roughness (e.g. from wind speed), or modelled from the (observed or modelled) diffuse versus direct irradiance.

Ed is defined as Lr corrected for the spectral reflectance of light by the plaque material, under the assumption that the surface is fully lambertian (diffuse). Users should take care to point iSPEX 2 at the grey card in the same orientation as used for water radiance measurements, while not shading the card. Grey cards must be kept clean and dry.

![4]

Questions to research: 
- Is a photographer's grey card sufficiently lambertian, or what is the measurement uncertainty associated with the use of the grey card [RQ_WP_01]
- Is there variability in spectral reflectance between grey cards, and how carefully should product selection be controlled? [RQ_WP_02]


Air
=====
Details to be added at a later stage.


References
====
Mobley, C. D. (1999). Estimation of the remote-sensing reflectance from above-surface measurements. Applied Optics, 38(36), 7442-7455. https://doi.org/10.1364/AO.38.007442 


  [1]: https://doi.org/10.1364/AO.38.007442
  [2]: https://mfr.osf.io/export?url=https://osf.io/download/qdhjb/?direct=%26mode=render&format=500x500.jpeg
  [3]: https://mfr.osf.io/export?url=https://osf.io/download/5czr4/?direct=%26mode=render&format=500x500.jpeg
  [4]: https://mfr.osf.io/export?url=https://osf.io/download/97xt5/?direct=%26mode=render&format=500x500.jpeg