iSPEX 2
===
![logo][1]

How does it work?
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iSPEX 2 provides foreoptics to the standard camera of any smartphone with Android or iOS operating systems. Light is captured through a narrow slit and divided into two channels. A series of optical foils work as grating, retarder and polarisation filters. The result is a projection of the light spectrum onto the camera phone. The optics elements are kept in place within a dark tube containing baffles to separate the two polarisation channels, and the whole unit is clipped onto the smartphone using a positioner specific to the dimensions of a particular make and model of phone.

Relative positioning of the spectrum across the y-plane of the phone camera is calibrated by pointing the phone with iSPEX 2 at a spectrally neutral and diffuse surface under a light source with known emission peaks, such as a fluorescent tube. 

A further series of image corrections are stored in the processing back-end for most phone camera types. 

![iSPEX 2 on various phones showing projected image from foreoptics][2]
Figure: iSPEX 2 on vairous phones showing the image from the foreoptics projects onto the main camera view.

![Cross-sectional diagram of iSPEX 2][3]
Figure: cross-sectional diagram of iSPEX 2

What can I do with it?
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iSPEX was originally developed to record light spectra across the sky to determine the absorption and scattering of light by aerosols as an indicator of air pollution. 

With iSPEX 2, sensitivity and accuracy are greatly improved to allow spectral measurements of water surfaces as well. Taking the reflectance of water, i.e. the amount of sunlight that is returned from within the water column, we can determine the presence of optical water constituents such as algal pigments. 

These types of measurements are already in widespread use using both satellites and ground-based instrumentation. iSPEX 2 brings a low-cost alternative to these procedures. 

See also [measurement protocols][4]. 


  [1]: https://mfr.osf.io/export?url=https://osf.io/download/undbp/?direct=&mode=render&format=120x120.jpeg
  [2]: https://mfr.osf.io/export?url=https://osf.io/download/u3svz/?direct=&mode=render&format=500x500.jpeg
  [3]: https://mfr.osf.io/export?url=https://osf.io/download/hsp27/?direct=&mode=render&format=500x500.jpeg
  [4]: https://osf.io/357bk/wiki/Measurement%20protocols/ "measurement protocols"