In order to measure tiny atmospheric particles, or atmospheric aerosols, with your iPhone, you will need the iSPEX add-on and the iSPEX app.
iSPEX consists of an add-on and corresponding app for the iPhone. By attaching the iSPEX add-on in front of the smartphone camera this transforms it into an optical sensor that is suited for the measurement of the macro- and microphysical properties of atmospheric aerosols. In order to achieve this, iSPEX measures the (intensity) spectrum and the degree of polarization for visible light. Its measurement principles are based on that of the Spectropolarimeter for Planetary EXploration (SPEX) that is designed to measure aerosol and cloud particles in atmospheres of planets within our solar system.
The add-on is essentially a slit spectrograph that uses a transmission grating foil and a plastic lens in addition to the lens inside the smartphone camera. In addition, it contains a combination of stretched (and hence birefringent) plastic sheets and Polaroid film to modulate every spectrum by a sine curve. The relative amplitude of this sine curve directly scales with the degree of linear polarization (DoLP), and its phase is determined by the polarization angle. As such, all the information on both the spectrum and the linear polarization of the light entering the slit is obtained in a single shot. The iSPEX app disentangles the spectral information from the polarization information.
This way, the degree of linear polarization (DoLP) of the cloud-free sky can be measured as a function of wavelength and, by pointing the phone at different directions in the sky, as a function of scattering angle. This DoLP as a function of both wavelength and scattering angle yields unique information on fundamental aerosol properties, including the amount of aerosol, and also the particle size distribution and the chemical composition (through the refractive index).
In order to measure tiny atmospheric particles, or atmospheric aerosols, with your iPhone, you will need the iSPEX add-on and the iSPEX app. The iPhone app is for free and available in the App Store. The app includes:
- fully guided, step-by-step iSPEX measurement procedure
- a demonstration animation
- a live map of preliminary iSPEX results
- background information on iSPEX and its measurement method
What does the iSPEX app do?
The iSPEX app guides you through the iSPEX measurement procedure. With the add-on in place, in front of the main camera of your iPhone, it instructs you to make a scan of the sky. As you are making the scan the app take pictures that contain the modulated spectrum and, simultaneously, adds your geographical location and the angles of direction in which you point the add-on. The app guides you through this process twice, as a consistency check for best results, then gathers all pictures – 10 to 25 in total – and information in one single package.
Next, it returns your preliminary measurement result, a color-coded representation of your sky condition based upon your iSPEX measurements. You are then asked to provide your name. This is optional – the name provided will only be used to show along with your preliminary result on the iSPEX live map, in the app on your iPhone and on this website.
Now, after your agreement to share your iSPEX measurements, they are submitted to the iSPEX database for further post-processing. Sharing your data is important to contribute to the campaign. Moreover, after the campaign, all iSPEX measurements will be analysed and processed carefully to obtain maps of iSPEX based aerosol information over Europe. This step is required to filter out pictures that contained buildings and to average your measurements with those made by others in close proximity, to obtain the most accurate result. The final maps of iSPEX based aerosol information will be published on this website.
The submission of your iSPEX measurements may take a few minutes depending upon your Internet connection (either 3/4G or Wifi). In case you have not switched on data roaming and there is no Wifi available either, you can still submit your data if you are able to reach a Wifi suited location within 10 minutes after you have taken your measurements. Please keep the app active until you reach this location, otherwise your data will be lost (they are not saved on your iPhone). After submission, a the live map with your preliminary result added to it is shown. You can view this map also on the live map on this website.
To much information here? The app comes along with an animated demonstration to give you a head-start with your first iSPEX measurement!