Measurement module

Here is the list of all the measurements made by LIMO. An innovative feature is that LIMO measures 75 spectral irradiances, from 390 nm to 760 nm, every 5 nm, with a mean FWHM of 25 nm. The time it takes to produce and transfer this information to memory depends on the illuminance: it may vary from 75 ms (100000 lux) to 750 ms (around 5 lux). In order to check whether the lighting environment was stable during that time, UV irradiance and photopic illuminance are measured twice: before and after the spectral measurements. Activity measurements rely on the information from a triaxial accelerometer since it has been used for a long time for the analysis of sleep quality. However, we have decided to investigate the use of the information from a triaxial gyroscope. The resolution and the range of the accelerometer and the gyroscope have been selected to describe sleep activity as well as sport activity.

The relative spectral response of light sensors has been checked using a monochromator. Absolute calibration and validation was done using side by side comparison or substitution with a reference spectrometer (JETI Specbos 1211UV) under skylight, sunlight and artificial light sources such as deuterium, incandescent and LEDs. The 75 spectral irradiances produced by the LIMO are used to produce alpha-opic irradiances according to CIE S026/E:2018 (System for Metrology of Optical Radiation for ipRGC-Influenced Responses to Light) as well as x and y CIE chromaticity coordinates and CCT (Correlated Color Temperature). Below is a comparison with the Specbos 1211UV under clear sky conditions. Overall correlation is very good (R2 >0.996, except CCT). Normalized RMSE (RMSE divided by the average) is below 4% except CCT and SSC (Short Cone opic-irradiance).

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Battery/Memory modules

Two battery/memory modules are available: one for the day and one for the night. The subjects switch modules when they move the device to the frame (in the morning) or to the wrist (when going to bed). The module which has been used is then plugged into a docking station which charges it and transfers its measurements to a secured datacenter using WIFI. Thanks to this design, investigators are able to monitor the progress of their study while the device is being used. As long as subjects switch from one battery/memory module to the other, there is no limit in terms of battery or memory and subjects can wear the device for any number of days suitable to the study. Real life tests have shown that a battery/memory module lasts about 28 hours, subjects are warned about low battery by the buzzer (2 short sounds).

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On a frame during the day

Once the battery/memory module is plugged into the measurement module, the LIMO starts measuring with a bip. When assembled LIMO has a total weight of 19 g, a cross section of 1.9 cm by 1.3 cm and a full length of 8.5 cm. It is worn on an eyeglass frame during the day. A frame without lenses is provided for the subjects who do not wear glasses all the time. Elastic straps are used to maintain the LIMO on the frame. The device and the straps have a dark gray color to be less noticeable. The side of the strap next to the temple has a white color in order to reduce the side contrast between the device and the environment. The subjects performing physical activities (running, fitness…) can use an additional strap to hold the frame properly on their head. Marking important events such as a change in activity, location, is done with a double tap anywhere on the device, it is confirmed by a bip.

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On the wrist at night

A large strap is provided to put on the wrist when going to bed. The LIMO with the night battery/memory module is inserted under the strap so that the white diffuser of the measurement module is not hidden. This way, the LIMO will be able to measure the ambient illuminance during the night (only if above 5 lux). Marking important events such as a wish to sleep is done with a double tap anywhere on the device.

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Cloud based analysis

In the morning and in the evening, once a battery/memory module is plugged into it, the docking station transfers its measurements to a secured data center. Starting fall 2023, an analysis environment will be developed around the data. The quality control service will check the measurements and provide alerts to the investigators (LIMO not worn, left in the pocket...). The visualization service will produce and plot illuminances, UV irradiances, spectral irradiances and alpha-opic irradiances at various time resolutions. The machine learning service will be used to infer enough information from the measurements that subjects will not have to bother filling diaries.

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A glimpse at raw data

The raw measurements produced by the LIMO are available for download, their time variations can be observed using the videos produced by the cloud based analysis environment.

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