NIR Measurement Device - incident angle test

Last week we were notified that our article “Low-Cost NIR Measurement Device”, about development and use of the NIR pixel module, was accepted to the 4th International Workshop on Optical Wireless Communication - IWOW - in Istanbul between 7th and 8th September, where it will be presented and demonstrated in a poster session. In preparation for the presentation of the device and to provide some additional information about its performance, we proceeded with some further testing.

In particular, we investigated the correlation between incident angle of the beam on the NIR pixel module and received optical power. Mechanical design was constructed, enabling controlled and automatised change of the angle, relative to the incident IR beam of the KORUZA unit. The alignment tool, consisting of 9 NIR modules placed in two concentric circles around the centre, was then rotated from -10 to 10 degrees in steps of 0.5 degrees, where in the neutral position the beam was horizontally and vertically aligned with the pixel located in the centre. At each position two power readings were made for each pixel and later the correlation between the angle and received power was analysed. We performed the experiment in two different settings, slightly changing the back focal distance between the receiver and lens.

For the first version of the device with back focal distance of 4.25 mm, the maximum incident angle, at which reliable reading can be made, was found to be +/- 5 degrees. When the back focal distance was slightly increased to 6.25 mm, the maximal incident angle decreased to +/- 2degrees, however more power was received.

(http://irnas.github.io/post_files/NIR-angle/nirPlot.jpg){: .img-full}

We concluded that based on the application of the NIR Measurement Device, one can change the back focal length to achieve optimal performance, weather to increase the incident angle and making the device more robust in applications where precise alignment with the beam is not possible, or to increase the optical power span and make the device more sensitive to IR power.