Photodiode EEG sensor

What it measures:

A photodiode measures light and translates it to a voltage signal that is recorded by the EEG amplifier. 

Research Applications:

The most common use of a photodiode is as a method of extremely accurate synchronization with visual stimuli, especially when presented on a computer screen.

For example, a photodiode can be connected to the corner of a computer monitor.  Alternating visual stimuli events can use a small box at that part of the screen, triggering the photodiode.  This would create a marker in the EEG amplifier data corresponding to the change in the stimulus and allow the time synchronization of all other data recorded by the platform.  Different intensity of light can be used on the box to denote different trials or parts of a test.

How to use:

The photodiode sensor is placed wherever the light must be measured (i.e. directly on a computer screen). 

Connector on EEG amplifier:

In order to connect the sensor to the amplifier, we have to look for the "Dig/Phot" or "Phot" input. 

Visualization filters:

The photodiode may pick up the refresh flickering of a computer monitor or florescent lights.  To avoid this in the sensor visualization, consider setting a low-pass filter up to 30-40 Hz.

Right-click on wave view in Bitbrain Device Viewer to view data-visualization filter settings.

Under "Notch-Filter" we recommend setting this to the preset matching your country's AC frequency (presets exist for USA/Japan at 60 Hz, and Europe at 50 Hz).

Under "Other Filter" click "enabled"
Set filter type to "Low-pass"
Set High (Hz) to 30-40 Hz 
Type is "Butterworth"
Order set to 2

Data file:

The output voltage roughly corresponds to light intensity recorded by the photodiode sensor.

Wavelength and Temperature Sensitivity

Operational temperature	-20ºC - +85ºC
Switch time	4µs - 6µs
Sense spectran range	400-700nm (peack 500 nm)
Acceptance angle	±60deg

 

Directional Sensitivity

The photodiode sensitivity depends on direction, and it will not see a signal as well that is too peripheral.  The following graph details sensitivity at increasing directional angle to the sensor: