photocell sensor

pettetmw · Post by **pettetmw** » Mon Jul 13, 2009 11:53 pm

Does anyone know about technical details of ActiveTwo simple photocell sensor?

I'm using it for "dry runs" to verify timing of codes in trigger status channel with respect to rapidly flashing stimulus on LCD monitor. At this point I've only used photocell by itself with no scalp electrodes connected. Timing of photocell response seems to be as I expect, but I'm unsure how to interpret the voltage trace in ActiView. The baseline seems to slowly drift around while flashing stimulus is on, then shoots off scale when constant white screen comes on, then re-centers when the trace wraps around. Amplitude of flash response seems unstable as well. I've seen amplitudes both in millivolt range and in fractions of a microvolt, using the same set up.

Does anyone observe similar behavior from photocell sensor? Is this related to fact that photocell sensor is "patient side" (as stated in Operating Guidlines)?

At this point, the timing information I'm getting is fine for what I need. I'd just like a little more general information about voltage response, so I can use the sensor properly. If you're interested, here's some additional details:

Phototocell is clamped in place about 5cm in front of LCD surface. Stimulus is a flashing blue box (about 20 Cd/m2) on black background, 10 Hz, 50% duty cycle (i.e., 50 msec on, 50 msec off). Stimuli and trigger codes are generated by Presentation software. Photocell is only sensor connected to AD box, i.e., no electrodes, no CMS/DRL.

Post by **Coen** » Tue Jul 14, 2009 3:10 pm

The photosensor contains a PIN diode with integrated transimpedance amplifier. Voltage output is approx linear with received light input. There is no high pass filtering applied. Peak spectral sensitivity is at 750 nm, with half sensitivity at 400 (violet) and 950 nm (infrared). Response (both rise and fall) time is typically 70 microsecond.

Make sure that the cell is not overloaded from background light (DC from sunlight, 60 Hz from electric lighting).

Best regards, Coe, (BioSemi)

maarcc2050 · Post by **maarcc2050** » Thu Sep 23, 2010 3:18 am

Hi coen.
What is the maximum voltage output we can expect from the photosensor? and the minimum detectable voltage (or light intensity input)?

Post by **Coen** » Thu Sep 23, 2010 11:33 am

And what are you planning to conclude from those max/min voltage levels ?

maarcc2050 · Post by **maarcc2050** » Thu Sep 23, 2010 1:01 pm

Nothing to conclude. Just want to make a comparison on the sensitivity of this device with other devices that we may want to use in the lab.

Post by **Coen** » Thu Sep 23, 2010 4:03 pm

Sensitivity is 45 mV/(uW/cm^2). Versions with higher and lower sensitivity are available, but speed is approx inversely proportional with sensitivity.

Best regards, Coen (BioSemi)