Hi.
I've been using LED for optogenetics and calcium imaging.
For now with the devices, I'm only able to adjust the currents or % power of the LED as out power can vary from patch cord to patch cord.
While it seems like people usually adjust the power of LED by W/mm^2, how would I be able to measure that?
What kind of meter do I need?
Please recommend me if you've been using one.
Thank you!
We use this to measure power:
https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=3341
We attach the the patch cord to the LED or laser source, attach the ferrule to the patch cord, and put the fiber against the sensor.
It's easy to use, but a little pricey (~$1500).
We also use the Thorlabs PMD100 (in combination with the S121C sensor). You can also connect it to a PC via USB and then use the software available here https://www.thorlabs.com/software_pages/ViewSoftwarePage.cfm?Code=OPM to get more accurate and elaborate measurements (e.g., high-frequency light trains). You can also use it to measure W/mm^2.
Niraj S. Desai , Roni Hogri
I appreciate you all for the nice answers!
Dear Kwanghoon,
I also can recommend the power meters and sensors recommended by Roni Hogri and Niraj S. Desai above (I find the S130C slim sensor quite useful).
However, these devices measure *power*, not intensity. In order to measure intensity with these, you need to know the total illuminated area on the sensor. That you can achieve in a number of ways, depending on what type of light source you are using.
1. If you have a uniform beam that has a larger diameter then the sensor aperture (9.5 mm diameter for the S130C), then you can estimate intensity as total power divided by sensor area (i.e. pi*r^2). Caveat: if you beam is not uniform, this estimate will be inaccurate.
2. If you have a uniform beam that is several mm in diameter, but difficult to measure the diameter exactly, you can place a smaller aperture onto the sensor, to limit how much light reaches the sensor. For example, if you drill a 3 mm diameter hole into piece of black material and place this over the sensor, and your beam is > 3 mm in diameter, then this would allow you to calculate the intensity as described in (1) above.
3. If you are illuminating a sample through a microscope objective, then these methods may not work well (especially if using a high magnification objective that typically generates a spot smaller than 1 mm). In that case, you would need to calculate (based on objective properties, wavelength, etc) the effective beam waist diameter at the focal plane. Use this together with the total power measured to obtain intensity.
4. If you are using a fiber to deliver the light, then all you need to know is the fiber core diameter (e.g. 0.2 mm) and the total power. As others have described, you just need to make sure that all of the light out of the fiber reaches the sensor, to get a measure of total power. Be careful because the fiber tip can scratch the sensor surface if they come in contact. In this case, you are estimating the intensity *at the tip* of the fiber. The intensity at the sample will most likely be much lower and generally quite difficult to estimate. But see:
https://web.stanford.edu/group/dlab/cgi-bin/graph/chart.php
http://www.openoptogenetics.org/index.php?title=Transmission_Through_Brain_Tissue
That website is currently unavailable, so here's a snapshot from 6/2018:
https://web.archive.org/web/20180606034802/http://www.openoptogenetics.org/index.php?title=Transmission_Through_Brain_Tissue
For consistency, if you are doing in vivo experiments with an implantable fiber, I recommend measuring the total transmitted power *prior* to implanting the fiber "cannula", as this can vary from implant to implant (and hence animal to animal). You can then adjust the command power to achieve the same power delivered to the brain in each animal.
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