Knowing the power output of your light source is crucial for interpreting and replicating your optogenetics experiments. Often, light power per unit area (mW/mm2) is a more important characteristic than overall power (mW), so one must take into account the diameter of the illuminated surface in addition to the measured power.
The basic unit of optical power is the watt, measured in joules per second, or energy per unit time. One watt is a very significant amount of optical power, although the concentration of this power makes a huge difference. A one watt laser beam can cut fabric and set fire to objects, whereas a one watt LED, which emits light in all directions, is safe to use in everyday lighting applications (although it may be uncomfortably bright to view directly).
There are three important characteristics of optical power that are derived from watts:
- Radiant intensity, measured in watts per seradian, or power per unit of solid angle
- Radiance, measured in watts per seradian per square meter of emitting surface, or power per unit of solid angle per unit of surface area
- Irradiance, measured in watts per square meter of incident surface, or power per unit of surface area
In the optogenetics community, intensity and irradiance are often used interchangeably, since irradiance is far easier to measure, while intensity is a much more familiar concept. Just be sure to keep your terms straight when discussing light power with someone with a background in optics, in order to avoid confusion.
The specifications of LEDs are often listed in photometric units, which are weighted by a luminosity function to account for differences in human brightness perception across the visible light spectrum. The basic unit of photometric power is the lumen, which is analogous—but by no means identical to—the watt. Photometric units are NOT acceptable for reporting light power used in optogenetics experiments, but they may be useful to understand before purchasing light-emitting devices.
In addition to the lumen, three important photometric units are:
- Luminous intensity, measured in candelas (lumens per seradian)
- Luminance, measured in candelas per square meter
- Illuminance, measured in lumens per square meter
To measure relative power during an experiment, a beamsplitter can be introduced into the light path. This allows a certain fraction of the light to be detected by a photodiode, the output of which can be read into a data acquisition system. An example setup includes the following parts, all from Thorlabs:
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