I am often asked to recommend light meters to use for testing the fiber optic implants that will be used in Optogenetic experiments. This is important in order to test the quality of each implant and make sure everything is working correctly.

Measuring the light correctly is not a trivial matter. The meter setting, the detector and the way the measurement is taken all makes a difference.

The light emitted from high NA fibers is highly divergent. Most photodiode-based laser power meters are expecting the light to hit perpendicular to the sensor. But light from LEDs and high NA fibers hit the sensor surface at an angle. As a result, a standard laser power meter will be underestimate the light emitted by a high NA fiber by up to 25-30% (for NA0.66 fibers). The best solution is to use an integrating sphere detector and power meter.  The integrating sphere  averages the various ray angles and provides a theoretically correct power value that does not depend on ray direction.   

Many of our customers use Thorlabs power meters and we purchased one so we can “be on the same page” as our customers. Which Thorlabs meter do we recommend?

Here is a cut and paste about light meters from a Prizmatix engineer:

For best measurements of LED we recommend detector with an integrating sphere and not the common laser power meters with the flat detectors.

We recommend spending more on the detector and less on the meter.

We recently started using:

• Detector:  2 inch Integrating Sphere Photodiode Power Sensor

(Some customers use a lower cost detector with one inch sphere but it is more difficult to get correct and consistent reading since it is more sensitive to the angle and position of the fiber)

• Meter: 
• If your setup is near a computer – we would recommend PM100USB - USB Power and Energy Meter Interface for C-Type Sensors
• If you prefer a meter with a human interface - PM100D - Compact Power and Energy Meter Consoleb...

Even with proper equipment, it is easy to get wrong readings. In a coming blog I will post our engineers advice about how to get correct power reading when using an integrating sphere.