Five Not Very Obvious Questions to Ask when Choosing an LED Source for your Fluorescence Microscope

More and more researchers are considering using an LED source for their fluorescent microscope. There are good reasons to do so, but what are the things you need to be careful about?

Some questions such as choosing a spectrum that will be effective are pretty straightforward. But some are not. You might not want to know about the finer details choosing a light source but to prevent disappointment – you are probably better off knowing some important factors to consider.

Here are some of the questions you might want to ask when choosing an LED:

1) What factors determine how much light will get to the microscope objective?

There are four major factors that determine how much light will get to the objective

1. The size of the LED emitting area
2. The divergence angle
3. The beam diameter
4. And of course - The power of the LED

You want highest power LED with the smallest emitting area, lowest collimation, and a beam diameter that will fill the optical path without overfilling it. These are the factors that will determine how much useful light gets to your objective. Keep this in mind when comparing different LEDs. The higher the power in mW per mm^2 on the surface of the LED, the better chance you will have of getting that light where you need it most, I.e. under the objective.

LEDs based on arrays usually have larger emitting areas, as well as less uniform light. Single emitter LEDs have smaller emitting area and lower divergence.

When testing an LED you might want to remove the objective and measure the spot of light to see that you are filling the back pupil of the objective. Hold a paper under the turret and measure the spot, comparing it to the size of the back pupil of the objectives you will be using.

2) Can the LED be used in a Faraday cage? In other words, “Is the LED Electrophysiology Friendly”?

Some LEDs light sources emit RF interference from the LED or cables. This can disturb delicate electrophysiology applications. One solution is to keep the LED out of the Faraday cage by using a light guide, but this will reduce the power to the microscope. A better solution is to use LEDs that can mount directly onto the microscope but not emit RF noise.

3) Can the LED be used for very fast imaging?

LED driver electronics often have a saw-tooth edge that results in a variation in brightness of up to 10%. For low speed imaging this is not a problem as the light is averaged out over time. But for high speed imaging in thousands of frames per second these fluctuations result in very problematic noise in the images. Special low noise driver circuitry alleviates the problem. You’ll want to make sure you have this feature if you need it.

4) Can the LED be used with a fiber, light guide or direct mount

Some LED systems lock you into one configuration, and LED light sources built with LED arrays can never be efficiently coupled to an optical fiber. Other LEDs can be configured for multiple delivery methods including coupling to optical fibers. You might want to spend more to enable your LED source to be configured to meet your future needs.

5) Can I test the LED and return it if it does not work?

LED sources are a relatively new and knowing in advance if a particular product will work can be tricky.

Ask your vendor if you can try before you buy or make a purchase with a right of return.

Your LED sales consultant should be knowledgeable enough to answer these questions and address your concerns.

I will be posting an article soon with the answers to these questions regarding Prizmatix LED available from Prizmatix and Goldstone Scientific.com.