What is a Dichroic Mirror?
If you’ve used a microscope or a filter based plate reader you’ve probably heard the term “dichroic” at some point. And if you’ve read the term “dichroic mirror” maybe you’ve always wondered what kinds of other words might follow that term in a sentence but have never made it that far. If you’ve made it this far, you might be thinking to yourself, “why am I bored right now?” but can’t quite put your finger on it. Let’s soldier on to see if we can find out.
Mirrors reflect light (duh.) But there are dozens of types of mirrors. In LCD projectors, mirrors may be designed to reflect some photons toward the viewer and others away, creating a display different colors or contrast levels. Perhaps just to the right of this post you are seeing a banner ad featuring an unsightly paunch with text telling you how you can lose 18 pounds in only two weeks? Depending upon your display, a mirror may be forcing you to see that.
But mirrors can do more than just ruin your internet browsing experience! A 50/50 mirror, or a beam splitter, can reflect half of the incoming light it receives and let the other half pass through. This is useful if you want to send a little light in two directions. It is not that useful if your purpose is damaged by having 75% your useable light headed in the wrong direction. With optical engineering, we can create mirrors that preferentially reflect some wavelengths and, as you might imagine, this has countless uses. Dichroic and dielectric mirrors that can do that are why we entertain our cats with laser light or produce telescopes that only operate in the infrared. And they have important roles to play in detection equipment.
In our PHERAstar FSX, we use optic modules that contain dichroic mirrors. They reflect low wavelength excitation light down to the sample, but let longer wavelength emission light pass through to the detector. Like so:
The dichroic here serves multiple purposes. Most importantly, it allows the instrument to send light to the sample without traveling through a series of tubes before it get there. Light guides, fiber optics, gratings and the like all have their place, but the fewer of those a photon has to encounter before fidning its purpose, the greater the chance is that it will arrive in the right place without being absorbed, scattered or otherwise detained. Just as importantly, the dichroic mirror allows emission light to pass through to the detector without being chaperoned by unwanted excitation light. Excess excitation light emanating from the well encounters the dichroic mirror and is reflected back the way it came from, reducing the chance that it will sneak into the detector and contribute to background signal. The dichroic does double duty, acting as an efficient optical path and a noise-blocking filter. Because of these dual and large advantages, most high-end filter based plate readers will use dichroics somewhere in their design. And most monochromator based readers don’t use dichroic mirrors at all.
The benefit of a monochromator is that it lets you conveniently tune to any wavelength you wish to use. If you had to buy and insert a dichroic mirror each time you wished to tune to a new fluorescence wavelength, that would certainly improve performance, but almost all of the flexibility advantages of a monochromator would be gone. For this reason, just about every monochromator-based fluorimeter will use inefficient half-mirrors or something like a fiber optic to send photons in the right direction. There are big sacrifices to performance in doing this, but the only way around would be if somehow a nearly magical, tunable dichroic mirror was available that allowed the performance of a filter based instrument while still allowing users to still tune to any wavelength. [polite cough] BMG has developed a nearly magical tunable dichroic mirror that allows the performance of a filter based instrument while still allowing users to still tune to any wavelength. And it’s a big reason why the CLARIOstar performs so much better than any other reader with a monochromator.
A tunable dichroic mirror means that the CLARIOstar’s monochromator can use direct excitation optics, just like a filter-based reader. The low background and efficient light transmission wouldn’t be possible without these cool mirrors.