First of all, does coherence of a light, which is used for illumination in a microscope, affect the sample, or the way we see/perceive it? I want to know what optical elements do I need to use if I wanted to change the illumination of a microscope from LEDs to a laser. Since laser light is more coherent than the LEDs, I want to know all the ways in which both, the spatial and temporal coherence of a light affects the sample and/or its properties we need to observe.
1
-
$\begingroup$ If its just for observation I would imagine two things: 1- It would be annoying to see everything in red or green (I am assuming you would not go for something fancier and super expensive like a white-light laser source), 2: speckles will also be annoying as hell (speckles are the graininess of laser light due to its coherence). $\endgroup$– José AndradeCommented Jul 22, 2022 at 19:08
Add a comment
|
1 Answer
$\begingroup$
$\endgroup$
In addition to spatial and temporal coherence there is also spectral coherence. This is also much higher in laser light than LED.
Coherent sources do have disadvantages though. Laser speckle results from interference patterns of diffusely reflected temporally coherent light. You may want a beam homogenizer (vibrating optical fiber coil or rotating diffuser) to reduce this.
High spatial coherence will result in a less uniformly illuminated field. This can be improved with flat-top diffusers. In a confocal microscope, the high spatial coherence is used to generate the highest power density at the focal plane.
The high spectral coherence of lasers will result in lower excitation efficiency of fluorophores if you are trying to extract maximum quantum yield (consider the large excitation bandwidth of fluorophores). On the other hand, this can be advantageous as there will be less spectral cross-talk as a direct consequence.
