chromatic aberration at different wavelengths for two-color simultaneous imaging

[haoran]

This is a question related to the on-going two-color simultaneous imaging updates:

could you enlighten me how you deal with the different chromatic aberration at different wavelengths?

Thanks!

[Daniel Aharoni]

Hi Haoran,
We are working on a few approaches to deal with the chromatic aberations of GRIN lens but each is pretty application specific. Some general ideas/suggestions:
1) You could try replacing the chromatic elements in the system with achromatic ones (achromatic lenses).
2) Split the optical path and use 2 CMOS sensors.
3) Replace the achromatic tube lens with a custom chromatic lens that removes the aberration caused by the GRIN lens.

[vluczak]

Hi Daniel,

I'm also interested in a two-color system, but I need to incorporate a relay lens to access deeper brain areas.

Regarding point 1, I think I'm missing something, but what chromatic lenses are currently in the system?

Using 2 CMOS sensors seems like the most direct solution, but I'm a bit concerned about the added weight from the extra sensor/filters/housing/torque from added height. Presumably, once the two sensors are adjusted to the same focal plane for a given GRIN lens, they will simultaneously adjust with the single focusing mechanism? I guess it depends on how the image path is set up. On a related note, it looks like this is the direction Doric has taken, but it's still not clear to me if you can adjust the focal plane at all. http://doriclenses.com/life-sciences/snap-in-microscope-bodies/956-2-color-miniature-fluorescence-microscope.html

I like the idea of correcting the chromatic aberration with a custom tube lens, but I'm assuming the aberration for a particular GRIN lens will vary from batch to batch (maybe not?). Do you have any suggestion how to begin searching for the right lens? I'll email Edmund optics to see if they can offer some advice, but I'm worried this will require some optical design that is beyond my qualifications.

Thanks!
Vince

[Daniel Aharoni]

Hi Vince,
The only chromatic element in the standard Miniscope design is the GRIN lens. The large objective GRIN lens has very large chromatic aberration that amounts to a significant focal plane shift when imaging red vs green.

Using 2 CMOS sensors and adjusting the optical path length of the different wavelengths would work but the relative path length will change depending on your overall working distance. This is how Doric solved the issue but they are only able to do this because they have no focal plane adjustment (working distance is fixed).

I don't think the aberration across GRIN lens batches would vary much but it might be significant work designing a robust tube lens that corrects for the aberration.

[vluczak]

Hi Daniel,

Thanks for the response!

I assumed you were talking about the GRIN lens but wasn't sure. Glad we're on the same page.

I know what you mean about the Doric system. It's a solution, but somewhat "rigid" in terms of lens pairing. Hence, the  S/L scope bodies.

Yes, I'm also concerned about the work needed to find the right tube lens. Unfortunately, I can't invest the effort but I'm sure someone else is close to solving this problem.

[olivier_at_doric]

Disclaimer: I am an engineer working on microscope systems at Doric Lenses.

In our case, we decided to correct the chromatic aberration by using 2 CMOS sensors, because we believe that this is the most robust solution that allows for two-color simultaneous imaging. As mentioned above, the trade-off is the increase of the weight on the animal. The weight of Doric’s two-color systems are 3.5 g for the model L (with GRIN), compared with 2.2 g for Doric’s single color microscopes. The 2-color model L was tested in one of our collaborators labs for more than 2 days straight. With a trained mouse, he told us that the mouse was moving fairly unimpaired in its cage even after wearing the microscope for 2-3 days.
 
You are right though that even when using 2 CMOS, one optical path adjustment will be valid for one specific GRIN lens length (actually the number of pitches). That is because the chromatic focal shift is proportional to the number of pitches of the GRIN lens. Two lenses having the same number of pitches will introduce about the same level of chromatic aberration, even if they are coming from different lot of the same glass with slightly different lengths.

Using a GRIN lens with a microscope body optimized for a different number of pitches will introduce a substantial chromatic shift. That is why we ultimately developed different microscope bodies, optimized for the different GRIN lengths.