Forum - GRIN lens specs

Great question Brendon. Our system has been tested with 0.25 pitch GRIN lens but should also work with (0.25 + 0.5*N) pitch lenses, where 'N' is an integer. The vast majority of our experiments were done using 1.8mm and 2mm diameter Grintech GRIN Lenses (respective part numbers: GT-IFRL-180-inf-50-NC and GT-IFRL-200-inf-50-NC). Currently Grintech seems to be the only manufacturer making GRIN lenses with the 4th order correction to the index of refraction as you mentioned. That being said, we are currently working with NSG to develop a 4th order corrected product line.

For a given lens pitch, smaller diameter lenses will also have shorter overall length. For example, a 0.25 pitch, 2mm diameter lens is around 5mm long where as a 0.25 pitch, 1mm diameter lens is about half that. While both lenses have similar optical properties, the 1mm diameter lens may be difficult to hold during implantation and limits how deep of a brain region you can reach. For these smaller diameter lenses we suggest getting custom lenses made with 0.75 pitch, 1.25 pitch, etc. to extend their overall length while maintaining the optical properties important to the miniscope. You will want to choose a GRIN lens length that, once implanted, will stick out of the skull by around 2mm to 4mm. This should give enough length for holding the lens during implantation as well as enough surface area to securing it to the skull.

So to summarize:

• Our system currently uses GRIN lenses with a pitch of (0.25 + 0.5*N), where N is an integer.
• We have successfully used 1.8mm and 2mm GRIN lenses from Grintech (which are about 4mm to 5mm in length) to image hippocampal CA1.
• GRIN lenses with a 4th order index of refraction profile will image significantly better than 2nd order lenses.
• The overall length of your GRIN lens should stick far enough out of the skull to allow you to hold it during implantation and cement it to the skull.
• The distance between the top of the GRIN lens and other miniscope optics is not too critical. You will want to cement the baseplate so that the top of the GRIN lens is somewhere between the bottom of the scope body and the internal dichroic mirror.

I hope this helps and I will also update out GRIN Lens page with this information.

Thank you very much. Perfect. I'll look into obtaining those, I guess I'll start with the 2mm ones (?). Thanks!

You're welcome. Yes I would start with the 2mm diameter lenses if possible.

Hi There,

Would a GRIN Lens with a 0.46 pitch work with this system?

Thanks!

Daharoni, A half pitch lens can't be used as and objective, but produce an intermediate image plane outside of the brain.

Sorry, Violinzju can you explain that again? Thanks Bradley

Hi Bmroberts, Violinzju is correct in saying that a 0.46 pitch (close to half pitch) lens won't work in our system. For a given index of refraction profile, a GRIN lens's optical properties are pretty much completely governed by the pitch length of the lens.

For example, if you were to image something very close (within ~100um) the the bottom surface of the GRIN lens:

• A pitch of 0.25, which is what our system uses, would produce roughly parallel rays coming out of the top of the lens. These rays are then focused by an achromatic lens into an imaging on our CMOS imaging sensor.
• A pitch of 0.46 or 0.5 would form an imaging close to the top of the lens with roughly a magnification of 1. Our achromatic optics would not be able to take this light and form an image onto our imaging sensor.

I have attached a ray tracing simulation of these 2 lenses:

What could be possible is to implant the 0.46 pitch lens (or anything close to 0.5 pitch) into the brain and image through a second, 0.25 pitch, GRIN lens and the achromatic lens in the scope housing. So basically you would be using the 0.46 pitch lens just as a relay lens and then imaging the relayed image using the standard optics of the miniscope system. This would be very similar to the optics in the Inscopix scopes.

By the way, Grintech hasn't answered in over 48 hours to my request for information/quote for GT-IFRL-180-inf-50-NC and GT-IFRL-200-inf-50-NC. I wonder if that means something? Usually companies are pretty punctual.

  I wrote to their "contact us" address of info@grintech.de.  Would you suggest I contact them some other way?  I'm basically not doing anything until I know I can get these lenses (since the rest is much more straightforward).

Hi Brendon,

If I were you I would send them a reminder email. If you run into problems acquiring GRIN lenses shoot me an email.

Did that a couple days ago. Last night I wrote them through their online contact form, maybe I should have done that the first time? If i don't hear early next week I'll contact you. Thanks a lot.