grin lenses working distance

[JCima]

Hi all!

I am working with GRIN lenses and I have some questions about them. I have read the Grintech datasheet and it says that the GT-IFRL-200-inf-50-NC lenses have a infinity Working Distance (WD), while for example the GT-IFRL-200-005-50-NC is 5mm. However you say the WD you are using is from 0 to 0.2mm, is it because the longer the WD the higher the scattering and the lower the image quality? if this is the case, why don't you use a lens with a lower WD?, is the pitch different?
Also when using a 0.25 pitch grin lens, all the rays come out from the lens parallel and the achromatic lens create the image in the sensor. How does the sliding the sensor up and down shift the focal plane, is not the focal plane determined by the WD of the grin lens??
Thanks for your help!!

[Daniel Aharoni]

Hi JCima,
Great questions.
All 0.25pitch GRIN lenses have an infinity working distance. What this means is a point source of light sitting on the bottom surface of the lens will produce parallel light rays (what is sometimes referred to as infinity optics or infinity space) coming out of the other end of the lens. This is similar to how a standard objective lens of a table top microscope works. The achromatic lens, or tube lens, then focuses this parallel light into an image on the CMOS imaging sensor. The combination of the GRIN lens and tube lens defines the functional working distance of the scope.

As the point source of light moves away from the bottom surface of the GRIN lens, the exiting light from the lens starts to deviate from being parallel. This effectively shifts the location of the image produced by the achromatic lens. Or thinking about this in reverse, if you shift the z-height of the CMOS imaging sensor, you effectively shift the plane in focus below the GRIN lens. Roughly, an overall shift of 6mm of the CMOS sensor will shift the focal plane in the brain by about 200um.

I hope this answers your questions.

[JCima]

Hi Daniel,

It answer my questions, thanks for the quick reply!!

[JCima]

Hi again!

I have done some experiments following your advices and I have been able to shift the working distance from 0-200um aprox. However due to the short focal length of the lens, sensor and lens ended up very close, and I was not able to increase more the working distance. Is it possible to increase the working distance to more than 0.5 by using lenses with longer focal lengths?  Or the working distance is not dependent on the focal length of the achromatic lens.

Thanks

[Daniel Aharoni]

Hi JCima,
A longer focal length achromatic tube lens will push the working distance further but I don't think you will be able to reach 0.5mm. In my opinion your best option would be to get custom GRIN lenses made that give you the additional working distance to image through your prism.

[JCima]

Hi Daniel,

The thing is that when I asked for lenses with a longer woking distance the pitch this lenses have is bigger (0.42) so they create an intermediate image which increase the tube lens size and the FOV to 1.6 mm. I have read that you are woking with Go Photon to make lenses with longer working distance mantaining a small pitch, do you know when they will be realsed?

Thanks again for all your help

Javier

[Daniel Aharoni]

Hi Javier,
Who did you ask for longer working distance lenses? Often times GRIN lens companies think you mean something different when you say "working distance". I don't really understand what you mean by " create an intermediate image which increase the tube lens size and the FOV to 1.6 mm".

[JCima]

Hi Daniel,

Sorry for my bad explanation. What I meat is that as the pitch is 0.42, these lenses act like a relay lens. They take an image of around 1.6mm of FOV from 1mm away from the top of the lens (so 1 mm working distance) and place that image in the bottom of the lens. What I am trying now, is to stick to the bottom of this lens the 0.25 pitch grin lens. This second lens will collect the image from the bottom of the first lens. At the moment I am having some good results, however both lenses should be well placed and super clean.

Thanks for your help
Javier