Optical Interferometry Part 2: Measuring Optics with a Zygo GPI LC
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- Опубликовано: 31 май 2024
- This is the second video on optical interferometry, which is dedicated to measuring the wavefront shapes of a mirror, 2 lens assemblies and 3 microscope objectives.
Contents:
00:00 Intro
02:04 Video camera upgrade
04:23 DFT-fringe software
06:57 Transmission Sphere reference calibration
12:29 Shape of a Zerodur Perkin Elmer wafer stepper mirror
14:55 Wavefront deformation of a Canon FD f/1.2 camera lens (1980)
18:10 Wavefront test of a modern Canon EF 24-105mm f/4 zoom lens
19:19 Microscope objective testing
22:44 Nikon Plan Fluor 10x / 0.30
23:17 Leica Fluotar 20x / 0.50
25:23 Nikon Plan APO 20x / 0.75
Link to Optical Interferometry Part 1:
• Optical Interferometry...
Link to the video on measuring MTF:
• Mirror Lenses Part 2: ...
DFTfringe forum page:
groups.io/g/Interferometry/to...
DFT fringe download:
github.com/githubdoe/DFTFring...
Dale Eason on RUclips:
/ @daleeason9687
lens config Canon F1.2 By Jake Low - Own work, CC BY-SA 4.0 (modified) commons.wikimedia.org/w/index...
Laser diffraction pattern By Wisky - Own work, CC BY-SA 3.0, commons.wikimedia.org/w/index...
Do you need to be referenced but I forgot to do that? Let me know and I will set it straight. 
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Yes, I am slightly more than averagely interest in optics... 😂
Wow that was unexpected! 🤔
@@HuygensOptics Top video as always. Sometimes I wish for more approachable topics, this one is again far above my pay grade.
Still waiting for a creator to cover the topic of synthetic holograms. Obviously needs to be a creator with a wafer stepper...
At this point, i think it’s part of being an alexander (my self included) 😂
@@graealex I agree, top notch, I think incorporating more real world examples of the impact of the aberrations (like of the marker in this video, or the zoom telephoto lenses in an older one) would be a way to give the concept more approachability... although as a pathologist I loved the fact that we are discussing microscope objectives!!
Precisely 😂
I recently made a poor-man's microscope for SMD soldering by 3D printing a Nikon F-mount to M12-mount adapter so I could mount my Nikon 50mm lens to a gopro clone. The results were very good, and I felt pretty good about myself, but this is on another level!
In any case, your channel is one of the hidden gems of RUclips. Not driven by a need to maximize engagement, just extremely well done deep explanations about optics. I learn something new every time you post a new video. Much appreciated!
That was great! I really appreciated the microscope objective testing and description. I've been working on this "minimum possible f/number" for a camera lens, and found that very fast camera lens designs have a lot in common with microscope objectives, especially in the glass near the image (or specimen) side. I never knew the cover slip was so critical!
Thanks Ben. You are right, the problems you encountered making a really high NA lens are similar to those in microscope design. By the way, there is a simple relationship that describes the relationship between focal ratio (F) and NA: F=1/(2*NA). You can increase NA quite a lot by working under water / oil because in that case you can avoid the refraction at the last interface, which is generally the limiting factor to increase the NA further.
@@HuygensOpticsapparently, the 1/2NA is an approximation for sin(arctan()), and works for f numbers above 1.2, but becomes very inaccurate below f/1. From geometry, this makes sense. But then, the Wikipedia article on NA further claims that high NA systems also have a really curved principle plane, which makes the definition of "diameter" complicated as used in f number. I think this is why scientists and engineers always use NA, not f number. Anyway there's both confusion from definitions and also the underlying physics, and finding clear explanations of anything in optics is pretty much limited to your channel :)
@@AppliedScience Yes, that is correct, it is an approximation. The way I look at it is that NA relates to the sin(theta) and focal ratio-1 to the tan(theta) and so the relationship is only a good approximation when theta is relatively small. However, in optics that are well corrected for spherical aberration and coma, the difference is less than the plain goniometric relationship suggests. I'm planning to do an upcoming video completely on NA and resolution.
A microscope objective not being perfect untill you add the coverslip is very cool, Ive seen a few home projects that use these in their laser setups so its good to keep in mind.
As a hobby astro photographer and tech nerd with a slight knowledge about that, this is sooooo interesting to me!
And so well made!
Thank you very much!!! 🙏
Somthing like this should be teached in shool!
Humanity could be so much further ahead.
Nice timing, just as Tom Scott dropped a video on the VLT and how that uses interferometry.
Not just interferometry. Optical interferometry the same as this. And 1 minute difference.
And top of that, one minute difference in length. Luckily, I did not have to travel that much to make my video.
This is the kind of stuff conspiracy theories are made of.
You have no idea how timely these videos are, I'm eating them up. The knowledge can directly be translated into the things I am working around on a day to day
The knowledge you're giving away for free in your videos is actually insane. thanks a thousand times
At the time of publishing your video, Tom Scott published one of this trip to The Extremely Large Telescope, in Paranal, Chile. I think it's super interesting. Quite a lot of ingenuity was needed to be able to do interferometry on light from multiple telescopes.
Thanks, I just watched it (and loved it) 👍
@@HuygensOptics Me too; just before. ps I may have go and lie down for a while.
To make you feel better about men vs women viewers stats you had. I think there must be a larger portion (Lets say >30%) in the 99% of men's list that actually their sciency girlfriend or wife were using their accounts to view your amazing content😄Thanks again for the great efforts of sharing all these. this setup pulled me back to those days back in the physics lab using Michelson interferometer...
This was an excellent video, as always. Would love to see a video going down the rabbit hole into why a high numerical aperture is required fro sharpness.
Don't worry, I'll meet up with Roger and his whole family in an upcoming video.
Your version of Schlieren photography is way cooler than the usual type. Very impressive stuff.
Not even close
There is something extremely addictive about these videos/topic. I wish we would not have to wait for another video for so long. 👍
Always look forward to your videos. Great work as always!
Wow. I work as a tech in a Photolithography workcenter of a wafer fab. Thanx for opening up the "black box" with your videos.
It was a little before my time, but one of our area's earlier fabs used to use Perkin-Elmer systems.
"...and have a variation in the diameter of about one micron. So they're not incredibly precise. " 😂
I am quite addicted to your RUclips channel ❤
"Free the fringe"? LOL! Does Mr. Dale Eason know he's running for president at 4:30?
Excellent video: couldn't have come at a better time for me. Looking forward to more of those gems from you, Jeroen!
Thanks Nobby, No I did not tell Dale, but I think someone will eventually notify him that he now is an official candidate and should probably start preparations for his move to Washington next year.
Fantastic video. I have no expertise in optics, just interest, but you present complex issues in ways that I can understand well enough to gain real enjoyment as well as education. Thank you!
The combination of 'requirements' for good-to-excellent optics has many parallels with good-to-excellent information conveyance, i.e. instruction.
Mr. Huygens, your elucidation, style, and clear understanding of the principals and details of your craft are exemplary. Please keep improving the world in your gifted style. 👍
This is so cool. I'm not in the optics field and the presentation is so good, It doesn't matter.
Excellent video, really well prepared and executed ... a master class really. Thank you.
love your explanation part of the microscope objectives. right now i'm also working on microscopes and interferometry. Thank you so much! very clear explanation and great for my students.
Wow! This was again such a fantastic slip for us normals into the strange world of optics. Just brilliant presentation and didactics, delivering a couchened introduction and a deep dive for the "more than averagely interested" at the same time. Well done!
Thank you! I learned so much about optics everytime I watch your video.
This was terrific - held my attention to the very end. Very clear and concise. (Reminds me of Open University lectures I used to watch on "Telly" in the 1970s and 80s when I was home sick / ill. ) [Real question below*]
I have a number of high precision photogrammetric lenses built back in the day by WILD (nominal 60 mm effective focal length) - Biogon symmetric layout. Usually shoot / shot at f-8 to f-11 "Sweet spot" beautiful imagery and very low (systematic)radial distortion (across the whole field of the order 10 micron for 6cm x 9cm frame.) BUT ;-) the vignetting towards the edges is fairly pronounced. [The instrument requires wide angle and low distortion .]
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* ["Question / video suggestion " ] (Don't run for the door ) Why is it difficult to build a low distortion high resolution ("Good" MTFs) wide angle lens with even field illumination (low vignetting) - [My Wild and Zeiss type photogrammetric lenses are decades old.
AND why " Distigon " layout (Retro
YT won't let me write what I need to write and turns it into RED text lol - so why retro focal lenses have high distortion and is it possible to design and build a low spatial distortion wide angle lens that had even field illumination - what IS the mutually exclusive tug of war going on here ? [Please and Thank You - for at least maybe reading this 🤣] - I had a university supervisor just brush it all away glibly saying it was "Pure Heisenberg" in terms of trade off and spatial uncertainty ? (maybe he had no clue ?) [This was over 20 years ago - so "We" are safe ;-) ].
Fascinating, especially about the glass cover slip....cheers!
17:55
Back in college i took a film photography class, and while I didn't have that exact lens I had a similar one that apparently had similar characteristics. No matter how hard I tried I could not get it to "properly" focus a test card (i.e. a flat plane at a fixed distance, which even with a very narrow DoF should have some setting that puts it in focus assuming proper distance) at the largest aperture. Stopping down a couple clicks and suddenly everything could be focused easily!
Thanks for (probably, or at least partially) explaining something that bugged me for over a decade!
"…and have a variation in the diameter of about one micron, so they're not incredibly precise…" Optics folks just live in a whole different world than us.
Your videos are so well done, maintaining interest and pace from start to finish. Thank you, and keep up the great work!
Love it when you upload a video sir. It's always deeply insightful. 🙂
What a wealth of information that I might never know to track down otherwise - many thanks!
I love your videos - so much detail and still remaining comprehensible to lay folk with "above average" interest in optics. Please keep up the great work!
Can you only measure spherical lenses with the spherical reference? I know the cannon lens had an aspherical element, but you end up measuring the 'distortion' of the entire optical train as if it were a single element - do you need an aspheric reference surface to measure aspherical lenses? how can you even make an aspheric reference?
Would you be able to use the same setup with different colors of lasers to more fully characterize the lens's performance? is that even a useful thing to do?
The interferometer actually measures spherical wavefronts best. How they are produced is relatively unimportant. A lens may incorporate aspheric elements to accomplish this. When measuring aspheric wavefronts retrace errors within the interferometer can be an issue. However its often possible to combine annular portions of interferograms obtained whilst traversing the axis of the caustic to accurately measure the wavefront whilst avoiding retrace error.
The wavefront error on JWST is something like 20-150nm (depending on field of view, and such), this puts that performance in a bit more context.
Fantastic! I wished this kind of content would have existed back in my phd days, It would had saved many hours of pain xD.
Wonderful explanation of interferometry and actually a very good intro of my DFTFringe program as well. I don't think I will run for president.
Thanks Dale, I had to keep the description of DFTFringe very brief because the video already got quite long. But luckily there are several videos discussing the program in way more detail, including some videos of yourself. The suggestion to run for president was of course not a very serious one, I just wanted you to know that your contribution to making interferometry accessible is very much appreciated!
Thank you. I enjoy your channel and admire you knowledge and video presenting and editing skills.
Awesome Interferometer! wavefront curvature analysis to the single nanometre range is incredible! Thankyou Jeroen!
Excellent information and test setup as always you do. Even though I am not in optic business, your videos help me to understand the light and optic, more deeply. Many thanks for your effort.
Hei! First of all amazing video! I really like your work! Idea for a future video: it would be very interesting if you can show us how to build at home a parabolic mirror for a Newtonian telescope (let's say 150 mm diameter, 750 focal length)? It would be nice to know the techniques to do it at home and how to make an in house Foucault or Ronchi test
I got a Zeiss microscope from the 70s which is in pretty bad shape and one of my hobbies is to make everything work again but it’s so difficult. I had to take all the lenses apart and put them back but the alignment is never right.
This video perfectly shows the level of precision required and I’m probably never going to restore it to its original performance. At least without spending thousands of dollars on equipment like your interferometer.
You are probably aware that he most difficult thing is to keep all the elements in the right order and orientation ;-). Taking these things apart takes quite a lot of courage, because reassembly is not easy and involves all kinds of tricks and tools that manufacturers have spend years to develop. But in the end, the only thing that matters is: what did I learn?
Это было безумно интересно! Огромное спасибо!!!
Delicious stuff, as always !!! I need to admit, I am still a bit jealous about your fancy toys 😊
Great explanations and presentation.
Excellent! A lot of effort was made to produce this very informative video! Thanks!
Your videos are always fascinating and informative
Wonderful video, esp final part with microscope lenses. Thank you!
Fascinating as always.
Amazing video. This is so interesting for me as I em researching a little bit of visual optics and aberrations.
Awesome video and explanation.. I was waiting with impatience for a new video 😄
Thanks for measuring the microscope lenses..
I'm curious to see Metallurgical microscope lenses too..
great video with a lot of details in practice😎
Top notch content, my hat is off to You kind sir!
Amazing video as always! Explanation how to create and use much cheaper version of interferometer ball calibration device was very useful for me.
You are amazing! Thanks for the video.
Fantastisch Jeroen, bedankt voor weer een mooie video.
Ik ben echt ongelooflijk benieuwd om te gaan zien waar dit naartoe gaat!
Looking sharp!
This was so cool! And I definitely didn't know that about microscope optics :D
You could create an adjustable thickness cover glass if you had a couple of wedge shaped cover slips. You could combine them into one equivalent cover slip and slide the overlapping wedges apart to adjust the thickness.
I wonder if anyone trying to get the best microscope performance could use this: perhaps to verify there is no quality problem in the microscope that can be corrected with a different cover slip thickness, or if the optimum cover slip thickness depends on focusing at the sample surface versus focusing 200 microns past the sample surface.
Always hectically waiting for next series 😊 thank you 🎉
Brilliant, as always
We're using interferometers from Zygo in labs testing optics for county college (sponsered by Thorlabs which is in our town so it's actually high end). I've been trying to get in touch with Zygo to get a test edition of the Mx software, the proper documentation and logged into their site (very difficult). At least I can look at these videos to get an idea what's inside even though our equipment is only about 3 years old. What you've gone over is what we'll be doing over the next couple of months in the optics lab (once again, Thorlabs so there's a whole building for it) and in Thorlabs itself. Even tho Mx virtualization is 'free', you need to be a member of Zygo, but they drag their feet on students and non-professionals memberships.
Great video!
Great video, thanks!
Superb video, thank you!
This is so interesting! Sadly I am bad at math 😅and understand only partially all the relationships. But is it sooo interesting. I would really like to see how you would design a high end UV-Vis spectrum analyzer. I want to build one myself but the Math and understanding of optics design holds me back 😅
I Love your Videos, thanks for taking the time to make this amazing content ❤❤
"Let's see if it actually definitively needs two or three cover slips" -> turns out the lens wants exactly two plus half or three minus half
That gave me hard good abdominal exercise of laughter
Very interesting learned a lot, great video
I picked up one of those unmarked Nikon apo objectives quite a few years ago. I was unaware of the info you presented about the ideal cover glass thickness so I was unimpressed with the lens and put it away. After seeing your presentation I decided to retest it. I set it up using epi lighting. Adding cover glass seemed to help a little, but what really made the image of an etched wafer dramatically pop was using the objective as a water immersion objective.
Thanks, that is good info. I discovered that you can buy the right coverslip thickness easily online. They are called "Haemocytometer Coverslips" and have thickness of 0.40mm. Using these brings you easily within the diffraction limit.
@@HuygensOptics I'll have to try that. Since these are oem lenses, I'm wondering if maybe the buyer may have ordered more than one lot with different specs, maybe differentiated by code numbers. We really don't know what the company was trying to accomplish. Anyway, I now have a lens that's useful! Thanks.
what a wonderful insight
Oh, cool stuff! Please make some more videos about microscopy.
This video contains a lot of knowledge packed into it! I am interested if you use some sort of vibration isolation system for granite base plate?
Out of curiosity, is using a linear correlation correct when trying to find the optimal glass thickness? I would imagine the correlation would be more complicated than just linear. It's absolutely already more than accurate enough, I'm just wondering if there is an expected mathematical correlation. (Also for the Strehl ratio, as it tends to 0 for large errors, we would expect any correlation to also tend to 0 rather than go negative for any thickness of glass far from ideal no?)
Whether the relationship with Strehl or wavefront errors is linear or not depends on how the correction is implemented in the objective. You might very well be right, because at detail level and large test ranges, the relationship is likely to be non-linear.
@@HuygensOpticsWhilst the SA contribution of a plane parallel plate is strictly proportional to its thickness, the strehl ratio is inherently a non linear function of the residual wavefront errors.
Amazing video :)
Oh neat i get to learn new stuff bout optics
Excellent, thanks
Yeaaaaaah
Thank you a lot for sharing
This is unbelievable
love it!
Thanj you very much!
DROP EVERYTHING HE POSTED
It seems that they key indicator of performance is the name of a product in lenses. Clearly what we need is a Super Minotar APO Plan ASPH II
If you ever feel the need to ditch the CRT, you can get a USB adapter for analog video pretty cheaply, and anything that supports UVD almost any software can open in a window for alignment and save you some space.
Thanks, I did not think of that. It might be part of some future upgrade.
A cheap car reversing display (4:3 aspect ratio) is usually a 3.5" 320x240 LCD, it's for NTSC video but i think it can decode PAL as well, i could check. This might do as well.
Yes you mean UVC video and the UTVF007 chip dongle is usually kind of bad (noisy video) but it's very cheap and easy to find. When you buy used composite video capture devices like the old ExCap or WinTV they tend to be normal quality but they play don't work on modern Windows version, no drivers exist, and UVC at least work.
Thank you.
Vielen dank
I've been waiting for this! (and first first comment for me)
Only now did I find the time to watch to the end. And you "buried" the very "photogenic" and click-bait worthy Schlieren clip of your hand in the end credits! This is somehow the essence of why I watch every video you make with great pleasure.
Excellent video. Very well presented. I'm surprised with the mount setup you showed and the thickness of the P&E mirror, you didn't see mount deformation of the 2 point support of the mirror in the interferograms. Did you stop down the image somehow not shown? Showing the effect of cover glasses on spherical aberration is also great.
With mirrors this thick, you generally don't observe any deformation due to the support structure, unless it is in a very unfavorable location. Placing the supports in the plane that also contains the center of mass is generally a good idea in this case.
gold!
24:57: I wonder if the distance of the lens to the cover slip thickness has any impact. These lenses are intended to focus just on the bottom of the cover slip, where Snell's law predicts the angle of the rays exiting the sample. However, if you change the placement of the cover slip, the intended focus position is again in air, where the angles are larger. I have a hunch it will cancel out.
The position of the cover slip doesn't matter, the focus can be in the back plane of the slip or the slip can be in the middle between objective and focus, the wavefront error at the focus is exactly same. I also discovered in the autocollimation configuration that it doesn't matter where you place your coverslip. In this case it can even be on the other side of the focus and it will still have the same effect on the measurement. I know it sounds unbelievable but you can check in Zemax and it will show you exactly this result.
@@HuygensOptics I was just thinking about this and it makes complete sense. The retardation of the wavefront from a certain angle of course doesn't depend on the position of the slip. The slip adds a certain optical path length error, but this error only depends on the angle of the light, never on the absolute position! Also from the raytracing model you get the same result. The ray is slightly displaced at the bottom of the slip (due to refraction) but then continuous normally. This displacement of course doesn't depend on the slip-lens delta. This even works if you place the slip behind the focal distance.
If I have some time today, I will calculate the wavefront error as a function of incidence angle.
Super fascinating as always! The calibration sphere method is really clever. Stochastic averaging to the rescue 🙂 Can't wait to see what's next with this instrument!
Thanks Zach! I guess I will be using the Zygo quite a lot in the coming months, but I don't think I will be making videos about it any time soon. The upcoming projects will involve photolithography again.
@@HuygensOptics Oooh, awesome! Looking forward to it! I've been doing a bunch of litho stuff recently and it's such a love-hate process 😅 So satisfying when it works, and so frustrating all the times something goes wrong and you have to start over.
Thank you for your excellent videos! If you evaluate the fringes without phaseshifting (e.g. using FFT-Method by Takeda), there still is the sign ambiguity. How does the DFT Fringe software handle this or do you have to use tricks like bending the surface to determine the sign yourself?
Yes the ambiguity is present in dft-fringe, because it cannot evaluate phase data. In averaging it can however determine the sign by comparison of the sign of specific aberrations and invert the wavefronts accordingly.
Make a second channel where you use interferometry to test different borrowed modern lenses,blow photography outlets and manufacturers lies out of the water and win the hearts of photographers,becoming THE lens review channel on youtube :)
If I had more time that would be a great idea! Unfortunately, I'm glad when I occasional get a video posted on this channel...
Very interesting video, well done.
I have a question. How did you manage to measure the deformation of the wave front due to the mirror in your auto collimation on the lenses?
Thanks. The mirror I used was measured separately in the past and is extremely flat, at least on the surface scales that it was used on here (it is 1/10th of a lambda over 250mm diameter). Also, it only counts one time instead of 2 for the optic under measurement so it is a bit less critical.
On the ball method, if you have an expensive ball refrence it would be interesting to compare
16:50 Looking at the wavefront error of this camera lens: could the full aperture image quality be improved by using about 1.0-1.5 mm thick clear glass in front of the objective? The wavefront error seems to have somewhat similar same to microscope objectives without the cover slips if I understand the graph correctly.
Very cool! As a photographer I do now wonder about the lenses I shoot my pictures with, just curious if my feel for the best aperture is backed by data. Is this a service you offer, or is the testing mostly just for fun/hobby? I am located in the netherlands btw.
Well I don't want to turn it into a business model ;-) But there are way simpler and better ways to check optics, just use a test chart and vary the aperture. That will tell you all you need to know about the sharpness of your lenses.
hi, i enjoy watching your channel but im curious if you've encountered the exact formula for correcting aberration by rafael gonzales: Their findings were published in the article General Formula for Bi-Aspheric Singlet Lens Design Free of Spherical Aberration, in the journal Applied Optics. if so how do you use it and how do you cut the curvature to the exact solution of the formula. thanks
What a great video. I am excited to see that you were able to figure out the exact coverslip thickness needed by your 20x 0.75 objective. May I ask what exact model number your objective has on its back side, if any? Different model numbers seem to represent different coverslip optimizations, so it would be nice to know which model your result applies to.
Hi Lou, first thanks for your post on the photomacrography forum, it initially helped me figure out what was "wrong" with the objective. The number on the objective is 1501-9398 for your info. I'm actually planning to make (and optically polish) a few cover slips of 0.42 mm to see how well the objective can perform. If you send me a message in an email, I can keep you posted if you are interested.
@@HuygensOptics I'm definitely interested. I bought one of these objectives because I didn't think there was any other way I could afford an apochromat. I currently have it attached to a digital camera with a 200mm FL tube lens and have been trying to get better performance out of it. So this is really timely!
Incidentally, the markings on mine are
Nikon
Plan Apo
20x/0.75
DIC N2
inf/0.17 WD 1.0 OFN25
002596
@@HuygensOptics Thanks, that is one of the versions that I have as well. Is the best email the one listed on the Huygens Optics website?
@@canonicaltom That may be quite different from the objective under discussion here. Yours tells you exactly what coverslip thickness to use, 0.17mm.
I am wondering how the canon compares to the earlier 50mm 1,2 aspherical noctilux from leica!
Ik heb 1 jaar biologie gestudeerd. De microscoop was een belangrijke tool. Maar ik heb nooit geleerd dat het dekglaasje zo'n belangrijke rol speelt in de schepte! Dank u! Dat is echt super interessant! En ook echt goed zichtbaar gemaakt!
Zou dat ook een issue zijn voor UV filters in de fotografie, of zitten die zo dicht bij het objectief, en zo ver van het onderwerp dat het geen rol meer speelt?
Overigens heb ik 2 hele goedkope ali express microscoop lenzen*. Mocht je die eens willen testen, dan breng ik ze wel een keer langs.
*) naast een rijtje fotocamera objectieven, ook 2 radioactieve, en een hele grote i37.
Het is dus alleen een issue bij hoge NA en vergroting. En zelfs dan moet je echt goed kijken om het verschil te zien. Waterdikte heeft een zelfde soort effect met een brekingsindex van1.33 dus als je wat dieper in een vloeistof kijkt klopt het sowieso allemaal niet precies meer. Voor UV-filters is het geen issue omdat het licht daar vrijwel onder een hoek van 90 graden doorheen gaat.
Is it possible to use this to analyse any of your previous lenses you created?
Sure, but as I stated in the video, this is only one of a large l number parameters that are important. So I actually prefer to measure MTF in the full image field above measuring wavefront errors for optical systems. I guess it is most useful for measuring surface shapes to see if they match with the design specification.
Well done. My only concern is the new lens possibly adding distortion to the interferogram.
Well, as long as the object is in focus in the image, there should not be a problem. Unless of course your lens has very high vignetting. Remember that it is not about the wavefront that passes the imaging lens but only about the phase differences introduced between reference and object under test.