Dear optics experts,
Recently, my colleague says that the colored band around the objective lens can regard as the location of the back focal plane.
Do you think it is reasonable?
Or, is there another rule of thumb to find the position of back focal plane of objective lens?
Regards,
**
Added :
Sorry for did not contains full information.
I already know that color means the range of the objective lens' magnification.
As you already know, the objective lens is not a singlet, and back focal plane is sometimes located in the inside of objective lens barrel. that's why I am trying to find the rule of thumb.
Use a sheet of paper to see where it makes a sharp image of an overhead light. If the light is not too close, that is the position of the focal plane.
Hello Yong,
as far as i know, the colored line is not supposed to indicate the plane of the objective back aperture, or at least i did not find confirmation anywhere.
You can measure the position of the back aperture if you have some simple optics tools, following this procedure:
- Mount the objective horizontally on the table
- Shine a low power, visible, collimated coherent light source in the objective from the sample side (a laser pointer is ok-ish if you don't have a proper laser).
-mount a vertical flat surface on the back of the objective, and measure the diameter of the beam and the distance of the surface from the objective. Repeat the measurement for several distances
- The diameter of the beam should increase linearly, if you interpolate and find the position at which your diameter would be 0, that is the back aperture position of the objective.
In alternative, if the manufacturer specifies the maximum field of view of the objective, you can try to have a rough estimate from the objective specifications, by just measuring the diameter of the physical aperture at the back of the objective. In this case:
- compute the focal f of the objective, which is the focal of the microscope tube lens divided by the magnification of the objective
- compute the angular field of view, as tan(θ)=FOV/(2*f), where FOV is the field of view of the objective
- estimate the back aperture diameter as d=2*f*NA/n, where NA is the numerical aperture of the objective, and n is the refractive index of the immersion media
- Measure with a caliper the diameter D of the last accessible aperture of the objective, it should be slightly bigger than d
- At this point the distance of the back aperture plane from the physical back of the objective should be (D-d)/(2*tan(θ))
EDIT: A quick edit two years later, once i realized this answer is the top google result for people looking for back focal planes, and having a student trying to do this for a 1.49 NA oil immersion objective making me realize the method i mentioned does only work correctly with dry lenses. Also, if the NA is very high, it is very difficult to perform the suggested measurement.
I have an alternative method you can try, make sure you use a eye safe laser, as otherwise the procedure could be very dangerous:
- if your microscope is a fluorescence microscope, select a filters cube with a emission spectrum which include the wavelength of the laser you want to use.
- unscrew the objective
- shine your collimated laser through the camera port of the microscope.
- on the objective flange you should see the laser focused in a tiny spot, note down the height of the tightest focus from the table surface, or any other plane which does not move with the objective
- mount the objective back on your microscope, add a drop of the immersion medium, and a flat glass coverslip on top.
- Adjust the focus until the laser beam comes out as collimated as possible from the objective
- in this situation, the back focal plane of the objective is coincident with the measured plane where the laser was focused.
Dear Yong, Paolo gave you right answer to you on back focal plane of objective lens. I will show you the meanning of color ring. They give you information on the magnification of the objective lens such as yellow for 10-12.5x, dark green 16-20x, bright blue 40-50x. Attached pls find a brochure of Olympus in Japanese. Regards, Shigeo
Dear Dr. Shigeo Kubota
Sorry for did not contains full information.
I already know that color means the range of the objective lens' magnification.
As you already know, the objective lens is not a singlet, and back focal plane is sometimes located in the inside of objective lens barrel. that's why I am trying to find the rule of thumb.
Dear Paolo Pozzi
Thanks for your answer. Interpolate the diameter of diverging light is really practical and also straightforward solution.
Dear Alan Scott
Thanks for your answer. I agree with your method. Actually I usually did same procedures when I dealing with some singlet or doublet lens. But this is objective lens, it means back focal plane could locate in the inside of the barrel, and precise locations are needed.
Take a far field source of the light. Take a mikroscope. Focus on the image in the focal plane.
There are already many excellent answers on how to find the position of Back Focal Plane (BFP) of an objective.
However, in practice, you may also take the BFP out of an objective lens using a 4f configuration, and use that BFP for further experimentation. A simple article on such a 4f configuration is given here: https://theonlinethoughts.com/2018/07/06/taking-fourier-plane-out-of-an-objective-for-angle-resolved-measurement/
To find the position of the BFP easily, and precisely place an object at BFP, I recommand using a Bertrand's lens to image it.
You start from the usual configuration where your camera is in the image plane, a lens (usually the tube lens) is focalising the collimated light from the objective on the camera/image plane. Let's call this lens L. If you add a second lens on that path such that the collimated light is focalised at the focal plane of lens L, the camera will receive collimated light again. What happens is : you are now imaging the BFP of the objective. (you reversed the optical conjugation)
You then see a sharp ring of light. If you have lights at the ceiling of the microscope room it may also happen sharp in this plane. (they emit ~collimated light which focalise at the objective's BFP)
For example here i imaged the BFP in a spinning TIRF configuration, where the beam is scanned in a circular fashion in the BFP, to produce different angular outputs.
The shining ring i believe is linked to the fact that the side of the objective corresponds to large angle ranges, which concentrate more power, eventually even supercritical (near field) light. I'm not sure if you still see it on low NA objectives.
- Badges
- Science topic
- Similar topics
- Biological Science
- Biochemistry
- Biochemical Techniques
- Analytical Chemistry
- pH