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Pitufo
Joined: 21 Jun 2015
Posts: 217
Location: United Kingdom
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 Posted: Sat Jun 09, 2018 10:18 am Post subject: The use of green filters with achromatic objectives |
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As I understand it..a green filter on the light source can be used to improve image quality when using achromatic objectives, because these objectives are best corrected for spherical abberation at this wavelength. I have done this experiment and it seems to work.
My question is a simple one and yet I struggle to understand it.
Green light goes in (lets say this is a perfect 550nm - this may be the mistake) and passes through the sample. The image arrives at the camera sensor (and appears green) and I reset the white balance on the green background.
I now have a full colour image. How?
If part of the sample appears red (if it is solely acting as a red filter), this would appear to mean that red light has been allowed to pass through it to the detector. And yet, there was no red light to start with as this was eliminated by the green filter.
Please put me out of my misery...  |
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Beatsy
Joined: 05 Jul 2013
Posts: 1478
Location: Malvern, UK
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| Posted: Sat Jun 09, 2018 11:45 am Post subject: |
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Your green filter probably has a very wide bandpass centred around 550nm so some red(der) and blue(r) light is getting through too. Using the "green" to reset the white balance will change colour temp and tint, and massively reduce green intensity in the process - thus restoring the colour.
I suspect the downside will be a huge loss of dynamic range and you could start seeing banding in areas that originally had smooth colour gradients.
Edit: I just had a quick go to see this for myself. An old "coloured glass" filter does as you describe, it's possible to get back to a white background. But I also tried a green interference filter with a very narrow bandpass and, sure enough, the image made with that remains resolutely green.
Last edited by Beatsy on Sat Jun 09, 2018 11:58 am; edited 1 time in total |
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Lou Jost
Joined: 04 Sep 2015
Posts: 3184
Location: Ecuador
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| Posted: Sat Jun 09, 2018 11:55 am Post subject: |
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If you really want to use green light to improve resolution, and if you are fine with a black and white final image, you might consider using truly monochromatic green laser light. There will still be some signal on the red and blue sensels, but it won't cause loss of sharpness. You should convert it to monochrome at the end. The red and blue channels will be noisier than the green channel, but there will also be some signal there. Alternatively you could just use the green channel but you would be throwing away half your resolution. Best would be to get a monochrome sensor-cooled astrophotography camera. With that, you could improve resolution even more, by using shorter-wavelength blue or violet lasers instead of green lasers.
Note that any time you use laser light for photography, you have to despeckle it. I use dilute milk-filled diffusers for that.
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Lou Jost
www.ecomingafoundation.wordpress.com
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Pitufo
Joined: 21 Jun 2015
Posts: 217
Location: United Kingdom
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| Posted: Sat Jun 09, 2018 12:49 pm Post subject: |
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Thanks for your answers Beatsy and Lou. That makes sense.
I've never noticed any banding artifacts or noticed a large reduction in intensity but it may well have happened and I haven't looked at any data.
Using a green filter did seem to improve the performance of achromats. This make sense if the longer and shorter visible wavelengths are mostly removed by the green filter - which I think would reduce the effects of both spherical and longditudinal chromatic abberation on the sensor.
However, if we are then removing most of our green light intensity when we reset the white balance on the background, aren't we then defeating the point of the process? (i.e. by effectively levelling the playing field and bringing all the wavelengths back to the same relative intensities).
Maybe this technique will only truly work with a green narrow bandpass interference filter as tested by Beatsy (or green laser suggested by Lou) and with a monochromatic output image.
EDIT: I found this comparison using a green filter and grayscale which seems to work http://www.microbehunter.com/microscopy-forum/viewtopic.php?t=2940
P.S. I don't really need to do this to improve my resolution, it is more something I would do if I was on an away trip with a portable achromatic microscope and wanted to make the images at least bearable.
Last edited by Pitufo on Sun Jun 10, 2018 3:02 am; edited 1 time in total |
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rjlittlefield
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Joined: 01 Aug 2006
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Location: Richland, Washington State, USA
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| Posted: Sat Jun 09, 2018 2:25 pm Post subject: |
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| Pitufo wrote: | | However, if we are then removing most of our green light intensity when we reset the white balance on the background, aren't we then defeating the point of the process? (i.e. by effectively levelling the playing field and bringing all the wavelengths back to the same relative intensities). |
Sort of but not really. Resetting the white balance can only bring all three bands back to the same average intensity. It cannot do anything to equalize the relative abundance of various wavelengths within each band. The resulting playing field may be level on average, but still contain some pretty strong hills and valleys.
For example, if your green filter passes some yellow but completely blocks deep red, then things that started off being deep red will still appear black even after resetting the white balance. Things that started off yellow will have their color altered, but the new color will still be clearly distinguishable from gray so the appearance of full color will be retained. Likewise for a filter that completely blocks deep blue/purple while still passing some blue-green.
It is usually the extreme wavelengths that give the most trouble with chromatic aberrations, so getting rid of just those can still make a significant improvement even though the final image may appear to be full color.
--Rik |
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Pitufo
Joined: 21 Jun 2015
Posts: 217
Location: United Kingdom
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| Posted: Sun Jun 10, 2018 2:59 am Post subject: |
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| Quote: | | Sort of but not really. Resetting the white balance can only bring all three bands back to the same average intensity. It cannot do anything to equalize the relative abundance of various wavelengths within each band. The resulting playing field may be level on average, but still contain some pretty strong hills and valleys. |
Thanks Rik - I think that is the important point, which I had completely overlooked.
I need to look into how white balance works exactly.
Thank you for helping me understand
Last edited by Pitufo on Sun Jun 10, 2018 3:49 am; edited 1 time in total |
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enricosavazzi
Joined: 21 Nov 2009
Posts: 1128
Location: Stockholm, Sweden
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| Posted: Sun Jun 10, 2018 3:39 am Post subject: |
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| rjlittlefield wrote: | ...
It is usually the extreme wavelengths that give the most trouble with chromatic aberrations, so getting rid of just those can still make a significant improvement even though the final image may appear to be full color.
... |
Exactly. Amateur astronomers have been using for many years "semi-apo" filters, which transmit only the middle parts of the red, green and blue bands and cut all the rest, for exactly the same purpose. Astronomik also markets L1, L2 and L3 filters that cut UV and IR as well as both extreme ends of VIS. L1 has cutoff points in the UV and IR, L3 has cutoff points slightly within the VIS range. These are dielectric coated filters with very sharp cutoff and roughly flat bandpass, potentially useful if achromatic aberration is highest at the ends of the VIS band.
Astronomy filters that selectively cut various VIS narrow bands (including "trouble" bands caused by some types of artificial street illumination) are also available. If you know which bands have the most troublesome chromatic aberration you could use one or more of these filters to cut these bands.
Using one (or more) of these filters is more likely to give satisfactory results that shooting with a green filter and trying to restore WB in post-processing. Of course, selectively eliminating some wavelength bands does alter the color rendering of the subject in ways that cannot be restored in post-processing, there is no way around it. But if sharpness is more important than an exact color rendering, these filters have their uses.
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JH
Joined: 09 Mar 2013
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Location: Vallentuna, Stockholm, Sweden
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Pitufo
Joined: 21 Jun 2015
Posts: 217
Location: United Kingdom
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| Posted: Tue Jun 12, 2018 1:32 am Post subject: |
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| Thank you Enrico and Jörgen - very interesting stuff. |
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