ND filter vs filter for phase contrast

Here you can discuss everything related to taking light micrographs and videos.
Post Reply
Message
Author
Sauerkraut
Posts: 204
Joined: Tue Apr 16, 2019 2:07 am
Location: Oregon, USA

ND filter vs filter for phase contrast

#1 Post by Sauerkraut » Thu Aug 29, 2019 3:37 pm

My Optiphot is missing the ND16 filter. I was thinking it might be more useful to use the slot for a green filter since it reportedly gives better phase contrast images.

Does anyone use the ND filters or can confirm/refute that a green filter would be worthwhile for PC?

Thank you for any input.

Heather

photomicro
Posts: 131
Joined: Thu Aug 03, 2017 10:28 am
Location: UK

Re: ND filter vs filter for phase contrast

#2 Post by photomicro » Thu Aug 29, 2019 5:13 pm

The ND filters are merely to reduce the light intensity. Whilst it might seem that this is possible just using the rheostat to reduce brightness, for photomicrography, the bulb is run at a certain voltage/current to attain a suitable (known) colour temperature. This was more important for film than digital, where we can set a colour temp on the camera, or indeed adjust in s/w afterwards. Another reason could be to attain a certain shutter speed at which vibrations were minimised.

You could perhaps also argue that with LEDs now, perhaps ND filters are less necessary.

A green filter for phase is because usually phase objectives are specified to operate optimally at one particular wavelength, commonly 550 nm. Light of this wavelength appears green, and is what our eyes are most sensitive to. This is why lens aberrations have been best corrected for this value too.

Mike

Hobbyst46
Posts: 2182
Joined: Mon Aug 21, 2017 9:02 pm

Re: ND filter vs filter for phase contrast

#3 Post by Hobbyst46 » Thu Aug 29, 2019 10:26 pm

Just to add to photomicro answers above.
There are 3 types of ND filters:
Least expensive , lasts forever is absorption filter. It tends however to attenuate blue light slightly more than green and red light. I think that is not so important.
More expensive, wears off with time is reflective filter. It is coated and attenuates at a fixed level regardless of the wavelength.
Forum membed mrsonchus uses a variable ND filter which consists of a pair of partially crossed polarizers. I do not have one, but it might be most convenient.
Even for LED illuminators, ND filters are good because some inexpensive LED dimmers cause banding in the camera image. Then, setting the LED to max intensity and using the ND solves the problem.
Zeiss Standard GFL+Canon EOS-M10, Olympus VMZ stereo

Sauerkraut
Posts: 204
Joined: Tue Apr 16, 2019 2:07 am
Location: Oregon, USA

Re: ND filter vs filter for phase contrast

#4 Post by Sauerkraut » Thu Aug 29, 2019 11:05 pm

Thank you photomicro and Hobbyst46. That answers my questions.

apochronaut
Posts: 2746
Joined: Fri May 15, 2015 12:15 am

Re: ND filter vs filter for phase contrast

#5 Post by apochronaut » Fri Aug 30, 2019 2:22 pm

The green filter also lessens the halo associated with most phase contrast systems. When photographing, adjusting to a black & white setting removes the green cast . Many specimens are not particularly colourful anyway , so converting to a b & w image doesn't really lessen the image's impact much with those.

Sauerkraut
Posts: 204
Joined: Tue Apr 16, 2019 2:07 am
Location: Oregon, USA

Re: ND filter vs filter for phase contrast

#6 Post by Sauerkraut » Fri Aug 30, 2019 3:54 pm

Thank you, apochronaut. Good to know.

Post Reply