Color Modulating Filters: How Useful?

[linuxusr]

Many microscopes, mine, for example, come with a set of color modulating filters--and no instructions. When I research their use, the information I find is rather thin.

It appears that the blue or yellow filters may compensate (balance) light from a tungsten or halogen bulb. Now that many of us use LED it would seem that this use is now obsolescent or obsolete except for those with older scopes. Sometimes a filter might be used in photomicography for emulsion films: again, obsolete. Or is there still a use case for digital imaging?

For the green filter, I get a different story but I'm not sure if I'm drawing the right conclusion. One source says that the Plan objective can benefit from green light spherical correction. I don't know if this means that that benefit is already built in or whether a green filter could augment the image quality. And P. Evennett of RMS who has excellent instructional videos on light microscopy conjugate field and aperture planes, notes that the human eye is strongly adapted to the green wavelength. I'm not sure if this means that a green filter could benefit. . .

I tried my filters on various permanent specimens and found no benefits.

[One filter that came in my pack, I don't know how to identify it. It will reflect a light that I shine on it, like a mirror, but when I hold it up to my eye, I can see through it. Does anyone know what this is?}

I have two locations for my lab supplies: one for stuff I regularly use and one for stuff that I almost never use. I'm thinking about relegating my color filters to the latter box. What do you think?

[BramHuntingNematodes]

The green filter is for achromat, not plan lenses. Achromat lenses do not fully correct chromatic aberration, so the green filter excludes wavelengths that are particularly troubling. This is why in the old ad literature the apps are often advertised as useful for color photography as you can take out your green filter and the picture will still be sharp.

On the other hand, a blue filter is said to slightly increase resolution as it excludes long sloppy wavelengths.

Filtering can sometimes increase contrast of colored objects in a differently colored field. So if you are looking for small grains in a contrast stained slide they might be useful.

Fluorescence microscopy makes extensive use of being able to select wavelengths of light that can be both emitted and transmitted.

Cheap LEDs will not benefit the same way from filters as they frequently lack some spectrum of frequency, even if they appear white.

[BramHuntingNematodes]

Oh and sometimes reflective films are used as neutral density filters, the most generally useful type of filter. It makes light dimmer if it is otherwise too bright.

[Hobbyst46]

Actually each filter has a well-known specific purpose.

The green filter improves the appearance of phase contrast images. Because the optical elements involved are optimized for green light (546nm wavelength). AFAIK this is true for all phase contrast (achromats and others) and is valid today as it was in the pre-digital era. AFAIK optimization is not identical to post processing of the image to leave only the green channel.

The blue filter removes some of the yellow and red hues from a halogen-lamp and other incandescent created images. It is useful for photography. I guess that white balance manipulations in software can reach the same effect.
In principle, a blue filter will improve resolution somewhat. In practice, this happens only when it is really deep blue, and the light is within a narrow range of wavelengths in the blue region. Yours is probably not that type.

A yellow filter will remove the blue component from the illuminating beam. This might be useful for cool LEDs, whose spectrum contains a very strong blue component. IMO an amber filter is better for such cases.

The mirror-like filter you describe can be either of two possibilities:
1) If the transmitted color is grey, you have a neutral density filter, that reflects some of the incoming light and transmits the rest; so it reduces the brightness as necessary. The percentage of removed light can be anywhere - 50%, 80%, 95%, 99.9% - but since you can see an image when you look through it, I will bet that it is a 10-20% neutral density filter. It is totally achromatic - same efficiency for all wavelengths. Very useful for control of the illumination when the power supply to the lamp (regardless if halogen or LED or whatever) stays the same.
2) If the color of the reflected light differs from the color of the transmitted light, you have an interference filter that cuts off the light within a given range of wavelengths. Such filter is of special value for fluorescence and of limited use otherwise.

[iconoclastica]

The blue filter removes some of the yellow and red hues from a halogen-lamp and other incandescent created images. It is useful for photography. I guess that white balance manipulations in software can reach the same effect.

I find that only a thin blue filter works as such. Darker blue filters are quickly overdone, changing the red to a blue cast. Setting the WB to Tungsten instead also works very well.

In principle, a blue filter will improve resolution somewhat. In practice, this happens only when it is really deep blue, and the light is within a narrow range of wavelengths in the blue region.

In digital colour photography, the effect of filtering out the long wave-lengths has a negative effect, since it deactivates the red and green subpixels of the bayer-filter. It ought to work with monochrome having the bayer removed.

[BramHuntingNematodes]

Phase also benefits from green filters but I haven't played around with phase apos very much or at all. I'm not sure exactly what the application is. Achromats were regularly made with higher NA in times past, and if you are filtering to monochrome frequency than it would seem that the advantages of the apochromat would be minimized.

[linuxusr]

@BramHuntingNematodes
@Hobbyst46

Thanks for sharing your experience and knowledge with color filters. It turns out that the filter I could not identify is a blue filter. It doesn't appear blue at a short distance but when I hold it up to the light it is indeed blue. And my original order confirms same.

In the examples you gave, I tried to find at lease one use case. The example I chose was yellow removes blue so that under LED illumination I might see improvement. The sample I chose were chloroplasts on Spyrogyra. With and without the yellow filter the resolution was the same. I found the yellow background a little annoying. On the other hand, the LED was overly luminous (for me) but I needed that much illumination to resolve. It could be that in the future I might benefit from a 10% ND reduction filter. And you do note that amber works better LED compensation than the yellow.

For the moment, I'm relegating these filters to my "maybe" box.