Filters Physical Explanation

[ciaran_mcmenemy]

<p>I hope this doesn't sound stupid!<br /> filters are a common thing, but I was wondering about how they actually work.<br /><br />If I use a blue filter then the result will be a blue photograph. If the filter passes blue light then why does it look blue when I look at it from the lens? is it because it's imperfect causing some of the light coming in to bounce outwards?<br>

Since it's blue then the red and green components of light get taken away. do the photons combine with the filter in some way? <br>

As for using filters for landcapes and so on, what are the effects of a red filter, green filter, and blue filter on sky? I know the last will make the blue sky lighter relative to the rest of the image, so I guess clouds would be less visible in that instance? </p>

 

[bebu_lamar]

<p>The filter looks blue because all you see is the light that passes thru the filter. The light of other colors are absorbed by the filter and you can not see it except if you have a dichroic filter then you do see the other colors reflected back. <br>

Color filters are mainly used for B&W photographs to change the relative brightness of different colors. When they are used for color photographs they are used to either correct the color balance or to make the photograph looks that way. </p>

[SCL]

<p>Colored filters are typically used primarily for b&W negative film these days. In simple terms they act by the laws of physics...passing greater amounts of colors which match (what we perceive as) the filter color and absorbing other colors. The end result is that subject material which is absorbed by the filter appear darker and subject material which is passed in greater amounts by the filter appears lighter. A blue filter appears visually blue because it has absorbed other colors. Effects of colored filters on B&W films will vary by the spectral sensitivity of the film itself. Today most B&W films are panchromatic, unlike in the 1930-40s when they were orthochromatic, meaning they have a more even sensitivity to color wavelengths across the visual spectrum. Skies are darkened somewhat by yellow and red filters....the amount of darkening depends on the strength of the filter and the time of day (which often determines the amount of blueness we see). This is often desirable to give more visual emphasis to clouds. A green filter, in landscapes, often brightens the earth landscape. The effects of filters, on human skin in portraits, used incorrectly can be disasterous, highlighting skin blemishes, wrinkles, and other features people (usually women) don't usually want highlighted. The best way to get a good understanding of filters is to get your hands on a basic B&W film photography book where you can get a full explanation as well as comparative pictures of the effects. Color film is a different issue entirely, and today colored filters are typically used for special effects rather than color balancing. Then there are polarizing filters, neutral density (or gradient ND) filters. All have a place in photography.</p>

[Alan Marcus]

<p>An optical filter blocks some colors and allows some colors to pass. As a rule of thumb, a filter passes its name. Thus a strong red filter passes red light and blocks green and blue. A strong green filter passes green light and blocks red and blue. A strong blue filter passes blue and blocks red and green. A strong yellow filter passes yellow (red + green) and blocks blue. A strong cyan filter passes cyan (green + blue) and blocks red. A strong magenta filter passes magenta (red + blue) and blocks green. When white light encounters a colored optical filter, the photons that are not allowed to pass (blocked) are converted to heat. </p>

<p>Most strong filters do not have 100% efficiency. They generally block some percentage of all colors. As an example, a perfect cyan should block 100% of the red light energy and pass 100% of green and 100% of the blue. In fact they likely block 98% of the red and pass 85% of the green and blue. The yellow filter is near perfect, followed by magenta with cyan the worst. A perfect red would pass 100% red and block 100% green and blue.</p>

<p>A red filter is used with black & white film to block some of the blue light of the sky. This renders the blue sky abnormally dark, allowing white clouds to stand out; thus clouds are enhanced. A green filter lightens foliage. A blue filter makes the sky appear very light.</p>

<p>The best is the medium yellow; it alters the normal way black & white film renders colors. Many believe black & white imaging is super enhanced by the medium yellow as it permits a more accurate monochromatic rendering of colored subjects. </p>

[ciaran_mcmenemy]

<p>Is it possible to buy dichroic camera filters then? I din't even such material existed!<br>

The ortho film wasn't so sensitive to red light, is that right? Was that on purpose to make developing easier? <br>

Thanks for all the replies! Alan hit the nail on the head (as the phrase goes); so the filtered light gets converted <em>heat</em>... how interesting. So the red frequency is 400-484 THZ, green is 526-606THZ, and blue 606-668THz. If you're using infra-red film (I have 120 rolsl of Ilford SFX, but no idea how to use it!) can you just use an ordinary red filter or do you have to buy one that is guaranteed to capture the lower frequencies?<br /> OK, next time I will try yellow with my b&w :D</p>

[ciaran_mcmenemy]

<p>as for the use of daylight filters, is it just down to choosing the exact complement colour to neutralise the tone?</p>

[bebu_lamar]

<p>I never see dichroic camera filters. Dichroic filters are used in enlargers where they are subjected to intense light source. If the filters were to absorb the unwanted light they would get hot and fade quickly. Dichroic filter reflects the unwanted light back and thus doesn't fade. </p>

[Alan Marcus]

<p>Early filters were water cells filled with various chemicals. These remain in use in laboratory applications. Frederic Charles Luther Wratten (English 1840 – 1926) was in my opinion the great filter maker. He devised recipes and catalogued them. These are the numbers you see on most filters. Wratten of the British firm Wratten & Wainwright was purchased in 1912 by Kodak. An employee, Kenneth Mees, moved to Rochester, NY. He was a renowned photo engineer who perfected Leopold Mannes and Leopold Godowsky’s film making method. This was Kodachrome. <br>

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Wratten mixed dye and pigment in purified gelatin and floated this mixture on water. The gelled filter material was scooped up from underneath on a wire frame. This filter material known as “Gels” is sandwiched between flat glass to make camera filters. Other styles are “dyed in the mass” optical glass and plastic. <br>

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A dichroic filter is optical glass over-coated with a thin layer of metal or mineral or mix. They resist fading as the blocked frequencies are reflected away. These work by the principal of thin film interference. The word dichroic is Greek for bicolored. As an example, a yellow dichroic looks yellow by transmitted light and blue by reflected light. They are often used as “cold mirrors” that reflect away infrared and pass white light. A “hot” mirror passes infrared and reflects white light.<br>

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[bethe_fisher]

<p>I was trying to find more examples of with and without a filter, but this is all I've got so far. These have good explanations, too, but Alan distilled it down really well.<br>

https://photographylife.com/lens-filters-explained <br>

http://www.digitalcameraworld.com/2014/07/24/5-essential-photography-filters-and-why-you-cant-live-without-them/ </p>

 

[glen_h]

<p>The first place I ever heard about dichroic filters was for color TV cameras. They can divide the incoming light into red, green, and blue much more efficiently than with light absorbing filters.</p>

<p>I know of a stained glass window where some of the glass pieces are dichroic filters. Some are blue transmitting, yellow reflecting, and look interesting both inside and outside.</p>

<p>Otherwise, for camera filters on the camera, you see the light that goes through the filter, then comes back though the filter, such as from the letters of the company name on the front. </p>

[glen_h]

<p>As for ortho film, in the beginning, silver bromide films were only blue sensitive.</p>

<p>At some point, to reduce the effect of light scatter in the emulsion, someone decided to add dye to the emulsion. I don't know if it helped light scattering, but it did add green sensitivity. You can search for information on sensitizing dyes. They adsorb (stick to the surface) of the silver bromide crystals (grains) and add sensitivity to other colors.</p>

<p>As the energy (frequency) gets lower than the bandgap (blue for AgBr), it is harder to get dye sensitization to work. That is why infrared film is harder to make, and red sensitivity harder than green.</p>

<p>It is rare to discus photon frequency, more usual to multiply by Planck's constant and get a photon energy, usually in eV, but sometimes in kcal/mole.</p>

<p>I suspect that the ability to use red light during developing delayed the move to panchromatic film. It was just too convenient, but eventually it happened. Maybe related to improvements in film developing tanks. It seems to have been 1957 for Verichrome to Verichrome Pan, I don't know about others.</p>