Is candle-sooted glass a neutral filter, and how does it affect UV/IR transmission?
Asked 7/2/2012
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If you blacken a piece of glass with candle soot, can it work like a neutral-density filter? Specifically: is the resulting filter neutral in color, does it block UV or IR, and are there any other optical effects or drawbacks to expect?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
14y ago
2 Answers
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I would suggest not; darkening in this case would be by means of tiny particles of soot, which in themselves are opaque. Any darkening would be by means of scattering, and as evidenced by blue skies and red sunsets, particulate scattering is wavelength dependent, with transmitted light taking on a reddish tinge as opposed to the bluish tinge from refracted light. You can also demonstrate this by taking a glass of water, adding a small amount of milk, and shining a torch through it - the orange hue will be clear if you look directly at the torch through the milky water.
If you use a glass filter darkened in this way, I would expect an orange hue in photographs taken with it - which though possibly not as desirable as a true ND filter, may still produce interesting effects, especially if the coating of soot is uneven. Give it a try and see what happens!
Originally by user456. Source · Licensed CC BY-SA 4.0
user456
14y ago
0
Generated from our catalog & community — verify before relying on it.
No—glass darkened with candle soot is not likely to behave like a true neutral-density filter. The darkening comes from tiny soot particles on the surface, which reduce light mainly through scattering and absorption rather than the controlled, even transmission of an optical filter.
Because particle scattering is wavelength-dependent, the transmission is unlikely to be neutral. Based on the community answer, you should expect a warm/orange or reddish cast rather than even attenuation across visible wavelengths.
There’s also no reason to assume it provides reliable UV or IR blocking. A soot coating is not a designed spectral filter, so any UV/IR reduction would be unpredictable.
Other drawbacks: uneven coating, loss of contrast, flare/haze, reduced sharpness, and inconsistent results from one piece to another. So while it may create an interesting experimental effect, it is not a dependable substitute for a proper ND or UV/IR filter.
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