emphasized text> Is it possible to design an tunable visible wavelength optical filter
for exclusively filtering intense coherent laser light while leaving the incoherent light background untouched?
Mr. Han-Kwang Nienhuys , the experimental physicist states that "If you want to protect a pilot against the most common high-power laser pointers (532 nm wavelength), you should be using dielectric mirrors as filters. Such mirrors have a coating that is a stack of thin layers, tuned to reflect light of a certain wavelength. The dielectric mirror can be designed to reflect only over a very narrow wavelength range, in which case they are called "notch filter". You can buy these off the shelf, for example Edmunds OD4 notch filter. A disadvantage for pilot protection is that the reflecting wavelength will depend on the angle of incidence αα. The only way to make them reflect 532 nm wavelength over a wide range of angles is by making them reflect over a large range of wavelengths. If you do that for green light, you will probably end up with something resembling very dark sunglasses, which only transmit deep red and deep blue light and block orange-yellow-green-cyan."
So another design criterion for the hypothetical tunable filter is insensitivity to large angles of incidence. It has been observed that large angles of incidence map to significant shifts in the center wavelength of the filter. What is the reason for this phenomena?
If so, how might one design such a thing?
There may be too much hoopala made today about the difference between coherent and incoherent light. If the source is fairly small (like sun, as it is very large but also at very large angle hence it is very small) , it will act as spatially coherent source. If you select a very small bandwidth it is also temporally coherent. Hence, one could stop worrying about coherent/incoherent thing.
The incoherent light form a background and the coherent light is a delta function riding over the background. If we have a narrow band reject filter(say 10 nanometers) could I exclusively remove the intense laser light without losing incoherent light visibility appreciably?
Please feel free to use formulas or drawings if you would like.
I spoke with the Edmund Optics application engineers in the United Kingdom this morning and asked how to exclusively remove red laser pointer light while letting red airport night runway light through untouched. The Edmund Optics application engineer stated that the spectral linewidth of incoherent red night runway lights is about 100 nanometers wide (50 nm FWHM) while the spectral linewidth of a red coherent laser pointer is about 2 to 3 nanometer. So , the Edmund Optics engineer recommended using either a custom notch matched filter with 2.5 nanometer FWHM similar to part number OD-86121 or an off-the-shelf notch filter with 25 nanometer FWHM. The off-the-shelf notch filter is less costly than the custom notch filter and is capable of blocking the variety of red laser pointers with slightly different center wavelengths.
The spectral linewidth is the first derivative of the optical bandwidth defined as 2 * pi * speed of light divided by center wavelength in nanometers.
Here is a proposed way to fix the weakness of Airbus and Nova Scotia's Metamaterials Inc.s Lambda Guard product where the only way to make them reflect 532 nm wavelength over a wide range of incidence angles is by making them reflect over a large range of center wavelengths because of the optical path length differences. One could design an array of 5 interferometers spanning 0 degrees of angle of incidence to 45 degrees of angle of incidence in 9 degree wide increments.
[EDIT 8/27/2016 11:46 AM Frank] The only way such an array would fix the weakness of Metamaterials Inc.s Lambda Guard product with regard to large incidence angles is to identify probable narrower angle ranges as I did in my volume of vulnerability analysis for the Boeing 767 which I will show here by Sunday evening August 28 2016, If I made the array wide band(0 degrees to 45 degrees) we would lose the transmission over all wavelengths hence defying our purpose.
Also , Metamaterials Inc.s Lambda Guard product may not be tunable with respect to wavelength causing it to be unresponsive to future frequency agile laser pointer threats.
I hope to show a drawing for this by tonight.