Old 06-11-2011, 12:12 PM #1
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Default Red laser spot filtering?

I'll make my apologies now if these questions have been answered before, however...

A recent topic of conversation at work with regards to a red laser mechanism has sparked some questions which could not be answered.
I have googled extensively to no avail and subsequently stumbled across this forum which would appear to be a good knowledge base for lasers and would hope someone can answer the questions.

To help explain the questions I will refer to the 2 people in the imaginary experiment as subjects 'A' and 'B' and if we can take the 'given surface' as a non-reflective white painted wall.

The questions are:

1. Can the red 'visible to the naked eye' dot projected on a given surface by a device used by subject 'A' be filtered by subject 'B' using optics that the red dot becomes invisible, without changing the colour or composition of Subject 'B's viewed scene?

2. If it is possible what equipment would be required to do so?

3. Does the selected surface that the dot is projected on determine what can be seen by the naked eye and through optical filters?

I thank you in advance for any wisdom.


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Old 06-11-2011, 07:18 PM #2
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Default Re: Red laser spot filtering?

I don't think so. Let's say you want to use glasses to filter the red dot from the wall; you have 2 types of glasses, one type filters the wavelenght of that specific laser dot by completely absorbing the radiation, and one type completely filters the wavelenght by completely reflecting that wavelenght. Both types will cause the redness dissappear from your compositon of your viewed scene.

Maybe some kind of indirect view with a camera and a computer that filters out that specific wavelenght, and only that wavelenght, and leaves the rest of the view intact. But no matter how you do it, you will always lose the filtered out colour.
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Old 06-11-2011, 08:08 PM #3
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Default Re: Red laser spot filtering?

1. Yes, subject B could wear an optical filter that blocks all of the visible red light produced by subject A's laser. However, that filter would also block the same visible red light that is naturally present in subject B's field of view, so the color composition of that view would also be affected unless subjects B's field of view was complete darkness.

2. I don't know of the choices, but it would probably be an expensive set of laser goggles.

3. Yes. But, the light source also contributes to this determination.

Last edited by yzer; 06-11-2011 at 08:18 PM.
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Old 06-11-2011, 10:16 PM #4
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Default Re: Red laser spot filtering?

Thank you for the quick responses.

To put a little more meat on the bones, as it were...

The laser in question is a military class 3B laser producing a red dot, I hear you ask why have a class 3B laser to produce a red dot? Well, its a simple case of old equipment that has never been upgraded.

The question of visibility of the dot came from a conversation to do with certain military vehicle optics not being able to see the red dot due to filters, however the viewed scene through these optics have no loss of colour perception through the sight including red light.

If the red spot was being filtered I would assume that a vehicles brake or sidelights (red) viewed through the optics would be filtered in the same way the laser spot is filtered? Would I be correct to assume this?

My next step in this 'investigation' is to get a hold of one of the said vehicles and a laser and try it for myself which is not practical due to safety implications etc. but for the sake of my sanity I will try to set it up.
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Old 06-12-2011, 10:54 AM #5
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Default Re: Red laser spot filtering?

Besides filtering color wavelength, there is another way to filter out laser light.

Laser light is polarized to varying degrees.. A polarized filter can block laser light effectively regardless of color wavelength. Polarizing filters do not need to be tinted, like sunglasses. The polarizing filters I use in photography do not alter color rendition.

You can prove this effect yourself with a 5mW 660nm red laser pointer and a good pair of polarized sunglasses. Hold the sunglasses by hand and point the laser through glasses. Look at the spot that comes through. If you rotate either the sunglasses or the laser pointer during lasing you will find an alignment that blocks nearly all of the laser light. I haven't tried this with other diode laser colors but suspect that they are subject to polarized filters as well.

Sunglasses with a single polarizing filter work great to reduce glare but for polarized filters to be effective against a variety of laser modes you would need a much more sophisticated approach. If anyone has done the research it would certainly be the military.

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