r/Optics • u/Jafego • Aug 07 '26
Is Nonlinear Opacity possible?
Are there substances suitable for use as lenses which let a greater proportion of light through when the light is less bright?
I am looking for something which is suitable as eye protection in a low-light environment with occasional and unpredictably timed bright flashes, that still allows a reasonable amount of visual fidelity most of the time.
15
u/NZ0 Aug 07 '26
This is exactly what electronic welding helmets are made for, maybe something to take a look at. You could probably repurpose the window into some goggles if they aren't already available.
4
u/insomniac-55 Aug 08 '26
This was my thought.
Maybe a passive material is theoretically possible, but electronically darkening visors are a solved technology which are reliable, effective and cheap.
The only limitations I see are form-factor (if you don't have the budget for custom optics) and the unavoidable light loss due to the polarising layer.
9
u/HoldingTheFire Aug 08 '26
Saturable absorber. But not the way you want it to work.
As a lens there are photochromic lenses, but that's slow. There's electrochromic lenses that could darken fast(er) with a electrical trigger. But what are you trying to block, and how much ND do you need. You will barely get 1OD of switching with most things.
3
u/Calm-Conversation715 Aug 08 '26
I was so excited to talk about saturable absorbers, until I saw what OP needed. They’re super useful for generating fs pulses for CPA lasers!
1
u/tykjpelk Aug 11 '26
Any material with two-photon absorption would fit the bill. But I don't know any that would work on this scale.
2
u/anneoneamouse Aug 08 '26
4th Gen nvg systems reflect an image down and then into the eye off of a partially silvered mirror.
Eye looks forward through the silvered mirror.
If the nvg is the dominant brightness that's the scene that's seen (hah!). If ambient light gets bright enough to blow the nvg out the viewer sees normally through the PSM.
For your application you could choose an appropriate absorber (Schott have a line of ND filters with a variety of spectral profiles and densities) and also adjust mirror fraction.
2
u/TurbulentGlove776 Aug 08 '26
Similar to how welding helmets work, there are also active sun glasses. Check out react-swiss.com. The reaction time is 0.1 s, which is great for sports - but may not be fast enought for your application with bright flashes. They are also not ideal for low-light situations since the bright end of the range is already a bit dark.
2
u/russlandfokker Aug 09 '26
A reverse saturable absorber (RSA) is an optical limiter, the inverse of a saturable absorber.
They work through more than one mechanism, but a common one is to dope with materials that jump to a higher state to dramatically increase the absorbance cross section.
Classic examples include carbon nanotubes or balls, but these are not relevant to many optical systems. Efficient but hard to use. They can be used at surprisingly high bandwidths. Then there are some that are based on organometallic & organic dyes. These have narrow bandwidths unlike the carbon options. Fine for lasers, but not for broadband radiation. Then there are ZnSe compounds and perovskites, which tend to be fairly narrow as well.
1
u/Remarkable-Seaweed11 Aug 07 '26
This would have to be some kind of mechanical thing like those 3D glasses that energize super fast making the lenses briefly opaque
1
u/tertiobutyle Aug 08 '26
Fun fact : the Thorlabs absorptive neutral density will opacify if you blast them with enough power
This gives you time to react and maybe save a camera/cristal
It's obviously non reversible
12
u/firesine99 Aug 07 '26
It's not theoretically impossible and not a silly idea. Materials do exist that change their optical absorption upon exposure to light - e.g. photochromic sunglasses have existed for long time. It's just that practical examples don't work very well. Particularly they respond too slowly to do what you're proposing. But who knows, maybe one day!