Are there filter stacks that balance the light intensity of UV, vis, and IR?

[Doug A]

Instead of cutting out certain ranges, is there a way to equalize the 3? In other words, reducing vis and IR to the same brightness as UV. Most filters cut out parts of the spectrum. Are there specific range neutral density filters?

 

Thanks,

Doug A

[Fandyus]

I have been wondering the same for months. The resulting images would have the potential to look quite interesting, though I'm not sure any conventional lenses would be able to deal with the huge focus shift.

 

 

[lukaszgryglicki]

Had the same thoughts, even asked commented in a quite similar way some time in the past - that would be really great filter.

It should be balanced to full sunlight IMHO - to maximize effect, we do UV handheld mostly in full sunlight anyway...

So it should transmit 100% below 380nm (ideally 200% below 355 :P :D), ten about 4-8% in visible and IR range...

[Andrea B.]

Assume you had such an "equal-pass" filter which would pass equal amounts of UV, Vis and IR light

together with a lens which would transmit those three light batches in equal amounts to the sensor.

There are still two things we don't know for sure:

• Would the three light batches pass in equal amounts through the Bayer filter?
• Would the sensor be equally responsive to each of the three light batches?

Assume now, for a moment or two, that the three light batches do pass in equal amounts through the lens and through the Bayer filter

and also that the camera sensor is equally responsive to each type of light.

 

We have seen that in most converted cameras both UV and Infrared get recorded by the camera mostly in the Red channel along with red Visible light. This seems to me to be the place where it all goes wrong. It doesn't seem like a useful or meaningful way to record three different but equal amounts of light. I think it would be better if there were a Bayer type filter which passed UV, Vis and IR light through UV cells, Vis cells and IR cells, respectively, and that each of the 3 cell types was recorded (mostly) in a separate channel. 

 

(Andy, where have I gone wrong here please? )

 

Only a real experiment with the proposed "equal-pass" filter will tell the entire story, methinks !!

 

Meanwhile......To save the probably very large expense of trying to get a custom "equal-pass" filter made, do the following.

Make separate UV, Vis and IR shots, align them and channel stack each frame into an R, G and B channel in some order.

 

We've been doing this around here for years. Most of you know this technique, but I mention it for anybody new to UVP who might not have tried it. Some call the result a multispectral stack, some call it a channel stack. 

[Andy Perrin]

Haha, so, what you are talking about is a Gain Flattening Filter and they definitely exist for monochrome silicon sensors. Lukas you should buy one, I think Omega makes them because I actually own one from the days when they sold the seconds on ebay. The situation is much more complex if there is a Bayer involved. 
 

https://www.photonicsonline.com/doc/omega-optical-introduces-the-next-generation-of-uv-gain-flattening-filters-0001

[lukaszgryglicki]

I've requested a quote for a few filters from then a few days ago, silence so far.

 

[Andy Perrin]

They will get back to you eventually — I have had good responses from them in the past. 

[lukaszgryglicki]

I know, I'm not saying that it takes too long, it's normal IMHO.

 

[Wayne Harridge]

On 7/13/2022 at 3:30 AM, Fandyus said:

I have been wondering the same for months. The resulting images would have the potential to look quite interesting, though I'm not sure any conventional lenses would be able to deal with the huge focus shift.

 

 

Yep, I've been thinking the same thing.  I reckon the problem is that a most of the IR ends up in the RED channel as does most of the UV, obviously something to do with the Bayer filters.  In this situation there isn't much you can do to discriminate between RED from VIS, RED from IR and RED from UV.  So you need to shoot 3 shots with different filters to get that discrimination.

 

Oops !  just read through some more posts in this thread and I'm thinking I'm repeating pretty much what Andrea said (though probably not as clearly).

 

 

 

[Fandyus]

8 hours ago, Wayne Harridge said:

Yep, I've been thinking the same thing.  I reckon the problem is that a most of the IR ends up in the RED channel as does most of the UV, obviously something to do with the Bayer filters.  In this situation there isn't much you can do to discriminate between RED from VIS, RED from IR and RED from UV.  So you need to shoot 3 shots with different filters to get that discrimination.

 

Oops !  just read through some more posts in this thread and I'm thinking I'm repeating pretty much what Andrea said (though probably not as clearly).

 

 

 

Yes, but I think the resulting images could still be interesting, even if only the red and blue channels would be affected significantly. Especially for flowers or faces. Or even food.

[Doug A]

Great points @Andrea B.Since most Bayer filters are similar, it might be possible to figure that into the equation. Even lens light loss in UV could be roughly accounted for. None of this would be exact, but it should work better than nothing.

Taking 3 shots works, but I struggle to avoid subject movement. Something always moves outdoors. 

 

Thanks,

Doug A