Strange spectrums with a diffraction grating

[Stefano]

Hello,

UlfW, thank you very much, very interesting to see it!

 

But - In any case in the photo with Baader - this can not be any error of the white balance.

Because this lens doesn’t pass UV in this part of the gradient, is not it?

 

Front 'yellow' UV part, there cannot be any UV part, green or orange or any 'color' of UV part - there should be a 'black hole' only.

So this 'orange' in the gradient - only if any IR leak.

 

Ok, but then, why if it is IR, why it is in front of the UV part? Then it should be test photos a spectrum 2-order -

in the test photo from Cadmium, and from Stefano also? Unlikely..

No, I didn't understand this gradient with leak. ¯\_(:-|)_/¯

Multiple order diffraction (2nd, 3rd, 4th, etc.) only occurs at multiples of the original wavelength. For example, a 400 nm violet light source will produce diffraction at 400 nm, 800 nm, 1200 nm etc, but not at 200 nm or less. So a second order diffraction from UV (ex. 365 nm) can be seen at 730 nm (far red), but not the opposite. I may try it since I have both a 368 nm and a 736 nm LED (measured values).

[Stefano]

The Baader U has some leakage.

It sometimes can be visible against an otherwise UV-black background if the motif is reflecting NIR well.

 

I played around a bit with my measurement results of my Baader U (new type), in the Schott filter calculator:

https://www.ultravio...dpost__p__28458

I never used an interference filter, but I personally prefer absorptive ones. In my opinion they look cooler and you can achieve very deep blocking (like OD 7 or even higher), at the expense of the peak transmission (I think it is difficult to make a filter with a "black" glass (ex. Hoya U-360) and "blue" glass (ex. S8612) with > 70% peak trasmission and OD ≥4).

[Stefano]

Multiple order diffraction (2nd, 3rd, 4th, etc.) only occurs at multiples of the original wavelength. For example, a 400 nm violet light source will produce diffraction at 400 nm, 800 nm, 1200 nm etc, but not at 200 nm or less. So a second order diffraction from UV (ex. 365 nm) can be seen at 730 nm (far red), but not the opposite. I may try it since I have both a 368 nm and a 736 nm LED (measured values).

 

Camera: Panasonic DMC-F3, f-stop: f/2.8, ISO 80, 1/800 s exposure, no filters

[Cadmium]

Again then, what lens is being used?

[Stefano]

Again then, what lens is being used?

Basically, this is my camera now (everything does make sense, including the paper cover). I use it to "mount" a filter, made of glass and paper too.

 

Detail of the lens.

[dabateman]

Interesting it has 6 elements in 5 groups. I am surprised by that, as the zoom range is 28mm to 112mm.

 

[Cadmium]

Stefano, Thanks! I didn't realize it was a camera with a built in lens, if I had looked up the camera then I would have realized this.

[Cadmium]

The Baader U has some leakage.

It sometimes can be visible against an otherwise UV-black background if the motif is reflecting NIR well.

 

I played around a bit with my measurement results of my Baader U (new type), in the Schott filter calculator:

https://www.ultravio...dpost__p__28458

 

I used Shane's Baader U graph a while back to plot the Baader U IR leak.

Of course his scan is old, but has a similar but not exactly the same IR leak and OD.

Here is his graph, top right graph, red plot:

http://www.beyondvisible.com/BV3-filter.html

 

Here is the Schott program plot I made using his graph, and sorry I left out the 730nm bump, my mistake, but it is in the same spot your 730nm bump is at.

[Ninjin]

Could it be reflections from the main pattern that are bouncing between filter and the lens's front element?

 

Hello, UlfW

 

I do not know if this is possible..

 

On these tests photos, with the ZBW2 stack, and with Baader,

for me, they show a spectrum of 2 order, and both filters with leak IR to 740 nm + _(from 1st order spectrum)

This is what I see from these tests.. But of course, if I think this is a Baader brand, I doubt what I see.

[Ninjin]

So a second order diffraction from UV (ex. 365 nm) can be seen at 730 nm (far red), but not the opposite.

 

Hello, Stefano

 

Yes, but this is what I asked - Why is the opposite in the photo then?

 

If orange is a leak - why is the leak (conditionally 730 nm) in place of 345 nm? I can't understand here. Cadmium in two tests shows a UV gradient through a UV incapable lens -

and sees an orange (conditionally 730 nm) in place of a black spot - 345 UV. How can it be? Ok, this is if a Baader, a "mystical mirror."

 

But in your test ZWB2 (2 mm) + BG39 (2 mm), also a UV gradient, and also orange (conditionally 730 nm) there should be a black spot in place - 345 UV. How can it be?

Perhaps, if any of position of grating, IR leak from 1 spectrum, and a 2-order spectrum is taken in the photo. That is all I could imagine.

Of course second option - If your lens and camera can record UV 350-300 nm inclusive, and the camera records all this part as 'orange'.

[Ninjin]

Here is the Schott program plot I made using his graph

 

Hello Сadmium

 

Thanks for the graph!!

 

I can only see by color. But I would suggest if this 'оrange' color of leak - should be well before 800 nm.

Here is my gradient for the infrared part of some filters.

 

[Stefano]

Hello Сadmium

 

Thanks for the graph!!

 

I can only see by color. But I would suggest if this 'оrange' color of leak - should be well before 800 nm.

Here is my gradient for the infrared part of some filters.

 

Hi, sorry if I didn't reply sooner but as you know the website went down.

The ~700-750 nm part of the spectrum (far red) appears yellow-orange when using a standard UV white balance. But the filter I am using should block infrared pretty well.

[Ninjin]

the website went down.

 

Yes, I was upset - thought if it's any ban. But now I see everything is working fine.

I am very happy everything is in safe :-)

 

Оk, I think I will try to take such a photo. I do not have BG39, but I have BG38.

I want to see if it is even possible to detect a leak from the first spectrum. However, I have never tried this before.

[Cadmium]

BG38 leaks a lot of IR.

BG39 and S8612 have the same IR cutoff/suppression at the same thickness.

S8612 2ill transmit more UV than BG39.

Chinese equivalents are not always the same as what they are said to be equivalent to. Only real BG39 is real BG39, but other glass is sometimes called something other than what it really is.

[Cadmium]

Any filter below 850nm (@50%) will transmit some 700nm range. That is why I use 850nm (RG850) when I want pure monochrome IR,

because the Bayer filters even out at about 850nm, and the footprint of the 850nm filter doesn't linger in the 700nm range.

830nm works pretty good also, but it crosses the 800nm line into the 700nm range at OD3, so I can still see slightly more coloration with 830nm than with 850nm when comparing to desaturated versions.

Of course these slight color casts are almost undetectable, unless you desaturate.

However, below 850nm the Bayer filters will 'see' slightly more red. Whether the red is visual red or just Bayer red is up to you to decide, either way it looks red to the camera.

The Bayer red filters transmit more IR in the 700nm range than the Bayer Blue and Green filters do.

 

The yellow/orange/red color seen on the RG/IR longpass filter grating tests are from below the 50% transmission point, thus in the 700nm range and below (depending on the filter).

RG830, for example, doesn't start at 830nm, it starts closer to 750nm, with 10% at 800nm.

At least that is how I am understanding it, I could be wrong.