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Confirming the wavelength of my 940nm flashlight + follow up to my previous post

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#1 Fandyus

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Posted 17 June 2021 - 21:38

Might be a bit of a boring post, I wanted to confirm that my 940nm eBay flashlight is in fact, 940nm, so I just laid two glasses on a pile of books, one filled with water and one empty, I put on my 950nm longpass GREEN.L filter and I illuminated the scene with said flashlight.
Attached Image: IMG_9470.jpg
The water filled glass appears significantly darker, almost tea-like, so I assume that confirms I got what I wanted.
Today I ordered another two flashlights.
https://www.ebay.com...ar=533326117053
https://www.ebay.com/itm/392804813218
One of them is just plain red, so it's probably 630nm-ish. Hopefully that won't be too much of an issue, I suspect it could bring extra color and saturation to any trichromes I make with this.
I also ordered this.
https://www.ebay.com/itm/133238203848
Just a single bandpass filter, I'll test it out once it arrives, and if it's good (doesn't decrease sharpness, doesn't leak too much of other wavelengths), I will buy a few other ones. As I said, I'll probably putty mount them on my Industar 50-2 right up to the front element.

#2 Andy Perrin

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Posted 17 June 2021 - 21:46

Yeah, the 980nm is right near the first water absorption peak so that should be fun. You may need to stack it with a long pass on the front depending how good the out of band blocking is in visible. There wouldn’t be much incentive to block well for a laser filter since most of the light will be at the design wavelength.

Edited by Andy Perrin, 17 June 2021 - 21:48.


#3 Fandyus

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Posted 17 June 2021 - 21:50

I figured, hopefully the filter isn't faulty somehow. They actually sell many wavelength versions of those bandpass filters and they're not that expensive. Would be fun to own all of them.

#4 Stefano

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Posted 17 June 2021 - 22:10

Dark water is a good sign. At 940 nm that's how water looks like.

A 970-980 nm LED should be the best for dark water with a silicon camera. The ~1200 nn peak is not accessible.

#5 Fandyus

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Posted 17 June 2021 - 22:11

View PostStefano, on 17 June 2021 - 22:10, said:

Dark water is a good sign. At 940 nm that's how water looks like.

A 970-980 nm LED should be the best for dark water with a silicon camera. The ~1200 nn peak is not accessible.
True, though I'm not sure how available those are. I'll just pray the bandpass works well and then I can just use incandescent lightsources.

#6 Andy Perrin

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Posted 17 June 2021 - 22:17

Incandescent should be excellent and worst case you stack with a long pass.

#7 Fandyus

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Posted 17 June 2021 - 22:25

View PostAndy Perrin, on 17 June 2021 - 22:17, said:

Incandescent should be excellent and worst case you stack with a long pass.
True, though that would get very annoying if I ever wanted to switch filters. Again, what I'm more afraid of are irregularities in the glass that cause spots of low sharpness as the light becomes refracted in a way it shouldn't. The bandpass is made for lasers so I'm not sure how does the expected quality for that compare to expected quality for photographic filters.

#8 Andy Perrin

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Posted 17 June 2021 - 22:26

Yeah, but at that price definitely worth a try. I would rear-mount that filter, though. Most dichroics perform better on the rear. And yes, you can't switch filters easily, but that's how it is sometimes.

Edited by Andy Perrin, 17 June 2021 - 22:27.


#9 Fandyus

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Posted 17 June 2021 - 22:26

Actually oops, I didn't notice you already addressed the laser grade glass question. I have no idea how did your reply go over my head. My apologies.

#10 Fandyus

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Posted 17 June 2021 - 22:28

View PostAndy Perrin, on 17 June 2021 - 22:26, said:

Yeah, but at that price definitely worth a try. I would rear-mount that filter, though.
I won't be rear mounting it, I'll be mounting it right on the front element, as the diameter is 15mm which is about the same as the front element size on the Industar 50-2.
Edit: I have a DSLR so I wouldn't risk putting it on the back.

Edited by Fandyus, 17 June 2021 - 22:30.


#11 Andy Perrin

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Posted 17 June 2021 - 22:29

I know, and I'm telling you that it's probably a poor choice on your part. I guess we'll see though! Sometimes they work on the front, but it makes all the problems with dichroics worse.

#12 Fandyus

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Posted 17 June 2021 - 22:31

I see, sorry for my confusion. I hope that the fact that my format is apsc and the lens isn't very wide angle will accommodate for the issues that could arise with a dichroic filter. I don't have any experience with them though so I don't know what to expect.

#13 Andy Perrin

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Posted 17 June 2021 - 22:35

For such a narrow bandpass, it won’t matter as much with regard to color shift, but you will make any issues with light hitting at an angle worse.

Ah, I see what you mean about the DSLR. Maybe your next camera will be mirrorless!

Edited by Andy Perrin, 17 June 2021 - 22:37.


#14 Fandyus

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Posted 17 June 2021 - 22:39

View PostAndy Perrin, on 17 June 2021 - 22:26, said:

Yeah, but at that price definitely worth a try. I would rear-mount that filter, though. Most dichroics perform better on the rear. And yes, you can't switch filters easily, but that's how it is sometimes.
Also I'm sorry if I came off as rude, I read your comment as "I WOULDN'T rear mount it", not that you would. So my answer was according to that.

#15 Andy Perrin

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Posted 17 June 2021 - 22:42

I see, no you didn't seem rude so much as missing what I was saying (which I guess you did!). Oh well.