Multispectral imaging of silver leaf (visible and UV)

[JMC]

Based on a discussion here I was really intrigued by the graph shown for silver with the strong dip in reflectance around 320nm. I ordered a sheet of silver leaf from ebay (edible silver leaf and it was about 2GBP delivered) and decided to test it.

 

Silver leaf was mounted on cardboard using double sided tape. Images done with my Monochrome converted Nikon d850m and Rayfact 105mm lens. f11 and ISO400 apart from the 254nm image (which was f11 and ISO6400). Light source was a Hamamatsu LC8 200W xenon lamp, but for the 254nm image I used a 4W UVP filtered lamp. I included a 99% Spectralon diffuse reflectance standard in each image and exposure time was adjusted to keep that as constant as possible for all the images. Images cropped and resized, but no further modifications.

 

Visible - LC8 light and room light, Baader UV/IR cut filter

390nm to 340nm - Thorlabs 10nm bandpass filter, Hamamatsu LC8 Xenon lamp

330nm to 310nm - Edmund Optics 10nm bandpass filter, Hamamatsu LC8 Xenon lamp

300nm - Edmund optics 10nm bandpass filter and Hoya U-340 4mm as the EO filter leaks light above about 600nm, Hamamatsu LC8 Xenon lamp

254nm - Sirchie 253.7nm filter, UVP 4W 254nm lamp

 

Here's the images.

 

The reflectance of the silver leaf does indeed drop sharply at around 320nm. I recorded the RAW files too, and extracted the channel responses for the Spectralon and the silver leaf. Ratioing the silver leaf against the spectralon gave an interesting graph which closely matched the behaviour shared in the original thread (down to 300nm at least).

 

 

Simple experiment, but took a while to setup. One of the images (the 360nm one) is a bit blurry, which I didn't noticed until I was processing the images. Also some of the dichroic filters show some evidence of light bouncing around between the layers (see the highlights on the 340nm one especially). Am I 100% confident with the 254nm image being 254nm and not leaks? Yes, pretty much as I've tested this filter before, but I suppose being ultra-picky that is the image and data I am least confident with as it doesn't match the published graph.

 

[colinbm]

Thanks for showing these Jonathan

[nfoto]

Perhaps the 254nm data point isn't perfect -- however, the genersl similarity of your curve to others clearly indicates you got the general shape correct.

[dabateman]

Looks good Jonathan. 

Thats what Stefano wanted to see.

So you need a 320bp10 to really see that Silver.

The best I have is a 310bp10 in 1/2 inch. Might be worth testing again.

But easier to wait for it to oxide for identification that UV photography I think.

 

[diant]

Very impressive (320nm especially) and well done!

The same thing we see with gold, only shifted to visual - no need in any UV stuff, all is obvious by naked eye.

[Stefano]

Nice indeed. This is what I would have expected, now nicely shown in an image.

[Andy Perrin]

This is a fantastic experiment! I’ve been hearing about this ever since we discussed the photoelectric effect in my high school physics classes. 

[JMC]

Cheers All.

 

The more I look at my 254nm image, the more I think that it isn't quite right. I leave it up for now, but with the caveat that I'm more confident with the 300nm and above images. I need to spend more time at 254nm trying out different settings. I believe that Spectralon reflectance starts to drop around 250nm, and I know from imaging at 254nm before that organic material is highly absorbing at that wavelength. As a result the 254nm image of Spectralon may be darker than it should be as my sample certainly could do with a clean. If this is darker than expected then it would make the silver seem more reflective than it is. However, as I say, more work needed on that.

[diant]

On 10/18/2021 at 8:16 AM, JMC said:

254nm - Sirchie 253.7nm filter, UVP 4W 254nm lamp

Jonathan, I think there is a simple way to verify this combination on possible longer-wave leakage:

- to shoot Spectralon through Sirchie 253.7nm filter under UVP 4W 254nm lamp

- to shoot Spectralon through Sirchie 253.7nm + usual borosilicate glass filters under same UVP 4W 254nm lamp

After that the second photo will show a possible leakage and difference between them will show a real share of UV at the region ~<300nm.

[JMC]

14 minutes ago, diant said:

Jonathan, I think there is a simple way to verify this combination on possible longer-wave leakage:

- to shut Spectralon through Sirchie 253.7nm filter under UVP 4W 254nm lamp

- to shut Spectralon through Sirchie 253.7nm + usual borosilicate glass filters under same UVP 4W 254nm lamp

After that the second photo will show a possible leakage and difference between them will show a real share of UV at the region ~<300nm.

Yeah I've done some testing of this filter before looking for blocking of out of band regions, and it is very very good, but the camera sensitivity is very very low, so of course that is a factor. See here for some of the early work on 254nm imaging.

[diant]

O, yes, good test. One thing, truly speaking, seemed to me strange: some leakage of WG305 and WG295 filters in 250-280nm region. But after some reasoning I agreed with such their property as a usual one for any borosilicate glasses.

 

I do not think that "camera sensitivity is very very low" is a real playing factor in a similar experiments - of course if one will shoot both images at the EQUAL camera and lens settings. The latter condition guarantee us that the difference between these two images will contain exact number of UVC-photoelectrons (however low camera sensitivity would be).

 

If a condition of "EQUAL camera and lens settings" will prove to be unrealizable - it is always possible to allow some scaling (exposure, aperture etc) and take it into accounts before subtraction.

 

BTW. Apart from it, a factor of low camera sensitivity in UVC or UVB or even UVA I overcame some years ago with a simple handmade wooden camera with intermediate fluorescent screen and two lenses - before and after screen. If a first lens is replaced with UV-dedicated lens or a suitable quarts lens with slow enough aperture - all work good in any UV rays.

 

Here is my camera without its upper cover. It rescued me more than once in such difficult conditions and I am not regret the time I spent on its building.

 

screen. If a first lens is

 

 

[colinbm]

Where do you get fluorescent screen from please ?

[diant]

Colinbm, I made (or obtained) them in a different ways. Some years ago I was deeply occupied with it.

 

As I remember, one way - UVA ready paints (I've bought them here). But for deeper UV it was possible to adopt another way suggested to me by one our Russian film engineers: to dissolve a dry luminescent powder with awater-base matrix and pour it on a glass plate with a high proportion of lum/matrix. In such way, after the matrix dries, you will have most or lum particles exposed in fact to air (with may be only thin film of matrix on them, optically not enough thick to block any real share of UV rays). It is the way used by him for creating a special UV-ready film emulsion.

 

For example, if you synthesize the usual gelatin film emulsion, you should keep the gelatin share to around 8% - or twice as much as the AgBr share.

 

You have a ratio of AgBr/matrix ~ 1/2.

 

I later diminished the gelatin share to 4% (1/1) and even to 3,2% (<1) without sedimentation microcrystals with fast plate cooling. If you diminish the gelatin share more, the  microcrystals will settle and aggregate. (And the photographic emulsion will be spoiled of course.)

 

But with luminescent powders there is not such danger - they may aggregate to infinity, and the more - the better! The main thing is not to allow a particles of lum powder to aggregate before pouring :) This will be a main restrict factor in diminishing of lum/matrix proportion.

 

You should only take care that they (lum powders) should be atmosphere- and moisture-stable and pour them on glass screen with any suitable matrix (gelatin, collodion, water-based UV-transparent lacquer etc) diluted with enough quantity of water.

 

I often used long-duration lum powders (my tasks demanded such conditions) and kept it in darkness for some time before exposure in my camera.

 

I did such screens around some 8-10 years ago and did not remember of course all details of my recipies. I remember only that I finally refused Chinese powders (as unstable and especially water-unstable ones) and instead got them from a reliable Russian maker with guaranteed quality.

 

Excuse me for my English please – it is not so easy for me to write on it.

[colinbm]

Thanks very much.
I gather I don't need quartz glass if the emulsion is on the UVC side ?
I would need glass on the visible side ?
AgBr fluoresces in UVC light ?
Cheers

Col 

[diant]

10 hours ago, colinbm said:

I gather I don't need quartz glass if the emulsion is on the UVC side ?

Of course.

 

10 hours ago, colinbm said:

I would need glass on the visible side ?

Yes.

 

10 hours ago, colinbm said:

AgBr fluoresces in UVC light ?

No. I speak about it as an example of technique of preparation.

 

AgBr(I) microcrystals (the usual compound of a photographic emulsion) are very sensitive to all UV rays in that they absorb or diffract UV rays, but they don't fluoresce.

The main problem with photographic emulsions and UV (especially UVC/B) - is the gelatin matrix. If you use a very clear "inert" gelatin and prepare the emulsion with a low AgBr/gelatin ratio, then you can get a very UV-sensitive emulsion. In some cases (air-exposed microcrystals) it will be very sensitive to UVC, but it demands special steps to prevent it from quickly degrading. I never made such emulsions myself and know about them only from my teacher.

 

The only thing I succeeded - the preparation high pBr emulsions (pBr around 7, when all microcrystals have a Ag-rich surface) with the AgBr/gelatin ratio ~ 1 (not so low for real UV-ready films) which are almost totally devoid of a Schwarzschild effect, and so, though the emulsions themselves may not be so strongly UVC/B-sensitive, they may work well in far UV at the expense of long exposures.

 

[colinbm]

Thanks diant
I would like to have a UVC fluorescent screen one day.
 

[Andy Perrin]

Diant, that setup is almost the same as what I originally used for SWIR imaging (with an upconverting anti-Stokes screen) before I got my TriWave:

 

[diant]

Andy, yes, such unsophisticated idea of course should have a broad circulation. I think there are many realizations of double lens cameras with intermediate screen (more or less similar) exist or existed in the world. But could you tell me some words about your upconverting screen?

[Andy Perrin]

Diant, the upconverting screen came as part of the relay tube apparatus. It is still sold by Edmund at enormous prices. It uses the anti-Stokes effect: two SWIR photons come in and one NIR photon at half the wavelength comes out. So 2x1500nm -> 750nm or so. It is not linear (closer to gamma of 0.5). It is also very inefficient. I had to use 600W halogens to illuminate. I was very glad to switch to my TriWave which can use normal lights. (The TriWave is a germanium sensor.)

[diant]

Andy, thank you!

I understand as concern the screen, and at the same time read a little about TriWave sensor - interesting thing. Germanium should generate a lot of dark noise electrons, but as I learned, TriWave sensor is cooled up to -70 -90C degrees (!!! unthinkable for a usual astronomical cooling cameras :) and so displays a moderate dark noise at 1/30s exposure.

Meanwhile I am dreaming about the time when there are come to exist inexpensive UV-Vis cameras... dreaming, dreaming...