Sparticle and Diffraction Grating Comparison

[Cadmium]

The Sparticle bandpass filter array is expensive to make because of the cost of the little 12.5mm bandpass filters.

Here is a comparison of my Sparticle with an inexpensive 1000 line linear diffraction grating found on eBay.

This is just an initial test outdoors in sunlight.

For these tests, the Sparticle was pointed at the sun, then lowered to not have the sun directly behind the filters, the diffraction grating was pointed at the sky, about 30 degrees to one side of the sun.

I used a D7000 full spectrum camera, Kuribayashi 35mm lens, with a Baader U on the lens.

These tests were shot 1 minute apart.

My diffraction grating is cut and mounted in a 52mm filter frame/ring and stacked on the front of the Baader U for the diffraction grating shot.

This test was not my idea, a friend recommended this test to me.

I have not tried this with other lenses yet to see if it shows the same kind of reduced transmission results for less UV capable lenses like the Sparticle will show.

The advantage of the Sparticle is that it still has a defined bandwidth for each filter, and does not rely on color, whereas the diffraction grating is color dependent, but this can be used.

 

[Andy Perrin]

Hm, that is nifty. Also, you can see some of the absorption bands in the sunshine! Kewwwlll!

[colinbm]

Good work Steve

[Cadmium]

Thanks, I take no credit for this, a friend showed me how to do it.

It is an interesting comparison, and I never thought these inexpensive gratings would work this way in UV.

I find the bandpass filters more useful, because I can tell how deeply a lens transmits with the Sparticle in live view, with no white balance, simply by seeing which filters 'light up'.

The grating needs to be white balanced first I would think.

[Andrea B.]

The diffraction grating thing has possibilities, though, as an easy way to get a quick idea of lens capability. Thanks for passing this along to UVP readers.

[Cadmium]

Someone asked if it makes a difference if the grating is in front or behind the U-filter, which I have not tried yet. This test above was in front.

[enricosavazzi]

Someone asked if it makes a difference if the grating is in front or behind the U-filter, which I have not tried yet. This test above was in front.

It may make a difference if the diffraction grating causes a significant angular spread of the incoming beam (we can assume the incoming beam consists of parallel rays if it comes from the Sun, because the angular diameter of the Sun is 0.5 degrees). With the filter between grating and camera, the left and right extremes of the "streak" pass more obliquely through the filter, which means they cross a higher thickness of glass. Dielectric coatings also behave differently with normal versus oblique rays.

[colinbm]

Yes that is a good consideration Enrico.

I have only ever used the filters in front of the gratings, but I have also had a slit in front of the grating.

Another way to capture a narrow line of sunlight or other light is to reflect off a chromed needle. This will give a narrow vertical line of reflected light.

The other thing is to focus at infinity, you should be able to get better absorption & emission lines this way.

Col

[Cadmium]

Thanks Enrico and Colin.

I want to point out to anyone who might be seeing these that these are not intended to provide good information about the filters or lenses used here, this is simply an experimental test, so don't base lens transmission or filters on these grating tests I am playing with here. I trust my Sparticle array much more than these experiments, and you can see quite a difference in the example below between the E lens grating tests and the E lens Sparticle test.

Here are a few more tests, setting up the same as before. Focus is infinity for all above and below.

I experimented with the difference between mounting the grating in front and behind the filter, and I didn't see much difference, and I have included some of those comparisons here.

I have used three lenses, two of which are not the best for UV but will transmit some UV. Personally I rate the Kuri 35mm #1, E 35mm #2, and Kuri 50mm #3 (of these three lenses used here) in real life, but these tests may not show reality.

 

Nikon 35mm E lens Sparticle with Baader U.

[Cadmium]

PS: Someone wanted to know what diffraction grating I am experimenting with here:

http://www.ebay.com/...r+roll&_sacat=0

 

They also sent me this link:

http://www.onlinesciencemall.com/diffraction-grating-sheet-1-000-lines-mm-linear-1-foot-x-6-inches-wide.html

 

I am using the sheet/roll version, 1000 line, linear, which I cut a circle from and mounted in a filter frame/ring.

The linear version is found in 500 line and 1000 line versions, and the double axis version is found in 13,500 lines.

Both linear and double axis are found in 'slide' size also.

The sheet/roll seemed the best for me because I can cut any size I want from it.

[Guest]

What is the Kuribayashi 50mm lens?

 

I can't find it mentioned anywhere else on this site.

 

Cheers

[Guest]

PS: Someone wanted to know what diffraction grating I am experimenting with here:

http://www.ebay.com/...r+roll&_sacat=0

 

They also sent me this link:

http://www.onlinesci...nches-wide.html

 

I am using the sheet/roll version, 1000 line, linear, which I cut a circle from and mounted in a filter frame/ring.

The linear version is found in 500 line and 1000 line versions, and the double axis version is found in 13,500 lines.

Both linear and double axis are found in 'slide' size also.

The sheet/roll seemed the best for me because I can cut any size I want from it.

 

Looking at the search results on eBay for 1000 lines/mm there are two types; the Holographic spectrum and at a slightly higher cost the Laser spectrum.

 

Which one should I purchase?

[colinbm]

Looking at the search results on eBay for 1000 lines/mm there are two types; the Holographic spectrum and at a slightly higher cost the Laser spectrum.

 

Which one should I purchase?

 

Holographic spectrum

 

What light source do you plan to use ?

[ulf]

These sparticle tests are great for comparing lenses, but the actual numbers are not that accurate.

First the filter are often production seconds that likely has deviations in both peak wavelength and peak width.

The shape of the actual transmitting peak is also unknown.

The width number is an indication of where a filter has half of it's transmission to the max at the peak.

It is quite possible that the filter has a substantial transmission away from the peak.

 

When combining a filter transmission with a transmission that is sloping, like the combination of decreasing sensor sensitivity and lens transmission the actual combined transmission of the test BP-filter gets shifted toward the longer wavelengths

 

The only really accurate way to determine a lens transmission characteristics is with a proper spectrometer setup.

[Guest]

Holographic spectrum

 

What light source do you plan to use ?

 

I have several uncoated monolight strobes.

[Guest]

These sparticle tests are great for comparing lenses, but the actual numbers are not that accurate.

First the filter are often production seconds that likely has deviations in both peak wavelength and peak width.

The shape of the actual transmitting peak is also unknown.

The width number is an indication of where a filter has half of it's transmission to the max at the peak.

It is quite possible that the filter has a substantial transmission away from the peak.

 

Ok, in general I would compare against a known lens and then with all other variables the same I could at least make some relative judgement.

 

If you have source to a to better quality but also reasonably priced grating I would consider that product.

[colinbm]

With a diffraction grating you need a light with a lot of emission lines like a mercury vapour lamp.

A BLB would be a good one for starters as it is only starts at UVA & higher.

There is a peak emission at 365nm & some in the 400s. These will help you to know what you are looking at.

[ulf]

An alternative to searching at the short end of the spectra for good UVA-capable lenses might be to look at the transmission at around 365nm only, instead and compare it to a reasonably good known lens.

This will not work for qualifying lenses for work closer to 300nm, with special filters and light-sources but will give a good idea for more common general UV-A photography. - Is the lens marginal or not? -

 

I would do such a comparison by taking pictures with the good reference lens and the lens to be tested, both at the same aperture setting.

In both cases some kind of UV-pass filter or filter stack should be used. (Baader U, U-360 + S8612 2+2mm, ZWB1 + BG39 2+2mm... anything goes as long as it do not leak wildly)

 

A good reference lens could be one of those, but others work too as long as there general transmission is good and well established:

https://www.ultravio...n-kogaku-japan/

https://www.ultravio...-metal-version/

A real Kuri 35/3.5 would likely be OK but not all "clones" as there are too much variation and some of them are not that good.

 

 

I would use a matted virgin PTFE-pate as a test target.

The target should be illuminated at the center by an UV-torch,

The torch should have freshly charged batteries and be placed at exactly the same distance for both exposures. IMPORTANT!

The images should be in RAW-format and the histograms could be analysed in the free FastRawViwer.

(or some other RAW-processing software that is good at showing the images histogram.)

 

I would first take several images with the reference lens with different exposure time and analyse the result to find really good exposure level.

Then I would change to the lens to be tested and take a test image with the same exposure time.

By comparing the exposure difference in the two histograms it is possible to know the transmission difference at 365nm.

 

The decay in transmission at shorter wavelengths is not identical for all lenses, but if the transmission is good at 365nm it is likely acceptable at 340nm.

 

For UV-photography in natural sunlight, the spectral range from 340nm and upwards is most important.

[Guest]

Thanks for the support, and I shall make something constructive of this.

 

I do have the Petri KC 35mm f3.5 you speak of.