Transmittance of Polyester Film & Teflon Film

[colinbm]

I have been wondering why diffraction grating don't show much in the UV, & they seem to have difficulty to show down to 365nm or lower ?

I am wanting to build another specrtophotometer with a web cam with the hot mirror removed, something like the RSpec Explorer but with a better response in UVA.

The RSpec at the moment cuts off at 380nm which is not much good for our purposes.

It would appear that the diffraction gratings are made of polyester film ?

I found this transmittance of Polyester & Teflon film of interest.

 

 

So what are the limiting factors in getting a webcam & diffraction grating to record below 350nm or even below 334nm so you can see those mercury emission lines ?

 

 

Cheers

Col

[dabateman]

Col,

This has many complicated variables to get to lower UV. The grating is only one part.

 

Most likely the web camera will not see below 380nm. I was disappointed that my Axis 212 barely saw any UV at all. You would have to test your camera.

 

Ulf ran the Israeli 1000 lines/mm film you can get cheap off ebay here:

https://www.ultravioletphotography.com/content/index.php/topic/2643-first-attempt-with-linear-filter-grating/page__view__findpost__p__19967

 

Then you have to get good fiber optics that would transmit the lower UV.

 

The cheapest option I was looking at was a Science surplus unit $200, and then getting a good fiber and better grading from on online seller for around $100.

 

But mostly my Em1 with 1000lines/mm film on pinhole pro lens works well enough. I can see to about 320nm, the endge of my 313bp25 filter and I know its lower than my 335pb10 filter.

[Cadmium]

An easy test would be to use a Sparticle, or just a 320BP10 and 340BP10 bandpass filters, shot first with a good UV capable lens,

then shot again with some of the chosen refraction grating material stacked with the bandpass filters, and note the difference. Easy to do if you have a few such bandpass filters around.

[colinbm]

Thanks David

That is good that you get a reading down to 320nm I'll have to try harder ;-)

Col

[colinbm]

Thanks Steve

I'll have to dig around in some boxes....:-)

Col

[dabateman]

Col,

Your camera may just be limited to 350nm/340nm.

Jonathan has found that many Sony and Canon sensors have coverglass that restricts the lower UV. This is with optical low pass filter directly on the sensor coverglass or AR coating used to improve our visible quantum efficiency.

So depending on the camera, you may not see into UVb.

[colinbm]

Thanks David

Yes I am aware of the cover glass pain too....;-)

Col

[ulf]

An easy test would be to use a Sparticle, or just a 320BP10 and 340BP10 bandpass filters, shot first with a good UV capable lens,

then shot again with some of the chosen refraction grating material stacked with the bandpass filters, and note the difference. Easy to do if you have a few such bandpass filters around.

Remember that a Sparticle, or its bandpass filters have wavelength tolerance variations both for center wavelengths and width.

Quality filters have ±2nm for both parameters. https://www.thorlabs...001&pn=FB340-10

Filters from eBay-sources can be rejects with bigger errors.

 

The width number, 10 in a filter like a 340BP10 bandpass filter, is the FWHM value where the transmission is 50% of the peak value. The filter transmits outside that 10nm gap.

The transmission of these filters is more or less bell-shaped and when that transmission is combined with the decaying sensitivity of the sensor the total sensitivity is shifted towards the more sensitive longer wavelengths.

 

Without knowing their characteristics of the filters the precision is not that good for determining a cameras real range with these methods.

[Andy Perrin]

By the way, can I recommend Thorlabs filters to everyone? They are fantastic and come in little tiny wavelength steps. They are half the price of Edmund for the corresponding product. They have full documentation including OD graphs across the whole spectrum. Sometimes you can get them even cheaper on eBay (including from Omega Bob). Also everything from Thorlabs fits together with everything else like LEGO.

[colinbm]

Thanks Ulf & Andy

I have a Sparticle. but Omega cheapies, at the moment.

Col

[Cadmium]

Ulf, it is not a spectrometer. Col has a 4 filter Sparticle he made, and he can easily do the test with what he has, and if using the same exposure time should indicate some difference.

Col, I would try it if I were you.

Col has a spectrometer also, so use that if you have a way to do that.

Don't be so fast to underestimate the usefulness of those little bandpass filters.

I have tested a lot of lenses with them and they work great.

[ulf]

Steve, I know you love your Sparticle method and like the results.

 

I do not underestimate the usefulness of the device and metod.

 

I am convinced that it is very effective for comparative tests, especially as it involves the full chain from light-source to camera.

It definitely help to judge if there is any possibility for photos at shorter wavelengths, with different lenses and cameras.

 

You have shown several times that it gives usable results and I do not dismiss the method at all.

 

I just want as many as possible to understand its limitations and weaknesses and how the method really works.

Then the interpretation of the results will be as correct as possible.

I am not sure that that deep understanding always is present.

[dabateman]

I would echo what Ulf is saying. The Sparticle method is good. But not as accurate as using the 1000 line/mm film method or next level up would be a calibrated spectrometer.

For the Sparticle method to be truly accurate you would need to spend $1000 on true known filters with exact wavelengths. The difference between the cheap omega filters and real expensive filters is night and day. There are visible leaks in the cheap ones, and lots of IR bleed. Thus why I always need to stack them with either a Baader venus U filter or my slightly less trusted 330WB80 improved filter.

 

1000 line/mm is better but limited to about 320nm. And you need to calibrate it with a light and lock it down. Moving anything changes your calibration.

[colinbm]

Thanks Guys

Steve, my Science Surplus Spectroscope isn't accurate now & I don't have the maths to calibrate, it wasn't good at lower then 365nm UV & I don't have the program & it would need to be upgraded to Win 10.

It is packed away & so is all my camera gear.

If I knew where to get it calibrated & to work with a program I would consider that, but I don't.

I have the RSpec Explorer & program & it is working but only down to 380nm. I don't know what is limiting it, perhaps the Hot Mirror or the program ?

So I want to make a webcam that goes down to at least 365nm but 350nm would be better & below 334nm then I can see the Hg emission line there & know what it is capable of doing.

I have some 500 lines/mm & 1000 lines/mm grating.

The webcam should arrive any day.

Cheers

Col

[dabateman]

Col,

I am surprised you didn't get your Science Surplus below 365nm. The linear sensor should be good in that unit to lower wavelengths. You just need a better, more expensive grating and fibers that support the lower wavelengths. That I would think to be your best bet. You will need to tilt the grating to the sensor.

To calibrate it you can download the more recent SpectrumStudio from their website. I think it runs on Windows 10. Otherwise VisualSpec will work. You will need a mercury bulb like the ExoTerra UVB.

[colinbm]

Thanks David

I'll look at all that thanks, plenty to do...

Col