Foveon Sensor Transmission Charts

[Andrea B.]

It seems as though as always sharing spectral sensitivity data has resulted in some passionate discussion.

 

You wouldn't expect anything less from devoted UV photographers (and the many PhDs) on this forum.

:D :D :D

[JMC]

My remote release adapter arrived today. I'd like to repeat this at a lower ISO when I get the time. Problem is at lower ISO it's going to take me 2-3 hours to take all the shots because of the longer exposure time needed. Maybe I'll just try it for the UV region given the visible data had better signal to noise.

 

Well that was pointless - bulb can only be used in ISO100 and 200, and even with a remote release the shutter closes after 30s. What a rubbish design. Wish I'd known that before I wasted an hour of life off my light source. A case of RTFM I feel......

[Andrea B.]

Most remotes have a lock setting for long exposures in bulb mode. The one you were using does not? If not, then you have to hold it in place manually for exposures over 30 seconds. It gets boring. :D

[JMC]

Most remotes have a lock setting for long exposures in bulb mode. The one you were using does not? If not, then you have to hold it in place manually for exposures over 30 seconds. It gets boring. :D

 

The problem, Andrea, is the camera not the remote release. This is from the SD14 manual;

 

CAUTION !!

• It is not possible to use Bulb photography for more than 30 seconds. The shutter will automatically close about 30 seconds after the shutter button was pressed.

[OlDoinyo]

The SD14s I used to have would keep their shutters open up to 2 minutes before cutting off, if a remote release was in use. The manual did not admit this--I discovered it on my own. Still, it could be quite limiting in dim light, especially with such poor high-gain performance.

[ulf]

If not, then you have to hold it in place manually for exposures over 30 seconds. It gets boring. :D

 

Andrea, you get bored already after 30 seconds, waiting for an interesting result? :D :D :D

That was quick!

[dabateman]

Jonathan,

Check the firmware on your SD14. If its 1.07 or 1.08, then the maximum bulb mode is 2 minutes. The last firmware version was 1.08.

 

Go here to update your firmware:

http://www.sigma-sd.com/SD14/software/firmware/

 

Update, just read the page on this. You need to activate extended mode. This is one of the hidden fuction setting. That may also be the issue for you.

But your right, at ISO 100, I think you would need about 10 minutes, to get adequate exposure. So 2 minutes may still be too restricting.

[JMC]

Yeah it's a firmware thing - the one on my camera is 1.05. Not the end of the world.

 

Right, it's been a busy 24 hour. I couldn't figure out what that weird peak in the sensitivity curve for the SD14 at 420nm was. As I wanted to repeat the experiment anyway, I also wondered if for some reason there was an IR contamination problem with my measurement (given the peak was biggest in the red channel). I decided to then rerun the same experiment, but also adding in a Hoya R72 IR filter to the front of the lens. The though here being to block the UV and visible and see whether there was IR coming in through the monochromator and integrating sphere assembly.

 

First then the repeat of my sensitivity experiment between 280nm and 480nm, with no R72 fiter. This is just with the 85mm UAT lens (at f4.5), camera set to ISO1600 and 30s exposure, image settings RAW and Low Res.

 

At least I saw the peak at 420nm again, so it was not a one off.

 

Now then, the repeat of the experiment but with the Hoya R72 filter on the lens (to filter out UV and visible).

 

Oh dear. I am definitely getting some IR coming through my setup, even when testing in the UV. Not good. But it's not the same at the different wavelength settings, so the amount of IR contamination in the image varies as I change the wavelength on the monochromator.Oh joy...... The really scary thing is how much the signal was in relation to the supposed UV signal in the first graph above.

 

To try and correct for this, I then subtracted the graph with the R72 filter present, from the one with no filter present. The aim being to remove the effect of the IR. This is what that it looked like;

 

The good thing is subtracting the IR data from the original scan does seem to be a simple way to correct for the problem. This now gives more like what I was expecting to see - no UV signal from the Red or Green channels - I couldn't figure out why I was seeing red and green signal in the UV with the Sigma, and it looks like the apparent signal was driven by the IR contamination.

 

I didn't see any obvious IR peak in my monochromator scans when I was originally setting up my system, but my spectrometer only goes up to 850nm. Could it be above this, perhaps? And why would it change so drastically as a function of wavelength with that huge peak at 420nm?

 

While I love working things out, I now need to go back and check whether this is also a problem from the work on my EOS cameras. I didn't see anything obvious which made me thing a correction was needed, but still have to check. Oh joy again.....

[dabateman]

Great work Jonathan,

As I say the SD14 is an IR camera. We just use filters to make it work.

If there is IR the SD14 will find it for you. This even includes when using the Baader venus filter.

[JMC]

Thanks Dabateman, but I am annoyed about the IR thing, and not spotting it sooner. I'll go back and change the info I have for the Baader U post above now.

[Andrea B.]

This is why scientific review exists.

****

 

That was so weird about the Bulb on the SD14. I'm happy to learn it was fixed in a firmware update.

*****

 

 

Andrea, you get bored already after 30 seconds, waiting for an interesting result?

That was quick!

 

I have such a short attention span!!!

No, it's just that sometimes UV requires many tries to produce a good image. So if one is on Bulb, it gets tedious. I have a programmable remote which greatly helps with any long exposure situation whether on Bulb or under 30". Highly recommended.

****

 

Lots of good info being presented on Sigma digicams. I have it on my list to add something to the Sticky.

 

Don't forget to Tag posts. That helps people find info.

[JCDowdy]

Jonathan, I am sorry you feel "annoyed about the IR thing, and not spotting it sooner" but you shouldn't. I think you should instead be pleased at having learned something.

 

What you have demonstrated is an example of how stray radiant radiation in low grade single grating or fixed grating monochromators can significantly confound results.

 

I have commented on this for many times on this forum as have others, most recently Pedro, and I am sorry to see my efforts to advise have failed.

[JMC]

Jonathan, I am sorry you feel "annoyed about the IR thing, and not spotting it sooner" but you shouldn't. I think you should instead be pleased at having learned something.

 

What you have demonstrated is an example of how stray radiant radiation in low grade single grating or fixed grating monochromators can significantly confound results.

 

I have commented on this for many times on this forum as have others, most recently Pedro, and I am sorry to see my efforts to advise have failed.

 

Don't get me wrong John. While it would have been good to spot it sooner, it didn't give a large effect with my Canon cameras to be obvious. It was only because of having the opportunity to play with the SD14 that it gave a strange result and made me wonder about it.

 

You may feel as though the advice has failed. I don't think it has at all, and without it I wouldn't have thought to look for IR contamination. All equipment has its benefits and drawbacks. I used my little second hand monochromator as it was a cheap eBay buy, and it was small and compact. Given my limited space where I work that is a big consideration when I think about equipment. I will continue to work with it, but will adapt how I do my experiments to try and minimise the negative aspects of its design.

 

I'm currently reworking my work on my monochrome Eos camera with a number of different filters, to try and understand the scale of the impact it is having on my initial data. Thankfully the effects on the monochrome Eos seems to be much reduced compared to the Sigma SD14, I presume because of the monochrome converted cameras higher sensitivity to UV. I shall of course write up and share what I find.

[JCDowdy]

I am relieved you have not taken my comment as an "argumentum ad hominem."

 

As I have stated a few times here before, I am a true advocate of making the best of the tools one has to hand!

 

If you do not already have a copy of ASTM E387, Standard Test Method for Estimating Stray Radiant Power Ratio of Dispersive Spectrophotometers by the Opaque Filter Method, you may wish to seek a copy. You may have other filters suitable for estimation of stray radiation from shorter wavelengths. A GG400 or GG420 type filter would be good for checking for contamination in your UV-A1 bands. Also WG360, WG335 and WG320 are suitable for looking a stray radiation in the shorter UV-A2 and UV-B. I doubt you have enough camera response below 320nm to resolve much of anything stray or real but a WG305 can show how much spillage there is in a fixed grating CCD array type spectrometer.

[JMC]

I am relieved you have not taken my comment as an "argumentum ad hominem."

 

As I have stated a few times here before, I am a true advocate of making the best of the tools one has to hand!

 

If you do not already have a copy of ASTM E387, Standard Test Method for Estimating Stray Radiant Power Ratio of Dispersive Spectrophotometers by the Opaque Filter Method, you may wish to seek a copy. You may have other filters suitable for estimation of stray radiation from shorter wavelengths. A GG400 or GG420 type filter would be good for checking for contamination in your UV-A1 bands. Also WG360, WG335 and WG320 are suitable for looking a stray radiation in the shorter UV-A2 and UV-B. I doubt you have enough camera response below 320nm to resolve much of anything stray or real but a WG305 can show how much spillage there is in a fixed grating CCD array type spectrometer.

John, I've spent many years trying to teach science to marketing. I take few things personally after that. I have some GG455, GG495, OG550, OG570, an R72 and Baader UVIR cut which I've been using to look at the problem so far. I do not have anything lower, although from what I can see online the prices for them aren't too bad, so it is something I can look at getting in.

 

I'll have a think about the ASTM document. I do begrudge spending $50 on something like that though. I'll have a re-think when I've had more coffee and am feeling less grumpy (I am so not a morning person)....

[Andrea B.]

John, I've spent many years trying to teach science to marketing. I take few things personally after that.

 

Well that got me to laughing for sure!

 

But thank you to both John and Jonathan for their gentlemanly interactions on the topic of spectrometric stray light leaks and amplification. And as an utter novice to spectrometric practices, I do hope I got the phrasing correct there.

 

***************************

 

.....spending $50...

 

In England can you deduct something like this from your taxes as a "business expense"??

[JCDowdy]

Jonathan, you marketing comment is so true, been there done that! I was in a meeting at Schering-Plough (worked there 1982-1994) and we were explaining about the different azole anti fungal agents used in our products. The lady from marketing referred to them as members of the Zole family! We laughed at that for a week.... guess you had to be there.

 

In the US the ASTM standards are sometimes in the collections of public and university libraries. If you have an academic affiliation which affords you library privileges perhaps you can score a photocopy.

 

The costs of standards is most certainly a business expense for our lab.

[JMC]

Andrea, John, yes, sure I can expense buying that as a business expense. But the fact is that I begrudge doing so. For some strange reason I see this as being different as to say, buying a filter. Perhaps because I'd end up with a PDF of something rather than a physical object. Unfortunately no academic affiliation at the moment. I was a visiting scientist at a local University but they have a time limit on those - can only do it for a couple of years and then you get booted out. Not quite managed to get myself a visiting professorship anywhere yet. I shall be biting the bullet and getting a copy of the article.....

[Andrea B.]

Print out the PDF and put it into a nice binder. Eh voilà -- a physical object on the bookshelf !! OK, tongue a bit in cheek.

 

But srsly, Jonathan, don't begrudge learning expenses. If it improves your knowledge, it is not at all wasted money.

 

Here in the US I could probably go to my local public library and request a copy through inter-library loan. The SigOth is now very retired and no longer affiliated with a Uni, so I can no longer use his various privleges for this kind of thing. (He is also missing those privleges.)

 

What surprises me these days, though, is just how much scientific material is available freely online. I have read myself silly about bee vision, for example. (Maybe I shouldn't have, she said laughingly.)