Jump to content
UltravioletPhotography

Flower Colours through the Lens...


Andrea B.

Recommended Posts

These authors want to recover the linear values from a raw file in order to analyze spectral reflectance.

I think Raw Digger might have made their life easier.

 

Flower Colours through the Lens: Quantitative Measurement with Visible and Ultraviolet Digital Photography

by Jair E. Garcia, Andrew D. Greentree, Mani Shrestha, Alan Dorin, Adrian G. Dyer

  • Published: May 14, 2014
  • DOI: 10.1371/journal.pone.0096646

From the Principal Findings:

Consumer-level digital cameras potentially provide access to both colour and spatial information, but they are constrained by their non-linear response. We present a robust methodology for recovering linear values from two different camera models: one sensitive to ultraviolet (UV) radiation and another to visible wavelengths.

 

From the Conclusion

However, and in spite of its advantages, the accuracy of the spectral information recovered from camera responses is subject to variations in the uncertainty levels, with larger uncertainties associated with low radiance levels.

 

 

Reference to spectral sensitivity curves of Fuji S3 UVIR.

I do not have full access to this paper.

Garcia JE, Wilksch P, Spring G, Philp P, Dyer AG (2014) Characterisation of digital cameras for reflected ultraviolet photography; implications for qualitative image analysis during forensic examination. J Forensic Sci 59: 117–122.

A methodology is presented for processing ultraviolet images based on linear responses and the sensitivity of the respective color channels. The methodology is applied to a FujiS3 UVIR camera, and a modified Nikon D70s camera, to reconstruct their respective spectral sensitivity curves between 320 and 400 nm. This method results in images with low noise and high contrast, suitable for qualitative and/or quantitative analysis. The application of this methodology is demonstrated in the recording of latent fingerprints.

Link to comment

Comment from the side line: A pity they used the Fuji S3 as it is well known* to be severely limited in its UV response. It is severely down already with a 350 nm filter.

 

* perhaps not to scientists only to photographers?

Link to comment
But we don't really know that because I cannot access that paper unless I pay for it.
Link to comment
Andrea, I do have access to it, but I obviously can't repost it publicly. Please tell me if you would like me to send it to an email.
Link to comment

Andy, what we would like to know from that paper is simply how deep into the UV the Fuji S3 UVIR camera can record?

 

When that camera was actively sold, the Fuji specs said only down to 375-380 nm, if I recall correctly.

This did not seem at all useful to most of us.

 

My error. It was the old newer* Fuji IS camera with the 380nm lower limit as Bjørn points out later.

 

 

* Bjørn corrects an error. Bed time, Andrea? :D

Link to comment

Andrea, it says on the relevant figure,

(a) Camera responses to varying amounts of energy (OECF) and Bezier fitting line for color channels sensitive to ultraviolet radiation for a modified Nikon D70s and a Fuji S3 UVIR digital camera. (b) Recovered linear camera responses from data shown in a. (c) Spectral sensitivity curves for the three col- our channels of a modified Nikon D70s digital camera equipped with a Baader U filter. (d) Spectral sensitivity curves for the three colour channels of a Fuji S3 UVIR camera equipped with a Baader U filter. Insert depicts the spectral transmittance curve of the Baader U filter incorporated in the two cameras as measured with a spectrophotometer.

 

And the figure (d) is,

post-94-0-21757000-1452126981.jpg

Link to comment

The Fuji S3 UVIR Limited edition was specified to respond to 350 nm. Says so in the camera's manual. Actual testing using a UV-Nikkor confirms this. Response drops very sharply at 350 nm and by 340 it is pretty useless (tested using 10 nm bandpass filters). These days, I rarely use my Fuji for anything else than falsecolour IR. My various UV-capable Nikons are far more responsive to UV than the Fuji.

 

There was a newer special model called IS (derived from a D200-lookalike, Fuji S5) for which Fuji claimed 380 nm as the lower limit. Not tested by me.

Link to comment

Oh foo !! I was indeed remembering the Fuji IS as being the 375 - 380 nm responder. I will go back and edit my remark. Thanks, Bjørn.

 

And you have tested with narrowband filters to confirm the difficulties at 340nm.

 

((I have to sign off now. I'm tired and making errors. Long day.))

Link to comment

Have we ever seen the RGB curves for a conventional DSLR like the D70s? I think it would be interesting to see curve 'c' if that does in fact show the response in the 320 - 400 nm region. The only curves I can remember finished about 380 or so I think. How was the D70s 'modified'?

 

I'm probably up the wrong tree as usual :rolleyes:

Dave

Link to comment

Why is this graph of figure "(c) Spectral sensitivity curves for the three colour channels of a modified Nikon D70s digital camera equipped with a Baader U filter", not show the colour peaks that we see in the UV Sparticle tests, ie green around 340nm, yellow around 360nm & violet/blue >380nm ?

Col

Link to comment

The Sparticle board photos are white-balanced, I think?

Although it is not clear that fully explains it?

 

Like how can there be no recording in the green channel? When I look at a UV photo in Raw Digger there is always info in the Green Channel.

 

Went and looked at Shane's analysis of the D70 (no S). Lots of red there too, and little green. So must be that's how the D70/D70S sensors rolled? So I go back and properly read Shane's D70 experiment and it clearly states the use of a UV-pass filter.

Link to comment
The 995 didn't shoot raw did it? I can't remember. You can't be sure what chennel response you have when just looking at a jpeg to which white balance of some type has been applied.
Link to comment
Hm, it is a good question. It has a maximum quality mode that spits out an uncompressed TIFF but it's hard to know if any white balance is applied. Does white balancing move intensity between channels, or does it just scale each channel by some multiplicative factor? If it's the latter, then the white balance won't matter because anything times zero is zero.
Link to comment

Jpgs are notoriously difficult to colour balance later as a lot of colour and data shifting has been applied to them and the process usually is not entirely reversible. These old Coolpix models did not support RAW output nor did they provide you with 16-bit TIFs.

 

W/b is destructive in the way channels are scaled up and down and you might get clipping or scaling into the noise floor(s) at some point, thus intensity changes might be introduced within the work flow. Only by starting from a RAW file (and ditching any earlier edits) can you be certain to preserve all the data.

Link to comment

As I understand the figures, they do not show the sensitivity of the sensor at all, but the sensitivity of the system Camera plus Baader U filter (?plus some kind of lens). That is the reason for the fall off at 400nm (there about should be the highest sensitivity of the sensor) and -as the Baader U "goes down" towards 300nm (and what about the lens?)- we can not say something about the sensor down there as well.

 

Am i right?

Link to comment

Andy, white balance is always applied to a Jpeg or a Tiff.

 

You can view a raw photo (with the help of apps like Raw Digger or others) without a white balance setting in which case the two Green channels predominate and the photo has a strong green cast.

 

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

 

Werner, that is the way I'm reading it. But I don't think that's what we typically expect to see for sensor response charts. We want the response unfiltered.

Link to comment

There are a couple of their other papers in open access journals.

 

I also have the J Forensic Sci, 2013 paper.

 

The lens is described as "A Micro Nikkor 105 mm quartz lens (Nikon) was fitted on both cameras to ensure free UV-A transmission." I assume they mean a UV-Nikkor but there is no photo of the lens to confirm that assumption.

 

The camera responses are with a "Badder U-filter" but there is no indication of which version.

 

The images were recorded as "native raw format for each camera and subsequently converted into the sRGB IEC61966-2.1 color space and encoded into an 8-bit scale employing Adobe Camera Raw plug-in (v 6.2) for Photoshop version CS5"

Link to comment

Andrea, I see. So the TIF, despite being the "high quality, uncompressed" setting, is not the same as RAW since it's been modified by white balance, etc. Oh well.

 

nfoto, I'm not sure why you say it does not provide TIFs though? For example,

post-94-0-20928800-1452194423.jpg

Link to comment

You missed the adjective "16-bit". The 995 TIFs were not 16-bit. They were 8-bit.

 

I know, it all gets a little crazy. TIFs are not raw files. Yet Nikon's raw file, the NEF, *is* a kind of TIF in basic structure. :D :lol: :rolleyes:

Link to comment

To my understanding, most if not all "RAW" files are within the specification container of TIF (structurally). TIFs can have various bit depth, layers etc. The main differential feature is that a RAW has no notion of colour and colour space as these are reconstructed later.

 

The Coolpix TIF is a file without the data massaging (and compression gains) of a jpg, but in practice won't offer much of an advantage in terms of later processing. It has also been subjected to in-camera white-balance.

Link to comment

Andy, thanks for the curve in #10. I think we all expected most of the signal to be in the red channel but we have never been able to track down response curves for the three channels either with or without the Baader-U in the UV region.

 

In response to John #19 I have found that CS5 / Adobe Camera Raw does not get anywhere near to being able to do a WB on my Pentax Baader-U DNG RAW files, there is always a very nasty red bias left. This probably doesn't mean anything in terms of the results in the paper.

 

Since I don't have Raw Digger here's the best I can do about the relative amounts of RGB, Nikon D750 UV-Nikkor lens

 

This is the visible flash image with a profile made in PN using the Colour Checker target, Baader UV-IR cut filter

post-28-0-57026000-1452220337.jpg

 

Spectralon target on left

 

This is the UV modified flash as used for my flower shots, Baader-U filter, RAW file opened in PN but not otherwise manipulated

post-28-0-82549400-1452220366.jpg

 

Tiff file gives R255, G4, B160 for Spectralon target

 

The same image after white balancing in PN on the Spectralon target

post-28-0-74643900-1452220382.jpg

 

Tiff file gives R 188, G 188, B 188 (+/- 1 or 2)

 

So there's minimal green in the RAW file as I have always thought

Link to comment

I am always happy to run a raw file through Raw Digger for anyone who would like to see the results.

Put a raw file in Dropbox and send me the link via PM or post the link in the thread if you don't mind others using the file too.

Link to comment

I tried all day to get my NEF file to upload to the Cloud, it seems that we must be connected to the Cloud by a piece of wet string.

 

So, I gave up and bought Raw Digger (25% off for the holiday)

 

post-28-0-95195000-1452398671.jpg

 

This was my first attempt with Raw Digger but on a selection of the Spectralon target it looks like I think you would expect. For some strange reason I didn't seem to be able to save the histogram as a jpeg despite doing what the manual asked, it would only let me save as png then convert it in CS5

Link to comment

Please sign in to comment

You will be able to leave a comment after signing in



Sign In Now
×
×
  • Create New...