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What is the real response of Panasonic Lumix G3? False colors vs. wavelengths. Spectroscopy.


sascha

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EDITOR'S NOTE: UPDATE 19 MAY 2022.  My full spectrum converted Panasonic S1R produces false colours in white balanced, reflected UV photographs which are slightly different from what we have seen so far with full spectrum conversions used with broadband UV-pass filters. There seems to be more dark cyan-green or green. We are thinking that this indicates a slightly different Bayer dye somewhere in the Bayer filter.

Anyway, I think I therefore need to add to the discussion the fact that false-color-to-wavelength charts are only good for the specific camera from which they are made in case that has not been obvious. Or, for those of you who think that false-color-to-wavelength charts are not good for general use, now you have another reason why. They may vary by camera.

 

EDITOR'S NOTE: 2015.01.13 We have chosen to carefully edit some small parts of this very interesting thread in accordance with our stated UVP Rules & Guidelines. The edited posts are marked.

During the time when the first part of this thread was locked, we continued the discussion in another thread. The two threads have now been merged.

 

 


 

 

 

Hello everyone!

 

My name is Sascha from the south east of Germany. I have been a "UV enthusiast" since four years now. My main interest lays in the area of dental research and to find useful applications for reflected UV imaging and autofluorescence.

 

During my research it was necessary to specify the actual response of a full spectrum converted Panasonic Lumix G3 (and Nikon D70). I have noticed similar charts on the excellent and extensive blog of Dr Klaus Schmitt here:

 

http://m8.i.pbase.co..._UVcolors_c.jpg

 

However, Dr Schmitt rarely comments on his equipment combination and I assume his standardized chart is meant to be more of a generalized reflected UV palate. Initial research results indicated that this chart was unable to project the false color effect accurately. My aim was to determine the UV sensitivity of a full spectrum converted Panasonic Lumix G3 and also to specify the false color response from the sensor.

 

I had received some great advice from Dr Enrico Savazzi on the aspect of white balancing and the general set up of the camera.

 

Materials and Method:

1. Light source: Stellarnet SL-3 Deuterium light source.

2. H10 UV Monochromator (Jobin Yvon) to scan through the different wavelengths (slit size 25 mµ)

3. Ocean Optics USB 2000 UV/VIS spectrometer to confirm the wavelength accuracy.

4. Panasonic Lumix G3 with full spectrum conversion.

 

Results:

This particular camera showed a UV response from 400 nm to 266 nm downwards. It was necessary to increase the exposure time from 4 sec to 60 sec while the ISO was kept consistently at 160, from 310 nm downwards. An increase in ISO did not seem to make any difference in the response at wavelengths lower than 310 nm. Merely the noise increased. As it so happens these colors are not 100% correct. They contain a small amount of IR harmonies which is difficult to cancel out. However, they represent a fairly accurate picture about the UV sensitivity of a converted G3 and its false color representation. Note, my chart differs quiet noticeably from that of Dr Schmitt. The influence of a UV transmissive lens is not considered of course. This is likely to have further effects: either weaker signals at wavelengths below 300 nm or a change of the false color interpretation or a combination of both.

 

Regards

 

Sascha

post-55-0-93820200-1420996033.jpg

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Hello, Sascha. Welcome to UVP!!

 

You have asked a rather complicated question raised a rather complicated topic. I will try to start answering addressing it with a few general remarks. And we will get more detailed as need be. Also other members here will have some input besides mine.

 

To begin...these remarks will be a bit general to you because I realize you are using a lab setup. But I still want to cover the basics. So kindly forgive me if I list items which are known to you, but perhaps this might also be useful to others.

  • Experimentation and sensor construction both indicate that the practical limits of UV photography with DSLRs or mirrorless cameras from which all internal filters have been removed does not go much beyond 300nm.
    Added: This is a consesus of opinion across various websites and UV photographers.
    Somebody please measure these sensors scientifically and 'publish' the results.

  • UV sensor response, in general, is also dependent upon:
    • Lens transmission
    • Filter transmission
    • Illumination, either ambient or direct
    • Illumination bandwidth
    • Bayer filter dyes
    • Microlenses, if present
    • Sensor pack covering (usually fused and unremovable)

  • Measuring UV response of a camera sensor or UV transmission of a lens
    requires the following (from our member Shane Elen):
    • spectrometer or spectrophotometer (preferably in a dual beam configuration)
    • monochromator
    • stable output UV-V-IR source
    • integrating sphere
      The integrating sphere helps ensure that the readings are independent of the incoming light ray angle. This will provide you with wavelength specific transmission response.
    • There is lots more about measuring UV response (from John Dowdy and Shane Elen) in what follows.
      See Post #21 for a summary (with ongoing edit/updates).

  • False colour is not a reliable indicator of sensor UV response for various reasons.

False colour is highly dependent upon digital white balance.

When shooting UV white balance can be set with diffuse reflective standards.

But digital white balance is not an absolute thing because:

* Each camera may have a different type of internal white balance algorithm or white balance sensor.

* Each editor may have a different way to set the white balance from photographs made with the reflective standards.

* Each editor may use a different colour profile for your camera.

 

False colour is also affected by

* choices of saturation and brightness both in-camera and in the editor.

* choice of ICC input and output profiles

* equipment colour calibration

* demosaicing algorithm used.

 

Mixed wavelengths may produce the same false colour as a single wavelength.

This is a relevant concern in the field, less so in the lab where monochromator is being used.

Another way to say this is that UV wavelength to false colour is not an invertible relationship.

 

So how do we attempt to standardize our "white-balanced false colour"??

These next two methods only partially solve the problem:

* Diffuse reflective standards for white balance

* Camera profiles made from known colour charts

 

Even when using white balance standards and profiles, the choice of lens, filter, illumination, and camera (dyes, microlenses, sensor cover) will affect the false colour outcome of the standardization attempt.

Even in the lab using equipment described above
,
the Bayer dyes
may
have UV-induced fluorescence which can skew the colour results. Possible fluorescence could be any one or more of the UV, Visible or Infrared wavelengths.

 

So my point with all this is that you should probably not attempt any colour charts as an indicator of UV wavelength.

  • There are too many uncontrolled variables.
  • There is no way to determine any kind of absolute white balance.
  • The relationship between UV wavelength and false colour is not 1-1.

***********

 

I need now to look at this from the point of view of using the measuring equipment you listed and see how to modify these remarks if need be. I cannot comment on your lab equipment as I have no experience there. But perhaps other knowledgeable folks will check what is being used and give their opinion.

 

***********

 

And now I will also turn this over to other members and also await any questions you may have.

 

***********

LAST UPDATE: 2015.01.13 3:35 PM GMT-5. Added additional false colour dependencies.

LAST UPDATE: 2015.01.13 3:04 PM GMT-5. Added remark about 'consensus of opinion'. Added links to subsequent posts.

LAST UPDATE: 2015.01.11 3:04 PM GMT-5. Added remark about Bayer dyes possible fluorescence and modified 300nm statement.

LAST UPDATE: 2015.01.11 2:14 PM GMT-5. Gotta stop updating this now. :D

LAST UPDATE: 2015.01.11 2:06 PM GMT-5

LAST UPDATE: 2015.01.11 2:04 PM GMT-5

LAST UPDATE: 2015.01.11 2:02 PM GMT-5

LAST UPDATE: 2015.01.11 1:55 PM GMT-5

LAST UPDATE: 2015.01.11 1:52 PM GMT-5

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It seems as brightness and saturation levels in some of your pictures is much higher than in the chart taken from Klaus's web site (did you get his permission to do so?), making comparison difficult.

 

By changing the exposure from 4 seconds to 60 seconds you increased it by 4 stops. It is highly possible that you are recording something else at those levels and not UV.

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Hi Andrea,

Very interesting in deed! Would you be able to quote this citation? I would love to find out more about this very interesting topic. I had a lot of fun doing this experiment, and other similar ones. If I understand you correctly, you are saying that just because the image sensor is able to depict something using a specific (false) color doesn't mean it is actually recording anything, even if the specific wavelength has been filtered using a dielectric mirror?

Regards

Sascha

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A good question to ponder:

  • Although a standardized false colour chart for specific UV wavelengths cannot be obtained across differing platforms, is it reasonable to think that for a given, fixed set of {camera, lens, filter, illumination} such a chart could be made and used?

 

Anyone like to try answering this?

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A good question to ponder:

 

Although a standardized false colour chart for specific UV wavelengths cannot be obtained across differing platforms,

is it reasonable to think that for a given, fixed set of {camera, lens, filter, illumination} such a chart could be made and used?

 

Anyone like to try answring this?

 

Isn't it what Klaus has done already for Panasonic cameras?

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I agree with you. This would be a matter of finding consensus regarding a standardized white balance procedure. I have used a standardized grey reference card (whiBal) to carry out a custom white balance using a UV-Nikkor and sun light. I took these measurements some six month ago and have failed to mention that I used an ocean optics integration sphere between the monochromator and the camera itself. This appears to be in accordance with the list provided by Shane Elen.
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Alex writes: By changing the exposure from 4 seconds to 60 seconds you increased it by 4 stops. It is highly possible that you are recording something else at those levels and not UV.

 

Are lab measurements using monochromator and spectrometer made in the dark? If so, then longer exposure should not record anything but the specific wavelength being produced by the deuterium source.

 

 

***********

 

Sascha writes: Would you be able to quote this citation?

I am not sure which citation you are referring to??

 

***********

 

Sascha writes: If I understand you correctly, you are saying that just because the image sensor is able to depict something using a specific (false) color doesn't mean it is actually recording anything, even if the specific wavelength has been filtered using a dielectric mirror?

 

No. I'm not sure how you got that?

I'm trying to point out that you cannot determine from the false colour which wavelength was used to produce it because false colour is not an absolute thing.

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Alex writes: By changing the exposure from 4 seconds to 60 seconds you increased it by 4 stops. It is highly possible that you are recording something else at those levels and not UV.

 

Are lab measurements using monochromator and spectrometer made in the dark? If so, then longer exposure should not record anything but the specific wavelength being produced by the deuterium source.

 

What about fluorescence of bayer dyes and microlenses?

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Alex writes: Isn't it what Klaus has done already for Panasonic cameras?

 

It is a nice idea, but there are too many points at issue.

 

And the problem of mixed signals producing the same false colour as a single signal cannot be solved.

So there never will be what we call a 1-1 (invertible) relationship between wavelength and false colour.

 

The furtherest I can go is this: such a false colour chart may indicate wavelength under very controlled circumstances such as when used with deuterium source, monochromator and spectrometer. But once you get out into the field making actual photographs, all bets are off because of illumination and possible mixed signals and so forth. So therefore it is always better to directly measure the UV reflectivity of the subject rather than attempt to determine its UV response via false colour charts.

 

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

 

ALex writes: What about fluorescence of bayer dyes and microlenses?

I do have Bayer dyes and microlenses listed, but that is a good point that even in the lab those Bayer dyes might be a very tricky business. I will add that to the initial topic for future reference. Thx.

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Sasha writes: I have used a standardized grey reference card (whiBal) to carry out a custom white balance using a UV-Nikkor and sun light.

 

I need more information.

Was this a custom white balance for Visible light? Did you put a UV/IR blocking filter on the lens?

 

Please note that WhiBal cards are useful only for custom white balance in Visible light.

 

WhiBal cards do not reflect "white" in a diffuse manner under UV illumination. When used to set custom white balance for UV work they will skew the false colours.

 

For customized white balance in UV you need specialized Spectralon or other diffuse reflective standards which reflect uniformly under all of near-UV, Visible and near-IR wavelengths. Sometimes teflon (PTFE) is used, but it is not diffuse and can cause misreads due to specular highlights.

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[OWNER/ADMIN NOTE: Because the pages of UVP are not a place for disputes, but rather for scientific interchange and debate and also for informal chat by UV photography enthusiasts, we have chosen to remove two sentences from this post in order for the interested parties to work out their disagreement in private.]

 

Ok guys, I agree with what has been stated so far. I was under the impression that this is a blog for UV-enthusiasts rather than a scientific peer review panel.

 

The purpose of the experiment was to obtain information regarding the ultra violet response of a converted Panasonic Lumix G3 camera which was white balanced in sun light using a reference grey card and using the scientific equipment that was available to me for quantification to the best of my knowledge. It is obviously correct that the interplay between the type of illuminant and its specific spectral power distribution, the ultra violet absorption and reflection properties of the specimen, the type of filter, lens and camera used, along with a million other factors will influence the false color effect in a considerable manner. The multitude of these cofactors and the absence of standardization make such results incomparable. Every study has its limitations and so does mine.

 

Considering the circumstances, it would be in deed impossible to carry out adequate comparison between my results (chart) and that of Dr Schmitt simply because the exact circumstances of how his chart was obtained are unknown. Dr Schmitt who is undoubtedly a major authority in his field rarely publishes a complete list of his equipment and method that he uses to obtain his results. My study was not an attempt to prove Dr Schmit wrong, which would be impossible to do, simply because the precise procedure of his measurement techniques are unknown.

 

On a final note and before leaving this blog, I would like to state that I do believe that the sensor of a Panasonic Lumix G3 is capable of faintly detecting ultraviolet radiation below 300 nm by itself and without the use of a lens, even if this view defies convention and if it presents a clash with expert opinion.

 

With kind regards

Sascha

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As a general photographic note, I remind everyone that for a specific camera choice,

the custom white balance operation depends on these four variables:

  • lens
  • filter
  • illumination
  • reflective standard

Therefore, whenever any one (or more) of those variables changes, you must perform a new custom white balance.

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We are UV enthusiasts, but many of us have Ph.D. degrees or similar, and we try to consider the technical aspects of UV photography seriously.

 

Everything that was published in this thread should be considered as Critique, and not as Criticism! Which are two very different things.

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[OWNER/ADMIN NOTE: Because the pages of UVP are not a place for disputes, but rather for scientific interchange and debate and also for informal chat by UV

photography enthusiasts, we have chosen to remove six short paragraphs from this post in order for the interested parties to work out their disagreement in private.]

 

We are a UV enthusiast website. We simply happen to have a great deal of accumulated knowledge and knowledgeable members. We also have lots of members who are learning and experimenting. We are all happy to share anything we know with anyone who drops by. We do not "judge" anyone by their level of knowledge.

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Shane, Bjørn, John, Alex, Enrico and others -- please review what I've written and let me know how I can improve it factually to help others.

 

 

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

 

Sascha writes: I do believe that the sensor of a Panasonic Lumix G3 is capable of faintly detecting ultraviolet radiation below 300 nm by itself and without the use of a lens, even if this view defies convention and if it presents a clash with expert opinion.

 

There really is no "expert opinion" because there are so very few actual measurements of sensors. And for any of us to accept such a measurement, we need to see also the methodology and equipment used.

 

There is a general consensus of opinion based on the properties of glass and the construction of sensors that the region of 300nm is a reasonable boundary. So your lab work has added to that consensus. You got something below 300nm in the lab and that is interesting.

 

Added Later: However, having to increase exposure time to 60 seconds from 4-6 seconds starting at 300nm indicates that the Lumix sensor response below 300nm is weak. This seems like it might cause difficulties in actual field work? So the fact that the practical limit for UV photography is not much beyond 300nm seems to be a useful generalization. Experimentation will eventually shed more light (UV of course) on this rule of thumb.

 

In the field the areas between 300 - 330/340nm may be difficult to record. I've had trouble with my cameras (Nikon, Lumix, Pentax) photographing below 340nm in actual field conditions.

 

Make some experiments if you have a lens and filter for the 300mm region and post them here!! We like to see any and all such experiments.

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Summary:

 

(1) The response of the Lumix G3 was demonstrated by Sascha under lab conditions.

It appears to have some weak response below 300nm in the lab test. Seems reasonable, although I'm not sure it would be useful in field work. It would be nice if we could determine whether or not possible Bayer dye fluorescence has interfered with that response in the UV response in the Lumix G3.

  • Any comments or questions about this?

(2) The various variables involved in determining white balance were discussed.

I have remaining questions about custom white balance.

  • When performing lab measurements of UV sensor response using deuterium illumination, monochromator, integrating sphere and spectrometer, how would we go about choosing or setting a white balance in order to produce a colour chart of the camera's UV response in such a way that the colour chart outcome is reproducible ?

  • How would you account for possible colour alterations when using filters and lenses in the field which, as I understand it, are not used in the lab measurement which produces the chart ?

  • Has anyone ever actually performed any mixed-signal experiments to verify that the invertibility of the false colour to UV wavelength is not 1-1 ? (Does one use a duo-chromator for this? :D A little joke there.)

(3) The pitfalls of correlating UV response to false colour were discussed.

  • In spite of the pitfalls, do you think that such a correlation for a specific fixed set of {sensor, lens, filter, illumination} may be somewhat useful in field conditions even though it is not a rigorous correlation?

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Wow, where to begin? I will start with what I know best. I have ranted about this before so some of you already know where I am heading next.

 

JY H10 UV is a scanning single grating monochromator with probably only ~10^-4 or 10^-5 rejection of stray light. So what that means is that outside of the bandpass, determined by the lines/mm of the grating and the width of the two slits, there is an underlying spectrum which is essentially that of input source. This underlying stray radiation extends in both short and long wavelength direction. This means that if the target material or detector (camera sensor) has for example sensitivity a few orders of magnitude higher to long wavelengths, present in this stray radiation, the response to the out of band radiation will dominate.

 

Most UV photographers are familiar with a manifestation of this phenomenon as the far red or NIR "leaks" in the secondary transmission lobe of Schott UG and Hoya U type filters which contaminates a UV photo with overwhelming NIR. A scanning double grating monochromator can provide ~10^-8 stray light rejection and with accessory filter wheel to block high order grating harmonics. This of course comes at the price of total throughput and adding an integrating sphere lowers throughput to a small percentage of the single grating monochromator. This is why such instruments typically use very very sensitive detectors.

 

Speaking of detectors, the USB 2000 is fixed (non-scanning) single grating spectrometer with stray radiation rejection of at best ~10^3. It is probably OK to use this simple spectrometer to confirm the wavelength setting of an untrusted manual dial on a legacy monochromator but it would not be reliable for quantifying the energy in that band, especially in the UV. This is not speculation, I have a USB2000 in my lab and these comments are based on experience.

 

I must point out that neither the scanning single grating monochtomator or the fixed single grating spectrometer are acceptable instruments according to the ISO standard for lens transmission in the visible. Quality spectral measurements in the UV are significantly more demanding requiring much higher level of instrumentation. Again I am not speculating here, I have 4 single grating and 6 double grating monochromators at my lab.

 

There is no ISO (or other) standard for testing lens transmittance or camera sensor response in the near UV so no one may be faulted for making the best use of equipment at hand or what is affordably obtained.

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Thanks, John. I am learning more about laboratory measurements and how to perform such an experiment.

 

I'll write a small summary here to check whether I've understood what you wrote.

  • Scanning single-grating Monochromators do not reject enough stray source light to ensure that the camera records the desired wavelength.

  • Scanning double-grating filtered Monochromators do reject enough stray source light (and also reject unwanted grating effects) to ensure that the desired wavelength is recorded.

  • Fixed single-grating Spectrometers can measure the location (wavelength) of a peak with reasonable accuracy even with incomplete stray light rejection and poor dynamic range.
    Only under very controlled conditions are these type of instruments usable for determining the total spectral power distribution (aka spectral irradiance) of a UV source.
    EDIT: The following statement was incorrect, so it has been crossed out and replaced with the preceding statement: Non-scanning single-grating Spectrometers do not reject enough stray source light to accurately measure a UV wavelength.

  • International Organization for Standards (ISO) requirements for Visible lens transmission specify:
    • scanning double-grating filtered Monochromators, and
    • double-grating Spectrometers.
    • ISO 8478:1996(E) There are other provisions.

  • Producing and measuring near UV light is technically more demanding and also requires scanning double-grating instruments as well as a double-grating Spectroradiometer to measure spectral irradiance of the pass band of each filter and UV radiation source combination.

  • Fixed grating array Spectrometers are generally accepted for field and survey work.
    Example: Ocean Optics USB2000

 

(ISO reference again please? Is there a ref somewhere in previous posts? I will look.)

 

Key words: scanning, double-grating, filtered.

 

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

 

Whew. I think I understood. :)

 

UPDATE: 2011.01.13 Added bullet point about field work spectrometer. Added ISO reference. Corrected a statement.

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  • Scanning single-grating Monochromators do not reject enough stray source light to ensure that the camera records the desired wavelength.

  • Scanning double-grating filtered Monochromators do reject enough stray source light (and also reject unwanted grating effects) to ensure that the desired wavelength is recorded.

Yes & Yes

  • Non-scanning single-grating Spectrometers do not reject enough stray source light to accurately measure a UV wavelength.

Not exactly. The fixed single grating spectrometers can measure the location (wavelength) of a peak with reasonable accuracy even with terrible stray light rejection and poor dynamic range. Only under very controlled conditions are these type of instruments usable for determining the total spectral power distribution (aka spectral irradiance) of a UV source.

  • International Organization for Standards (ISO) requirements for Visible lens transmission specify:
    • scanning double-grating filtered Monochromators, and
    • double-grating Spectrometers.

ISO 8478:1996(E) specifies a double grating monochromator with filters - OR - alternatively a series of narrow bandpass filters providing the series of filters have high order out of band blocking. Although not explicitly stated it may be assumed the out of band OD of the filters must be at least as good as provided by the double monochromator. The maximum bandpass using either approach is 10nm. There are lots of other provisions dealing with collimators, beam diameter relative to aperture, integrating sphere and how the whole assembly is calibrated and used.

  • Producing and measuring near UV light is technically more demanding and also requires scanning double-grating instruments.

Yeah, it is not as simple as one might assume.

Producing really clean high order out of band rejection requires a double grating with order sorting filters.

One can approximate using filters with very high out of band OD.

But in practice you would still need some way to measure precisely how much radiation is being delivered.

This requires a calibrated radiometer to measure the irradiance or a better still double grating spectroradiometer to simply measure spectral irradiance of the pass band of each filter and UV radiation source combination.

 

The filter method is in my opinion sort of a holdover from the past. While one would need to employ a double grating instrument to properly characterize and calibrate a filter based system there may be some advantage with respect to total throughput.

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Please can I be allowed to make a digression from the specific thread topic. Please divert it to a new thread Andrea if it is too far away.

 

I have noticed in a couple of papers the use of the Ocean Optics 2000 spectrometer to measure the UV-Vis spectrum of flower petals and leaves. On the basis of your experience with this type of instrument John would the results be meaningful. I thought it would be nice to have a system to measure the spectrum of some of my "UV black" flower petals etc. I've not investigated the cost of the spectrometer so that might rule it out at the first fence anyway.

 

Dave

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The OO USB2000 and other similar fixed grating array spectrometers are generally accepted for field and survey work. We used ours as a survey tool, better than a meter to sniff out UV sources that need proper measurement in areas where UV needs to be abated. I also know people use these type instruments as simple spectroreflectometers with bifurcated fibers and a small portable light source in the field. In the UV these will suffer all the same problems in reflectance mode as for anything else. It is just not a bench grade instrument, that said, a $50K 50lb instrument is a bit much to go mobile even though we do make onsite measurements. If you really want to set one up for UV reflectance try to find a UV only grating and also try using a UV pass filter which blocks Vis/NIR on the input fiber. That is sort of like filtering an excitation source for UVIVF, it restricts the wavelength range but cleans it up some.
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