This link has been posted somewhere else in the forums but it was ages ago. This guy has some good notes on how he built his light source for an 8mm projector rig:
(click the “lighting” link in the nav menu on the left)
I’ve just come across this, so thought I would add my experiences, which might be useful to someone.
I built my own 8mm telecine system about 10 years ago. It was moderately successful though could definitely be improved. I wrote some notes - http://www.nightshade-arts.co.uk/telecine/index.html.
The biggest improvement would be the light source. I used white light LEDs. Problem is, from the published spectrum, they aren’t really that white. The spectrum is still fairly narrow, which seriously reduces saturation. I would use a source with RGB LEDs in a redesign - important to get mixing even. Better still take three monochrome images - one with each LED colour source and combine at the processing stage. Easier on the camera.
I used a Sankyo Dualux 1000 as the base - because I had one. If I were to do it again I’d design from scratch and look at using a sprocketless drive using a stepper motor driven capstan and pinch roller optically detecting the sprocket holes, and separate low torque DC motors for take up and rewind. Much simpler, and would give much more scope for things like triple exposures as above.
Hey Nick!
The current Moviestuffs have the same problem with the light not being close to full spectrum. The YUJILEDS High CRI 95+ White LEDs perform very well though, you can also get the full-spectrum CRI 98 LEDs in strips and build a light out of those.
RGB is still better if you want to adjust the light for the film.
You shouldn’t need to do triple-exposure either any more (unless you mean sequential R/G/B) the Sony Pregius chips are excellent as they are.
I was using Lumileds. The spectrum had a double peak. From what I’ve read this may be the natural wavelength of the LED, plus some fluorescent material, giving the illusion of white. There was a big dip in the green region. I tried adding extra green LEDs but it didn’t really work. Greens were muddy.
I think I’d probably go with an array of R/G/B LEDs in future for maximal colour options, although getting even colour balance across the image may be awkward without so much diffusion there is no light left (wondered about one or more pieces of Cokin diffuser filters). What you describe is interesting, though.
– that is absolutely correct. Have a look at the spectra I posted here - you clearly can see the rather sharp peak of a blue LED plus the additional broad peak of the photoluminescent dye creating the impression of a “white” LED. Depending on the material parameters, nowadays you can get rather flat spectra afterall (see the spectrum 5000K posted by @PM490 just above my post). So nowadays, you do get white-light LEDs which have decent flat spectrum.
Actually, I think one topic is often overlooked when discussing LED illumination with respect to scanning color reversal film. First, no every white “LED” is created equal (see above). Than, even a perfectly flat spectrum will no correspondent to the spectrum of the projection lamp used to view the film in the old days - neither a halogen or xenon lamp does have a flat spectrum.
Now, the topic I would like to discuss is the following: film dyes were designed with a certain light source in mind - within that setting, a very good color balance throughout the full color space was obtained. However, using a light source with a different spectrum will introduce subtle color shifts within certain color ranges, depending on the spectral filter properties of the film’s dyes.
Well, from a signal-processing point of view, that should not really matter too much - you can adjust all of this in post production, provided you work with a sufficient bit depth along your signal path.
However, if you do not use a broad-spectrum LED setup, but a LED-source composed of single red, green and blue LEDs, you do end up with a spectrum composed of three rather narrow peaks. If these peaks are perfectly matched to the dyes of the film and the filter curves of your scanning camera, you will be rewarded with a great color definition (in terms of saturation mainly). But: what will work well for one film stock will not work that well for another one, at least in my experience. I am using for scanning a light source of different red, green and blue LEDs, mixed together in an integrating sphere. The Super-8 material I am scanning shows noticable variations in terms of color definition. While this is expected for different film stock (like Kodak or Agfa material), you notice even within a single film variations at that point in time when the material came from a different 15m-cassette. I think that effect is related to the difference of scanning a film at three well-defined spectral peaks (combination of red, green and blue LED) versus scanning the film with a broad spectrum (white LED, sampling over a larger spectral range for each color channel).
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Interesting. So you basically think I’d do better with a white light LED source providing I managed to find one with a flattish spectrum (not like the ones I had which look a lot like your graph), as compared to narrow band R/G/B sources? I guess the response of the filters on the camera image sensor must come into it as well.
It’s a lot easier with white light sources - no problems with casts across the image from poorly mixed R/B/G sources.
I see you said you used an integrating sphere to even out the light source. I wondered about that when I did it - something like some old enlargers used to have. Couldn’t think of an easy way of making one at the time so used opaque diffusers. Worked OK, but the diffusers do soak up most of the light output. I suppose these days one might be able to 3D print something.
There is an excellent spectrometer app called Colour Grab for Android devices. It can be calibrated using a white card in natural daylight. The result is as accurate as necessary in my opinion (1%).
Well, yes. Whether this is something worthwhile to consider is a different issue. What a scanner’s camera sees in its three color channels depends on how much energy the light source is supplying at a specific wavelength, multiplied by the camera’s filter response at the same wavelength. The actual output of each camera color channel is than this combined value integrated over the whole spectrum of wavelengths.
Now consider hypothetically a narrow dip in the curve of a dye of the film stock. Further assuming that by chance your narrow red LED just happen to sample in this dip, you will get wrong color values. With a broader spectrum (white light) the risk of such alignment is less, if not non-existent.
Well, that’s certainly a heavily constructed example, and a highly unlikely situation given the spectral curves in place. But averaging over a broader range of wavelengths will reduce the risk of such a thing happening.
On the other hand, with strategically chosen LED wavelengths, you should be able to increase color separation for a specific film stock.
When I started to design my light source, I actually tried to get hold of the camera’s sensor curves (which I achieved) and the spectra data of the LEDs used (same). What I did not achieve at that time was any good information about the spectra of normal film stock. Nevertheless, I created a little program to choose the “optimal” wavelengths for my red, green and blue LEDs. The result was ok, but not “sparkling”. I ended up “tweaking” my setup simply by trying out different LEDs. The color definition of highly saturated red tones was still not satisfactory - I solved this by exchanging the stock IR-filter of my camera (which is a Raspberry Pi HQ camera) with a more expensive block filter.
Given all the possibilities we nowadays have with respect to color grading, I actually do not think that all these consideration do have a big influence with respect to scan results.
Well, while this is a nice app, it is only locating colors in RGB-space. This will be of not much help in the context of the spectral curves we are discussing in this thread. What you really want to use is something which can measure the energy present at a specific wavelength. These units are called spectrometers; basically, these are units with a diffraction pattern which is projected onto a camera’s sensor surface. Here’s is a link to one based on a Raspberry Pi camera
and here’s another one based on a smartphone/webcamera
The little attachment connected to the smartphone contains actually the little diffraction pattern you need for spreading out the light spectrum onto the camera’s sensor. Here’s
as a 3D-printed version of this tube, and here’s
an example of the software which is needed to calculate a spectrum out of the camera image. There are example spectra on this website as well.
A few of points of information for context, from what I have seen.
- Color film density curves are typically based on 3200K.
- Sensor sensitivity curves are typically based on 3200K.
- Film structure is not additive (dies are Yellow Magenta Cyan), so one starts as white light, and the film substracts.
If one is setting the illuminant to a color (R G or B) the capture would be the resulting product of the LED and the sensor sensitivity for the particular wavelength. And the goal is to capture what the film density did not substract.
For illustration, below is the sensitivity chart for the Sony IMX477 (sensor used by the Raspberry HQ) overlapped to the density chart of Kodachrome.
As pointed by @cpixip using a narrow led band would then make the capture very dependent of the wavelength chosen. Selecting the LED basically selects what one is scanning, but more importantly what is not.
Another alternative, White + RGB. That’s the path that @matthewepler chose for this iteration of Kinograph selecting an RGBW LED, and constant current to control each channel as needed.
All of the above is in the context of a color sensor. But I think illustrates that when selecting the monochrome camera + RGB LEDs it shows how critical it is to select the right LEDs, which in turn decides what is not scanned.
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I thought it would be interesting to combine all this information in one chart, combining Light (LED), Film (Kodachrome), and Sensor (IMX477/Raspberry Pi HQ minus filter).
For the purpose of illustrating the substracting nature of the negative, the Diffuse Spectral Density was plotted upside-down to what the datasheet presents.
In short… It all starts with what the light source can provide, in this case the White LED, then…
- Cyan let that portion pass (blocks Red), then captured by the Blue+Green pixels.
- Yellow let that portion pass (blocks Blue), then captured by the Green+Red pixels.
- Magenta let that portion pass (blocks Green), then captured by the Blue+Red pixels.
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Fantastic! Looking specifically at the red part of the spectrum, it seems that one might encounter issues here. First, the input from the magenta dyes will end up in the green channel as well, because the green filter is not at 0%, but somewhere above the 10% mark. Second, both the white-light LED as well as the Sony IMX477 feature a fall-off, starting at 600 nm for the sensor and at about 640 nm for the LED. That is about the same wavelength were the magenta dye is starting to become relevant (with diffuse spectral density going over 90%).
Indeed, I encountered some time ago issues with the color definition of intense reds using the HQ camera - albeit with a discreet RGB-LED setup (not a white-LED). Now, replacing the stock IR-Filter with a better one resulted in improved color definition.
The blueish stock IR-Filter is responsible for the linear fall-off in the red channel of the Sony IMX477 starting at 600 nm. Here’s the plot of the IR-filter of the Raspberry Pi HQ camera measured by myself (the Raspberry Pi foundation published a very similar filter curve some time ago, but I can not find their data on their website any longer):
One can clearly see how strong the filter blocks light in the red color parts. Just for fun, I replaced the stock IR-fitler with this one,
which resulted in a better color definition in the red part of the spectrum (at least in my setup).
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Glad you like it. Thought it was an interesting representation of the issues with light.
Can you elaborate on the setup to do this measurement?
I was watching at this youtube Pi Spectrometer and was considering putting one together for doing a bit more light measurements, rather than relying on manufacturer specs.
On the IR HQ filter note, I recently got the camera, but haven’t had the time to play with it, and was thinking about removing the filter and leaving the sensor exposed.
Well, the machine which was used for this measurement is a professional spectrophotometer which is able to measure transmission values from the UV to the near IR (specifically, from 185 nm to 900 nm).
Basically, it uses different lamps, a cascade of different diffraction patterns and variable slits to single out a specific wavelength. This light is than passed over two separate paths to a photomultiplier. One of the path has the filter to be measured inserted, the other is empty and is used as 100% transmission reference. The photomultiplier is switched optically between the two measurement channels. It’s a quite complicated machine.
Be careful - the RGB-filters used in the sensor are not functioning in the near IR range. They are all basically transparent here. That’s the reason for the IR-blockfilter. Without an IR-blockfilter, you will get a dramatic color spill ruining your colors. Check some example images taken with the Raspberry Pi foundations “NoIR” camera to get an idea how the removal of the IR-filter affects the colors. You will absolutely need to put another IR-filter in place when you remove the stock filter. Also, of course the color balance will be shifted if you have replaced the original filter. So the original color matrices which are used in the Broadcom image stack are strictly speaking no longer valid. Of course, potentially you could try to handle that within the context of the new libcamera-stack - for my taste, this new “option” is however still something I will avoid; I do not consider the existing implementation to be production-ready.
that is actually the spectrometer I linked above. Seems to be a decent development. Certainly usable for checking LED curves. However, for various reasons I do not think that transmission measurements with such a setup would be very meaningful (in short: non-linear camera sensitivity, noise floor, constancy of the light source during measurements, etc. ). You would need to measure (as described above) first the pure light source (to get the 100% transmission line) and than measure the filter in question. Transmission would be measurement 2 divided by measurement 1.
Thank you @cpixip.
That’s what I thought, but worth checking.
Good points. I was thinking about experimenting with IR sensing, some scanners use it to detect dust. But you are right, would have to consider the light source or a filter as part of the lens.
I do see as problematic the factory filter response for red. The filter you suggested is great, thank you.
Understood.
Primary use I was thinking would be to have a real life measurement of the LED, mostly for qualitative comparison of light sources rather than for quantitative measurements.
Great exchange, thank you for the information.
You can read about the YUJILEDS here and here. They have peaks in the spectrum, but I think it’s more about the spectral coverage than whether there are peaks or not (I’m not a lighting expert). The white LED lights of the past were not capable of this so they were missing a lot of the spectrum and then even if you pair a light like that with a really good top-of-the-line camera it doesn’t matter it will me missing a lot of detail and unable to reproduce the colours.
Thanks, very interesting.
These LEDs employ the same technique as classical “white”-LEDs: pair a single LED with a dye converting part of the LED’s light into other wavelenghts. It’s “just” an optimization to obtain a desired spectrum. In the extrem case, you take an UV-Led and create the whole visible spectrum purely through the appropriately designed flourescent dye.
I’ve found some other relatively high CRI (97) white LEDs that I’m going to try - they were quite cheap, so it’s worth the experiment before looking at anything more expensive, I think.
The benefit is I can easily fit these LEDs into the current mechanical and electronic design. However, unless it works absolutely brilliantly, I’ll probably then experiment with a 5x5 matrix of R/G/B LEDs driven by three constant current sources for comparison.
From what I’ve read above which is most ‘illuminating’, I’d expect the R/G/B approach to give better colour separation than high CRI white LEDs, ie saturation, but possibly with worse colour accuracy. In the end though, it’s completely subjective, because I am just converting old family home movies so all that really matters is the family’s opinion.
An added problem is that the films (mainly 1960s) have not been well looked after, and in some cases there is fading. Not only that, in a lot of cases the original exposure was atrocious, and transparency film like Kodachrome or Ektachrome has a really narrow tolerance of under/over exposure, but there’s not much one can do about that.
As a point of interest I do have a 35mm film scanner I still use occasionally for slide scanning. It gives phenomenally good results (on well exposed slides). Must look into what light source that uses.
The White LEDs I selected have a Vf of 9V for 100mA. I would speculate that these use more than one LED junction under the die, perhaps of different wavelengths to achieve the better response. I have no information to support this speculation but it is consistent with the high Vf.
Possibly, though I have seen LEDs that are simply two or three identical LEDs in series - higher light output at higher forward voltage. Could be either, I guess.