The Backlight

Machine Design
22

Maybe you are right, I do not know, I am not the expert in chemical film processes and a simple increase of the amplitude could be enough.
I am also here to learn.
I just assumed from black an white material, that can shift in intensity due to oxidation of the silver.
In the colour layers there is no silver anymore, but maybe other chemical processes which affect the spectrum.
And another thing is, that yes there are layers, so a 3 dimensional structure, but in most cases the film layers seems to form a kind of irregular bayer pattern itself, for directed light.
That is what I saw mostly under the microscope.
So if the illuminated particles are mostly beside each other and not above (technically they still are), the G and B or C and Y parts will only affect the corresponding particles and not the red ones. In six passes you would be able to reconstruct the right colours, if there is no crosstalk between the R, G, B, C, M, Y bands of the illumination.
Film was ever projected with a continuous spectrum and that gave the best results for colour reproduction in projection, so it may be a also the right way for digitisation.

23

Seems like it is an aliasing effect of the PWM and a rolling shutter sensor.
You already hat the idea to use a RC filter. That should smooth the current through the LEDs.
Direct PWM can damage LEDs because they are intolerant to fast current changes.
It may also help to increase the PWM switching frequency.
The artefact can be a form of EMI from the PWM module to the sensor too.
On our Korn film scanner we had an EMI problem between the illumination and the magnetic audio head, resulting in a high frequency noise in the recorded audio signal.

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24

Another approach to avoid PWM would be to use 3 I2C DACs (MCP4725). They could be multiplexed together using a TCA9548A I2C mux connected to your uC. The max current output of the DAC is 25ma, so if additional current is needed, an op amp or transistor circuit could provide the boost.

25

I was able to make a test system using a 3W RGB LED and a White LED, with the help of a friend. It seems to be OK. (I will post a video soon with details),

Now I need to build the lightbox. With a single RGB LED the colors are not mixed perfectly, even with a diffuser. Your ideas are welcome.

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26

For diffused light, you might want to build a diffusion dome, like this one: https://www.metaphase-tech.com/diffused_dome_tube_lights

27

Thanks for the link, Martin.

28

And found this thread on the forum.

29

Thanks for linking that thread. I re-read the info on Frank Vineā€™s webpage and it was immensely helpful. Take a look and let me know what you think: Frank's Cine2Digits project page

TL;DR -

  • R, G, and B LEDs on separate constant current circuits
  • When the shutter opens on the camera, all the circuits are on, then each one shuts after a set time according to the userā€™s color balance settings.

Is anyone interested in building one of these and testing it?

30

You might consider a WS2811 controlled RGB driver like this one. WS2811 is easy to control with 1 data line and gives you individual RGB/brightness control. No need for handling PWM or rolling your own constant current drivers. Rob offers 10, 20 & 100W boards and these can be chained if you want to have more than one for mixing. These are popular for use in flood lights for x-mas light shows.

31

@todd1814 thanks for the suggestion. I had mentioned a WS2812 LED option further up in the thread. I looked into the datasheet and see that the integrated driver is also constant current. To me that means that the PWM used to control the color balance is just a data signal. There shouldnā€™t be any flicker due to the constant current circuitry. That means the Adafruit Neopixel ring could still an option. But Iā€™m an amateur and learning as I go so if anyone sees flaws in this logic please let me know.

To the group - can someone help me understand why something like Frank Vineā€™s system (see link in my post from earlier today) is superior to the Adafruit setup?

Thanks to all on this thread. Really helpful.

32

OK. this is the first test run with the RGB LED set up with my attempt at a diffuser.
https://vimeo.com/258221254

33

Looks great! Can you remind me what LEDs your using and what the diffusion setup is? Pics would be great.

34

Sorry I was away for a while. The LED is a normal RGB 3W LED and a White LED. Could not find a RGBW LED in Sri Lanka. Basic testing with a No PWM method was OK as you can see in the video. No rolling bars. The diffusion box is just a white cardboard cylinder with a diffusion dome from a LED domestic light bulb at the end. Nothing fancy. The contraption is hilarious. But sort of worked. Anything will yield better results than this. I will post some photos ASAP,

35

Thanks @Udayarangi! Would love to see photos. When you say ā€œa no pwm method,ā€ is it one of the ones discussed in this thread? Schematics would also really help. Iā€™d like to reproduce your results.

Thanks for sharing. This is exciting.

Matthew

36

For whats its worth I used three of these constant current source to control one of these RGB led

I tried a 50mm cardboard tube on the output of it to try to reduce light loss - this was a Very bad idea giving specular reflections. I also tried a table-tennis ball as a diffuser but this just reduced the output of the leds. The light produced by the led I chose already generated (relatively) a wide diffused field.

If you use this the circuit above the lm317s need a bigger heatsink than I thought, the led needed little.

-Steve

37

Steve,
Yes, I said that anything would yield better results. But on the recorded video I published above, I did not see any specular highlights.

38

I think we misunderstand each other.

if you point your led, diffuser and tube at a white matt surface (painted wall) it is easy to see of the lighting is uneven - the problems I had where very obvious. if the illumination looks even it may be ok, but if you can easily see specular highlights then it is very bad.

I had lined the cardboard tube with reflective mylar film from ebay, hoping to increase the light hitting the film. This was a big mistake. FWIW the idea came from the cintel modification from ITK which used a cylindrical mirror to collect light scattered by scratches in a flying spot telecine to reduce their visibility. Interesting experiment, lesson learnt.

I think I might try a black velvet lining to minimise reflections in the tube.

-Steve

39

Steve,
I was rather confused. Did you watch my sample video of the results I obtained with that contraption?

https://vimeo.com/258221254

Are there any specular highlights in that?

40

I just ordered a bunch of high-CRI LEDs (link) and will run some experiments with them, various diffusors and a spectrometer.

41

Oh dear.

I got specular highlights due to my bad design, I am saying nothing about what you have done.

I used a reflective lining inside a crdboard tube to try and collect more light but this went badly wrong for me.

your video looks great, no obvious highlights, though I found it easier to see them by just pointing the light source at a painted wall. This attenuates the light output enough that it is quite easy to see highlights with the naked eye.