Thanks @Udayarangi! Would love to see photos. When you say “a no pwm method,” is it one of the ones discussed in this thread? Schematics would also really help. I’d like to reproduce your results.
Thanks for sharing. This is exciting.
Matthew
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
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.
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
Steve,
I was rather confused. Did you watch my sample video of the results I obtained with that contraption?
Are there any specular highlights in that?
I just ordered a bunch of high-CRI LEDs (link) and will run some experiments with them, various diffusors and a spectrometer.
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.
Martin, it will be interesting to see what results you get but I would recomend using three seperate leds rather than high CRI, broad spectrum LEDs.
I have explained why on the LED light source thread here.
Anyway, as you have bought the leds already, best of luck.
-Steve
Apologies for not posting any diagrams or photographs of my LED unit. I will do it soon. It cost me 20$.
Steve, I agree (at least) 100%. This is for testing an alternative workflow with a Bayer-patterned sensor; where the primary goal is speed. For quality/color I would and will go the monochrome sensor, individual lightsource route.
Noob here. I’ve seen this before, stumbled across it again, had something useful to share.
Just wanted to mention biaslighting.com / medialight. They make bias lighting for direct view monitors, color timing, etc. They have a high quality LED that is a D65. A group buy might do, as you have to buy them as a “kit” of several feet (the spacing of the LED’s is about 1 LED every 33.33mm or 1+5/16"). I use them for diverse things from their intended purpose (back lighting of direct view screens), to illuminating a Conductor’s podium.
Hi all. Back again with a little update (bigger one coming soon). I was able to install a first prototype for the lighting system for version 2.0. Definitely still in the rough stages.
The white planet looking thing is a two-part dome. I made it by vacuum forming a half dome twice and then gluing them together. I cut a square out and attached a tin-foil tunnel (3D printed part to come) to the LED, which is mounted on the other side of the acrylic plate. Then I cut off the top to match the aperture of the gate (35mm).
Here’sa link to the LED unit I bought and assembled. Would love to get some feedback on that unit. Do you think it’s what we need for quality results?
My next steps will be to get the camera mounted and do some tests:
- flicker
- maximum shutter speed
- other? Tell me what you want me to test and I’ll post the results!
CC @Greg @Andreas @quintile @Udayarangi who have been of great help in this channel
My only comment is that I would shy away from using PWM due to possible interference with a CCD camera. You might get beat frequencies. IMO analog control of the RGB channels is best. If you do resort to PWM, I would at least consider a first order filter on the output.
https://www.youtube.com/watch?v=LQDS6z-90sE
Just my 2 cents.
@matthewepler That is a really cool integrating sphere - I would love to see/know more about the vacuforming process you used. I’m also curious about what the edge-to-edge falloff looks like with it (if it’s even detectable!).
I’d echo @Greg’s concern about interference, depending on the imager and integration time, but there are a lot of variables. I didn’t see it on the LED’s site, but it’d be cool to know what the response looks like when it’s “tuned” to common targets like 3200K, 5500K and 6500K. It might have a sweet spot for color.
Thanks, Greg. I totally messed up and bought the wrong one, then. For some reason I had it in my head if there was constant current to the LED then we would avoid flicker. Back to the drawing board.
Can you point me in the right direction? What specs should I be looking for?
@johnarthurkelly what LED did you buy? I remember it not being RGB, but maybe it has a sibling?
Also - if anyone can explain to me like I’m 5 what qualities make a good light in this situation and why, I’d be very grateful. My knowledge of how PWM and power affect light quality is limited. Thanks so much!
My buddy has one of these: https://www.mayku.me/. I bought an acrylic half-dome from Canal Plastics and used that. It was really easy!
I bought the Yuji Led VTC Series High CRI COB LED - 135L 3200K model, the published curves for it looked pretty close to the original illuminant that would have been in the projector that the films in my collection would have been viewed with - that and I’m pretty sure I like the response from the sensor in my build with a white balance around 3200K.
Like all machines that are involved in making, storing, manipulating and viewing pictures, the ones we’re building can be thought of like a system or an equation - each part/variable/factor (or illuminant, lens, medium or sensor) changes what comes out on the other side of the equals sign. To keep things as faithful or neutral as possible, I’m trying to pick imaging parts that are compatible, so as to reduce conflict or disagreement in components that may manifest as image artifacts. The light is just as critical a component as the lens, camera, etc.
When all this film was designed, the lighting was assumed to be incandescent (or discharge even) from the use on set to the lab to the projector. This the lighting was pretty much all homogenous. Now, since we’re working with aged material in certain cases, we have to be much more careful - those old hot lights can damage or increase the rate of damage to the film. However, the sacrifice made is that in figuring out how to eliminate the heat issue, we’re using lights that don’t produce light across the spectrum in the same way as the old ones.
LED’s typically are either on or off - they do not dim quite like older incandescent lights, but like older incandescents, their quality of light changes as they are dimmed. Thus there is an “optimal” condition in which to operate them to get the desired quality of light. If dimming is required (say you want to adjust relative brightness of a Red, Green and Blue LED separately), the most common way to go about doing that is with PWM. PWM is less desirable in our application because of the possibility of interference with the shutter on the camera - you may see banding or flickering due to the timing of the PWM and the timing of the shutter. That’s not to say it can’t be done - it just requires a great deal of effort to implement correctly.
Ideally (aside from dimming the light source) you’ll want to consider how well the light matches the film and how well it matches your sensor. Some films prints were designed to be viewed/projected with light of a particular color temperature and color sensors are designed to have a color response that is best at a particular color temperature. N.B. - I’m using color temperature as an umbrella term for a lot of concepts about how the energy of the light is distributed throughout the spectrum, but 6500K or “daylight” etc. are familiar terms that are understandable in a broad context.
For my build in particular, I picked a light source that at full brightness wouldn’t meet a damage threshold for my film during a likely exposure time in the gate and then decided to set my exposure with the shutter and aperture on my lens. Since I had a light that matched the film and sensor well, very minor white balance adjustment was able to be done “on-chip” in the camera. (There’s a wonderful set of papers that I don’t have the link to right now on how color records fade in many film stocks based on temperature, humidity and exposure to light.)
Matt, you might still be OK with PWM and a filter as I mentioned previously. Here’s a nice little article on the topic. This is where a scope would definitely be helpful.
Matt, I think you should be ok with the path chosen for the LED driver. I think what Greg is referring to is using PWM directly to control the LEDs. Not sure about the module you are using. I couldn’t see the spec for the three ICs in the module, but I presume they use PWM input to control constant current output. Which is exactly what you want to avoid flicker. In essence a digital input to control analog output.
I’m impressed with your design for the integrating sphere. After my first attempts with a regular diffuser I’m now using an integrating sphere (of sorts) to achieve better diffusion. 3D printing didn’t seem like a good way to get the smooth surface required. So my current implementation is very crude: I took an old broken LED light bulb and figured the bulb was close enough to the required shape. The former base is the output opening and at a bit of an angle also the input opening. And to make it reflective instead of translucent I spray painted the whole bulb with silver rattle can paint I had lying around. Not great, but I wanted to see results first before going further down this avenue.
I think it would depend upon whether or not there is any sort of filtering on the chip itself because you would expect that controlling a constant current driver via PWM would pass that ripple on to the current driver output as well. It would be hard to know for sure without a scope or maybe just try it out and see if there’s a problem.