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Author Topic: DSLR testing sites like DXOmark and Imaging Resource use HMI and LEDs for color  (Read 56273 times)

sandymc

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To somewhat clarify the questions going around what is a module and how to connect it:

  • Modules are a string of individual LEDs connected in series, with a resistor connected in series with each string.
  • The resistor does two things - firstly, limits current to the string, and secondly, prevents thermal runaway if you connect modules in parallel.
  • 12V modules usually have three LEDs in series, 24V modules 6 or 7, depending on LED color, type, etc. On the photos in previous posts, the LEDs are the yellow blocks, the resistors the black blocks
  • The amount of current drawn depends on the module power - you have to look that up on the data sheet.
  • You can wire as many modules in parallel as you want, subject to the power supply being able to provide enough current.
  • Usually, the modules are internally wired such that if you just connect them end-to-end, that gives you an parallel connection.
  • If you want constant intensity, any old DC power supply of the right voltage will do. If want to dim LED modules, things get a bit more complicated.You need a power supply that can vary current smoothly, not voltage, such as the one linked to in a previous post.

I don't recommend trying to deal with individual LEDs without specialist equipment and knowledge. But anyone that's reasonably competent with wiring should be ok with modules.
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Alexey.Danilchenko

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12V modules usually have three LEDs in series, 24V modules 6 or 7, depending on LED color, type, etc. On the photos in previous posts, the LEDs are the yellow blocks, the resistors the black blocks
Yuji D50 LED strips referenced above have individual LEDs running at 3.3-3.4V with LED nominal current 120mA (at these specifications their output and CRI corresponds to the specified one). The specification of the individual D50 LED is here https://cdn.shopify.com/s/files/1/0344/6401/files/YJWJ011-1.1_YJ-VTC-5730-D50-G01.pdf?5186286384022133237

If you want constant intensity, any old DC power supply of the right voltage will do. If want to dim LED modules, things get a bit more complicated.You need a power supply that can vary current smoothly, not voltage, such as the one linked to in a previous post

It is not that simple for colour critical applications since for the LEDs to stay tightly within their specs the power supply needs to be constant current power supply. That is hardly any old DC one :). But generally this is true for a strip solution yes.
« Last Edit: June 04, 2018, 12:09:54 pm by Alexey.Danilchenko »
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sandymc

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It is not that simple for colour critical applications since for the LEDs to stay tightly within their specs the power supply needs to be constant current power supply. That is hardly any old DC one :). But generally this is true for a strip solution yes.

<non-techies tune out now>

Yes, that's true - a constant current supply would be marginally better. But practically, the bin variation (aka the variation you'd get from module to module) is more than anything that you'd get from temperature variation. (Temperature variation is the only reason why current would vary given a constant voltage supply - temperature variation causes variation in the LED forward voltage, which in turn causes current variation)

<non-techies can tune in again now>  ;)
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WayneLarmon

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Thank you sandymc and Alexey!

It is not that simple for colour critical applications since for the LEDs to stay tightly within their specs the power supply needs to be constant current power supply. That is hardly any old DC one :). But generally this is true for a strip solution yes.

Does the Yuji Flicker Free Dimmable Power Supply, 48W qualify?

I'm looking at making lights for family memories reproduction work.  Assuming that the Yuji modules and power supply have reduced the problem to plug-and-play, the next step is figuring out how to fabricate some semblance of

https://dtdch.com/dt-photon-custom-cultural-heritage-lighting/

Looking that site over, I need just about everything.  (But not at those prices.  This means going to Home Depot, walking past the Cree bulbs and looking in the lighting fixture, electrical, and building materials departments.)

However...earlier Doug discussed sidestepping the camera's Luther/Ives divergence by using the variable illuminants used in the IES TM-30-15 color rendition method

Any ideas on fabricating these kinds of illuminants?  Or would this take me down a very deep rabbit hole?  (Should I forget about Luther/Ives divergence and stick to emulating D50 as best I can?)
« Last Edit: June 04, 2018, 03:23:42 pm by WayneLarmon »
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sandymc

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In my view, the Yuji power supply looks fine.

Also in my view, Luther/Ives is in of itself a massive rabbit hole. But I know that others on this forum would violently disagree.

 
Thank you sandymc and Alexey!

Does the Yuji Flicker Free Dimmable Power Supply, 48W qualify?

I'm looking at making lights for family memories reproduction work.  Assuming that the Yuji modules and power supply have reduced the problem to plug-and-play, the next step is figuring out how to fabricate some semblance of

https://dtdch.com/dt-photon-custom-cultural-heritage-lighting/

Looking that site over, I need just about everything.  (But not at those prices.  This means going to Home Depot, walking past the Cree bulbs and looking in the lighting fixture, electrical, and building materials departments.)

However...earlier Doug discussed sidestepping the camera's Luther/Ives divergence by using the variable illuminants used in the IES TM-30-15 color rendition method

Any ideas on fabricating these kinds of illuminants?  Or would this take me down a very deep rabbit hole?  (Should I forget about Luther/Ives divergence and stick to emulating D50 as best I can?)
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Alexey.Danilchenko

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Yes, that's true - a constant current supply would be marginally better. But practically, the bin variation (aka the variation you'd get from module to module) is more than anything that you'd get from temperature variation. (Temperature variation is the only reason why current would vary given a constant voltage supply - temperature variation causes variation in the LED forward voltage, which in turn causes current variation)

The D50 LEDs that I ordered came from a singe bin. From my experience so far - the variations in the current do get chromacity shifts. It is also a reason why I don't use power supplies with analogue dimming that just vary current. The power supplies that sense and control the current in LED chain in my view are far more stable approach than chain of LEDs with a resistors and constant voltage DC power supply.
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Alexey.Danilchenko

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Does the Yuji Flicker Free Dimmable Power Supply, 48W qualify?
No idea - since it is powering LED strips (with resistors) I would guess it is not the constant current power supply. I build my own but this approach of course is not for everyone.

Their supply though is perfectly fine to drive their LED strips and panels - it will still give you a much better light source than other off the shelf solutions in the same price range. If you look at utmost quality and stability then I am afraid no off the shelf components do exist in the price range Yuji offers - unless you do build them yourself (I have chosen that road).
« Last Edit: June 04, 2018, 05:17:59 pm by Alexey.Danilchenko »
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Doug Gray

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No idea - since it is powering LED strips (with resistors) I would guess it is not the constant current power supply. I build my own but this approach of course is not for everyone

Resistors in series with a constant voltage supply is commonly used to drive LEDs.  Selecting the right resistor and supply voltage will automatically compensate for LED output with temp drift since the forward voltage decreases with temp.
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Alexey.Danilchenko

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Resistors in series with a constant voltage supply is commonly used to drive LEDs.  Selecting the right resistor and supply voltage will automatically compensate for LED output with temp drift since the forward voltage decreases with temp.
I know they do as well the drawbacks of this approach (as well as the fact that its the cheapest way to drive strings of LEDs). Variations of resistance, variations caused by temperature makes the current through the LEDs not that well controlled as in constant current approach.
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Doug Gray

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I know they do as well the drawbacks of this approach (as well as the fact that its the cheapest way to drive strings of LEDs). Variations of resistance, variations caused by temperature makes the current through the LEDs not that well controlled as in constant current approach.

Voltage driven LEDs can cause thermal runaway at higher levels because the forward voltage drops as the LEDs heat up.  Constant current approaches completely avoid thermal runaway but the right resistor, in series with the LEDs is very good. As the LEDs heat up and the forward voltage drops, the voltage across the resistor increases and that increase is also seen as increased current to the LEDs. Goal is to increase the current just enough to compensate for the lower efficiency as the LEDs heat up. By balancing the resistor and voltage, one can compensate quite well for the drop in light that would otherwise occur with a constant current source.

Sometimes the cheapest way is the best way.
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Alexey.Danilchenko

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Sometimes the cheapest way is the best way.

Its a good thing manufacturers of the LED monitors do not agree with you and use constant current for LED backlighting ;)

Driving LEDs with resistor as current limiter and constant voltage does nothing to control chromacity variations as current varies in that case. It also does not help that current and voltage through LEDs have non-linear relationship - tiny variations in voltage can cause substantial shift in current and changes in chromacity. For kitchen lighting it may be ok, for target shooting and profiling - well, I would not use anything but constant current power supply. Less variables to control is always better.

There are quite a few articles on a subject out there for those interested in technical aspects - for example this https://www.maximintegrated.com/en/app-notes/index.mvp/id/3256

For my application driving four Yuji VTC 5600K LED COBs with constant current supply on full output at rated current (well below maximum incidentally) - there is almost no variation in spectral output (there is of course some variation as LEDs get hotter to the working temperature which stabilizes after first 10 mins of running). I did periodic spectral measurements over 35 mins period starting from turning the LEDs on from cold.
« Last Edit: June 05, 2018, 06:32:43 am by Alexey.Danilchenko »
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Tim Lookingbill

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Interesting read up on the thermal dynamics within LED lighting constructs. I'm sure some of the members of Underwriter's Laboratory (UL) would probably find this interesting as well.

But as a photographer, and this is a photography forum, I'm sure we are now all in agreement that build your own Yuji LED full spectrum lighting is not as easy as Alexey points out.

And I'ld like to add for the photographers interested that the Cree 100w equivalent, 1600 Lumens, 90+ CRI, 5000K bulbs I bought at Home Depot yesterday are about the same in color rendering as the Walmart Great Value branded 5000K, 60w's at 800 Lumens. The Crees get VERY HOT after a while.

Cadmium Yellow turns Lemon Yellow. WhiBal gray is a slight greenish yellow.
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Alexey.Danilchenko

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But as a photographer, and this is a photography forum, I'm sure we are now all in agreement that build your own Yuji LED full spectrum lighting is not as easy as Alexey points out.

Nothing is easy until you actually try it :)

The solution I am building with constant current is comparable and aiming more towards high end solutions. In this aspect it is far easier than paying small fortune for the equivalent.

There is very little point in comparing it to LED bulbs as those offer neither predictable and repeatable lighting nor the control over brightness and chromacity. If comparing those then yes Yuji LEDs solutions are very comparable and easy to build (and due to spectrum will be better).

There is of course always a compromise - for example attempting to find a constant current supply (readily made) that matches the existing panels/LEDs. Practically no soldering (perhaps to de-solder resistors from panels). I considered going with this approach when building LED light from COBs but in the end doing my own solution gave me better control over the whole circuit.

An example of this ready to use approach is this constant current power supply (one of many) matching Yuji VTC 5600K COB LEDs (this power supply should be enough to support 5 LEDs in series - that's 3000lm). All that remains is attaching them onto radiator with these Molex connectors. No soldering whatsoever and gives you quality LED driver.
« Last Edit: June 05, 2018, 05:56:03 pm by Alexey.Danilchenko »
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WayneLarmon

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And I'ld like to add for the photographers interested that the Cree 100w equivalent, 1600 Lumens, 90+ CRI, 5000K bulbs I bought at Home Depot yesterday are about the same in color rendering as the Walmart Great Value branded 5000K, 60w's at 800 Lumens.

When I lasted tested hardware store quality LED bulbs several years ago, CRI was in the low 80s, including, Walmart bulbs.  But I hadn't tried Walmart bulbs lately.  I'll pick some up and test them.  Recent Home Depot house brand LEDs were in the mid 80s (but rendered skin tone in paintings very well.)

Did you measure CRI?  Or are you going by how well the colors you are interested in were captured?

Quote
The Crees get VERY HOT after a while.

Cadmium Yellow turns Lemon Yellow. WhiBal gray is a slight greenish yellow.

Duv measured on the high side for the Crees.  Duv is the amount of tint away from the Planckian (white) locus.

What I'm interested in is if the increase in measured CRI for cheap bulbs (was low 80s several years ago, now (Cree anyway) is in the 90s) correlates with colors being captured more accurately. 

Otherwise, what is the point of measuring CRI (etc.)?
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WayneLarmon

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An example of this ready to use approach is this constant current power supply (one of many) matching Yuji VTC 5600K COB LEDs (this power supply should be enough to support 5 LEDs in series - that's 3000lm). All that remains is attaching them onto radiator with these Molex connectors. No soldering whatsoever and gives you quality LED driver.

Thank you again.  I know you are frustrated but, until this post, this information was illusive.  For example, your earlier posts referenced constant current power supplies but I didn't know if you were, for example, making a CC supply for each LED (rather than connecting them in series.  Or if connecting in series caused problems.)  Or that prepackaged CC supplies and COB  connectors were readily available.  (Why didn't Yuji have CC supplies instead of CV supplies?  Why didn't they offer COB connectors?) 

I know that this component sourcing information must seem elementary for you but it isn't for those of us whose electronics skills are rusty.

An open question: Have other forum participants built lighting as Alexey has described?  If so, could you share any construction tips?
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Alexey.Danilchenko

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Thank you again.  I know you are frustrated but, until this post, this information was illusive. 

I am frustrated by the people spending time posting here about how difficult that is and complaining that no cheap solution exists (not referring to anyone specific here but rather generally) where a fraction of the time spent on writing those posts could be spent to research the subject. These days information is at everybody's fingertips - it took me less than 2 mins in Google yesterday to find the power supply and connectors for COBs and I was not searching for some special terminology (LED, COBs, constant current LED power supplies etc).

Yuji as well as any other LED manufacturers provide enough packages to cater for various levels of application. All you need to do is look at what's there, research a bit: pros and cons of various solutions, whether the cons outweigh for you the pros and then select the target LED package and the rest follows from there (power supplies, necessary skills if needed, youtube searches etc).

LEDs by far the easiest in this kind of research (try to build a lighting solution with pulsed point Xenon arc lamps for example to see what really difficult lighting looks like) since the information is out there in spades.

For example, your earlier posts referenced constant current power supplies but I didn't know if you were, for example, making a CC supply for each LED (rather than connecting them in series.  Or if connecting in series caused problems.)
You didn't ask. The constant current supplies are done with the goal of LEDs connected in series simply because the current regulation will be met this way for each LED regardless of their small variation in forward voltages. Connecting them in parallel means that unless they are precisely aligned there will be always one LED that passes more current than the other and you still need to balance them with resistors. Basically connecting them in parallel should only be done if the power supply has multiple channels. For example, VTC COB LEDs that I am using have 450mA per LED at 19-21V, my power supply can provide up to 50V at 450mA and has 2 channels individually controlled. So my LED wiring is two strings of two LEDs in series - one string on each power supply channel.

If you go down the COB route - the most important thing to remember is that COB packages do need good heat dissipation (they can produce substantial amount depending on a COB). So unlike panels from lost of small LED chips where board itself may provide enough heat dissipation, COBs do need a heatsink (be mounted one one).

Or that prepackaged CC supplies and COB  connectors were readily available.  (Why didn't Yuji have CC supplies instead of CV supplies?  Why didn't they offer COB connectors?) 
None of the LED manufacturers offer connectors simply because the cannot cater for every single application. A typical search for the LED COB connector reveals that Molex (a most common connector manufacturer) has lots of variations. Short research from there using the size of the COBs (from spec) gives you a couple of variants to try. And they are really cheap to try and see what fits.

I know that this component sourcing information must seem elementary for you but it isn't for those of us whose electronics skills are rusty.

There are no electronic skills involved in searching for  mechanical connection. A little research into how LED work is needed to make necessary connections. I had no electronic skills (apart from soldering an occasional wire to the battery connector) at all before a few years ago when I decided to build my spectral measurement device. Now I do have some rudimentary skills (and most of that is just research and practice) but none of those were needed in search for LED COBs attachments.
« Last Edit: June 06, 2018, 04:43:49 am by Alexey.Danilchenko »
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Tim Lookingbill

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When I lasted tested hardware store quality LED bulbs several years ago, CRI was in the low 80s, including, Walmart bulbs.  But I hadn't tried Walmart bulbs lately.  I'll pick some up and test them.  Recent Home Depot house brand LEDs were in the mid 80s (but rendered skin tone in paintings very well.)

Did you measure CRI?  Or are you going by how well the colors you are interested in were captured?

Duv measured on the high side for the Crees.  Duv is the amount of tint away from the Planckian (white) locus.

What I'm interested in is if the increase in measured CRI for cheap bulbs (was low 80s several years ago, now (Cree anyway) is in the 90s) correlates with colors being captured more accurately. 

Otherwise, what is the point of measuring CRI (etc.)?

I have no instruments to measure CRI. Like you I'm interested in very good reproduction of color appearance in a controlled lighting environment (studio work) where I can shoot a scene containing a wide variety of artificially and naturally colored objects lit under these "Daylight" balanced LED bulbs and do the least amount of editing in order to develop a turnkey process. So far it's not possible but some of these LED lights get pretty close excluding metameric failure of the object being photographed.

I haven't tested this on artist's paints which might have the potential for metameric failure no amount of spectrally accurate light will fix. Most of the subjects lit by these daylight light bulbs photograph well enough but do require HSL/White Balance and some SplitTone edits (including some Adjustment Brush work) in ACR to get it to look right but then that's not a turnkey process and not very efficient.

I really don't need instruments to tell me when something doesn't look right. It really doesn't make any sense since I have to edit every image I shoot anyway even lit by the sun. But what I've seen, your 80 CRI measurements seem in the ballpark but I'ld favor maybe a bit under 90. But I don't have any visual to relate the scaling of the numbers to color appearance matching so I don't find any use in measuring instruments.

I have enough trouble visualizing Delta E differences. Different colors are more pronounced than others when they look off but the number doesn't indicate how much so I don't find that system very useful.
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Tim Lookingbill

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I am frustrated by the people spending time posting here about how difficult that is and complaining that no cheap solution exists (not referring to anyone specific here but rather generally) where a fraction of the time spent on writing those posts could be spent to research the subject. These days information is at everybody's fingertips - it took me less than 2 mins in Google yesterday to find the power supply and connectors for COBs and I was not searching for some special terminology (LED, COBs, constant current LED power supplies etc).

You're referencing me and you know it. I don't take that personally though because I respect those who have that level of technical expertise.

I just think you're wasting your time here. Like you, I see something I say something. You don't seem to grasp the more you post explaining how to build your own LED lights the more complicated it appears. And as a musician I do understand some electronics since I installed my own sound system in my car and wired my two 10in. subwoofers in series to my 250w Alpine amp in order to raise its crossover max peak on the low end because the subs were rated to reproduce signals down to 10Hz meaning peak (where most of kick bass resides around 50Hz) would barely be heard if I wired them in parallel (at 2 ohm impedance instead of 4), but that's DC current so I won't add any more complexity to this subject. See, I know some technical electronic stuff, too, but I know when I've gotten too deep to make it worth while. Wiring for AC to drive DC current is above my pay grade.

This is a photography forum, not a structural engineering forum. Every post you make explaining the intricacies of electronic regulatory issues with resistors, power supplies and wiring in series verses parallel makes your comments useless to those that spend most of their time making images. I'm one of those photographers and I've got over 3000 edited Raw images under my belt and my car's audio system sounds incredible especially digitally recorded wide dynamic range EDM (Electronic Digital Music).

I'm frustrated that you aren't making any headway in convincing a whole bunch of photographers to build their own full spectrum LED lighting on the cheap. But maybe if you keep explaining, it will come across a lot more simple outside of entertaining the retired engineers and technical folks who like to collect and talk about gear instead of making images.
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Alexey.Danilchenko

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You're referencing me and you know it. I don't take that personally though because I respect those who have that level of technical expertise.

Not just to you

This is a photography forum, not a structural engineering forum.
And I am not one either - I write software.

Every post you make explaining the intricacies of electronic regulatory issues with resistors, power supplies and wiring in series verses parallel makes your comments useless to those that spend most of their time making images.
Every? I have not started about resistors even - someone else did. When discussion goes that way I can support it. What I did start with were simple things (perhaps you just fail to read them).

The thing is - I need nothing of this. I already built my lighting systems (not one)and it works. I was merely responding here to the statement that LED lighting is rubbish - it is not and provided the reference. The technical bits came out after people were asking questions.  It is completely fine with me not to answer any of those so I will stop from now on.
« Last Edit: June 06, 2018, 05:56:46 am by Alexey.Danilchenko »
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Tim Lookingbill

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But I did start with simple things - you just fail to read them.

I did read them and my understanding of electronics tells me there's nothing simple about the subject. Wayne pretty much backed that up with his questions which was just as much over my head.

I have gotten shocked working on simple AC electronic components in my youth. That cured me of ever attempting a DIY project involving AC wiring no matter how simple it may appear. It just makes me nervous.
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