Understanding Additive and Subtractive Mixing

NateTheRiddler

Well-Known Member
Premium Member
Morning, CB! Time for Newbie Nate’s Question of the Day!

Reading through my textbook again, came across a description of RGB/RGBW fixtures that featured gel frames (particularly cheapie units).

Wut?

Hold the phone, I thought the point of a tricolor or quadcolor LED system is to additively mix the color you desire? Isn’t adding a subtractive filter (like a gel or dichroic) the equivalent of using a water filter to remove minerals you previously added for taste?

I understand that, with a pure white engine such as a Source 4WRD, using subtractive filtering is still a legitimate thing; after all, you’re producing as much of a full spectrum as you can, and filtering so you can get nice saturated colors. But filtering the white light produced by weak RGBW or worse yet, RGB, seems absolutely silly.

Can someone either 1) confirm that RGB/RGBW+gel frames is moronic or 2) confirm that I’m silly and don’t understand the point?


Unrelated P.S.: I’m compiling all of my “Question of the Day”s into a little guide that I might post somewhere at some point so other learning non-degree-holding LDs can learn something. Thoughts? (Or redundant?)
 
Great question, I have thought the same. I do not know the answer, but here are some guesses to think about.

When I use RGB or RGBW, I sometimes have a hard time getting that exact color blend I want using low end LEDs and boards. I usually just end up getting close and saying that is good enough. I could see using a filter that I like with full brightness might get me closer to the color I want.

Also, when I do color blend with an LED, I sometimes find that the brightness drops way down. Especially if I am wanting a really saturated color. I could see using a filter with full brightness could actually give me higher lumens than using the LED colors alone.

I like the idea of your question of the day guide. Once you post it somewhere, let me know and I will definitely subscribe. I might also throw you some questions to ask and post.
 
You wouldn't want to put a saturated color in the gel slot. I have on rare occasion seen people put a cto filter in to try and warm up the whites and ambers when using an rgb fixture. But the main purpose is to hold a shaping or diffusion filter. There are all kinds of different filters you can get. Various diffusions to get different beam angles. And even different shapes, oblong if you want more of a traditional par look, or more linear filters if your trying to was a cyc. There are all kinds of options out there depending on what you want. You may even want to stick a top hat or some other accessory in front of one for some reason like masking. Having a gel frame give you options that you won't have without it. Some lights have other ways for attaching filters, and tape is sometimes an option. But being able to just drop in a gel frame makes things a lot cleaner and simpler.
 
It may be that you're silly and don't understand the point. ;)

I wouldn't buy an LED par that didn't have an accessory slot slot for things like holographic lenses, barndoors, top hats, half hats, and louvres. I use the ones on my ETC Desires all the time for lenses and half hats.
 
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yes, there are a lot of uses for that frame slot besides color gel. Outside of the "W" channel, you are not going to be able to bend the color too much as RGB LED's have a pretty tight bandwidth and a gel outside of that bandwidth will only dim the fixture. Still, the biggest problem with inexpensive LED fixtures is dispersion so you may very well be putting in a diffusion or spun filter to get a wider angle.
SIDE NOTE: One of the best ways to widen the dispersion on an LED fixture is a chunk of those old plastic florescent light light diffuses which can be found in abundance in dumpsters as office buildings trash the old 2x4 trothers.
 
It may be that you're silly and don't understand the point. ;)

I wouldn't buy an LED par that didn't have an accessory slot slot for things like holographic lenses, barndoors, top hats, half hats, and louvres. I use the ones on my ETC Desires all the time for lenses and half hats.

Durrrrrrrrrrrr I’ll go back to my newbie corner now. I heard they want their LX3 back. :doh:o_O I feel extraordinarily silly for not having considered accessories. Although, to gain myself +0.1% reputation back, I have to admit that I was completely thinking cheapie LED PARs and the like, which I would probably never bother with accessories for. OK, I’m making excuses now, I’ll leave. *whistles innocently*
 
Durrrrrrrrrrrr I’ll go back to my newbie corner now. I heard they want their LX3 back. :doh:o_O I feel extraordinarily silly for not having considered accessories. Although, to gain myself +0.1% reputation back, I have to admit that I was completely thinking cheapie LED PARs and the like, which I would probably never bother with accessories for. OK, I’m making excuses now, I’ll leave. *whistles innocently*
Your not the first person to wonder or ask about that. After all, it's called a "frame holder" as in Gel Frame Holder.
I can't even be sure the knock-off manufacture designer didn't think, "gee, it's a theater light, shouldn't it have one of those thingies on the front of it?"
I chuckled the first time I saw one, but after all, yea other stuff fits so I gave them a pass!
 
@JonCarter You’re killing me, Smalls. I’m not supposed to laugh this hard during my apartment’s quiet hours. Damnit.
 
Just to throw fuel on the fire here. If you put a gel in the frame would that gel effect all colours and therefore really only work if you were using the fixture for one colour only? I thought the beauty of LED fixtures is the colour mixing even if they are cheep ones. having said that I had not really considered accessories either (another rookie mistake avoided by reading CB forums)
Thanks for listening
Geoff
 
Well... the color range or envelope for *most* inexpensive LED fixtures is not always linear...

Hence the need for RGBW, because if the range was *good* 255,255,255 would yield a lovely pure white at about 6-8K.

And 128,128,128 would yield the same lovely white at 6-8K with about half the intensity (depending on the *curve* DSP the manufacturer may have inserted to "help" you...)

But it doesn't.

So we get gels from folks like LEE's Zircon series of filter packs to try to reduce the inconsistencies associated with LED lighting.

In my limited experience with less-expensive fixtures they are, in fact, pretty linear within production batches.

Once upon a time we got about 150 SlimPARs in at one shot and you could see the differences when we were checking them in 10 at a time. (I recognize that SlimPARs do not have accessory ears, it is an example of the *quality* of the light tho)


I’m compiling all of my “Question of the Day”s into a little guide that I might post somewhere at some point

More knowledge is rarely a bad idea. IMHO
 
would that gel effect all colours

It's complicated.
Colored LEDs are narrow band emitters. Gel is continuous.
Pretend these 2 spectrum graphs actually line up. The first is a theoretical RGB LED emitting "white" light. The latter is Rosco R23. The R23 will pass no blue or green leaving only red, so the result should look closer to R26 than R23. The RGB LED is incapable of providing any of the spectrum between the peaks.

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It gets even messier with RGBW because the white LED is typically a blue emitter stimulating a phosphor to produce a more full spectrum "white". While it might come closer to R23 than the RGB LED it will still be missing a lot of blue, red, green and the frequencies in between. RGBL, RGBAW, ETC's x7 and other LED combinations are left to your imagination to work out.
 

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It's complicated.

Yes, yes, yes!

The truest issue is that the full visible spectrum is not (generally) provided by LED instruments.

Especially inexpensive ones.

It is why LED "blacklights" consistantly suuuuck. Tru-nuf they have the portions of the UV spectrum that makes stuff flouresce... with the penalty of portions of the spectrum that ARE visible... hence spoiling (maybe "making less effective") the effect.

sk8rsdad hits it on the head.




It all boils down to what is "good enough".

All decisions are compromises. Especially theatre ones.

Your mileage may vary.
 

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