# How To Read Lee Filters Data Sheets?

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### #1 Macks Fiiod

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Posted 21 April 2018 - 06:23 PM

Would anyone here happen to have a resource or insight on this?

I feel like I SORT OF get the math, but then I see some numbers which go against my assumptions so I just want to make sure I fully understand the math behind it all.

If you don't know what I'm referring to, reading these:

An issue I always end up running into is buying a gel with a color I like, but then it isn't nearly as strong as I wanted it to be so I have to double or quadruple up on it.

Thanks!

Edited by Macks Fiiod, 21 April 2018 - 06:25 PM.

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### #2 Stuart Brereton

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Posted 21 April 2018 - 09:33 PM

Color saturation with gels is connected to exposure. If you put a gel on a lamp, and then expose that lamp at key, you'll see very little saturation. You have to underexpose in order to see the color, which is in effect, what you are doing by doubling the gel without altering the exposure.

In terms of reading the data sheets, remember that gels are subtractive. A bright red gel doesn't add red light, it subtracts everything else.

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### #3 Gerald King

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Posted 27 April 2018 - 02:10 PM

Color saturation with gels is connected to exposure. If you put a gel on a lamp, and then expose that lamp at key, you'll see very little saturation. You have to underexpose in order to see the color, which is in effect, what you are doing by doubling the gel without altering the exposure.

In terms of reading the data sheets, remember that gels are subtractive. A bright red gel doesn't add red light, it subtracts everything else.

So this means, Lighting (down), gel saturation goes (up)? Basically?

EDIT: also what do you mean bright red gel doesn't make the light red, what does? Is this just for in terms of reading the sheets?

Edited by Gerald King, 27 April 2018 - 02:11 PM.

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### #4 Stuart Brereton

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Posted 27 April 2018 - 06:27 PM

what do you mean bright red gel doesn't make the light red, what does? Is this just for in terms of reading the sheets?

That's not what I said. I said a Red gel doesn't ADD red light. Gels are subtractive, so they remove light frequencies. A red gel works by removing all the frequencies of light except the red end of the spectrum. That's why heavy color gels lose so much in terms of light levels, because they are cutting so much of the visible spectrum. If you look at the transmission graphs above, you'll see that the Lavender gel is transmitting about 40% of the blue/purple spectrum, 30 % Red, and virtually nothing of Green & Yellow.

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### #5 Adrian Sierkowski

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Posted 27 April 2018 - 07:59 PM

Another thing to note; unless you're working with rather broad spectrum lights, you can't be certain what color the gel will be-- for example, if you put a red gel on a pure blue light-- well you'd have basically next to nothing.

this is one of the reasons gels don't perform as well on cheaper LEDS (the modern stuff is ok) and I find all the "yellow" gels look rather crap on HMIs.

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### #6 Toni Vucic

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Posted 14 May 2018 - 03:34 PM

I was looking at this today.

I really don't understand the metrics on the right.

What is transmission Y?
What is "x" and why is it that value?

What is "y" and why is it that value?

What does absorption mean? I thought 0.7 would be "absorbs 70% of the light", but then the gel on the left has absorption 2.1??

I've had math, I know how to read a graph, but the numbers make no sense. Like, if x = 0.509 on the magenta graph means the middle of the x-axis. Then reading the y-value on the graph for that x reads 0. Yet it says y is 0.234.

Edited by Toni Vucic, 14 May 2018 - 03:40 PM.

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### #7 Gregg MacPherson

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Posted 15 May 2018 - 02:57 AM

Without knowing the definition of x,y it is confusing. It may be defined somewhere and we all missed it. Or so obvious we are supposed to just get it. I asked on the Lee help page and invited them to send someone to enlighten us.

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### #8 Gregg MacPherson

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Posted 17 May 2018 - 12:45 AM

I got an email back from Adrian Marsh, Technical Development Manager, LEE Filters UK.........

"Thank you for your enquiry regarding the x and y chromaticity co-ordinates that we list for our lighting filter products.

A system was developed in colour science some time back by the Commission International Eclairage (CIE) as a way of expressing colours mathematically – I won’t go into the details of how it was developed or the calculations involved, but the result of their work was an x-y graph and curve on which theoretically, all colour can be expressed. (The calculations also take into account the light source being used (Tungsten, Daylight, etc.) to observe / measure the colours as this obviously has an effect on how the colour is viewed / perceived.)

When we manufacture our products, the individual colour spectrums are measured using a spectrophotometer and the results are fed into a calculation which produces the x and y chromaticity values. We can then plot these values on our own small reproduced section of the CIE graph, together with tolerance boxes, to ensure that the colours we produce are consistent. The x and y values we list in our literature can be used to distinguish different colours from each other.

I have attached an image of the CIE 1931 chromaticity space which I hope illustrates what I have been trying to explain – there are also some notes on the diagram itself which may help with the explanation.

I hope this helps with your enquiry – let me know if you need any further information.

Best Regards
Adrian Marsh – Technical Development Manager, LEE Filters UK"

One can read a bit about chromaticity and the meaning of the x,y values here...
https://en.wikipedia...ki/Chromaticity

His graph is looks the same as the one linked to, withh added notes. I'll try and attach that...maybe in another post

EDIT: fixed the formatting in my added text...

Edited by Gregg MacPherson, 17 May 2018 - 12:53 AM.

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### #9 Gregg MacPherson

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Posted 17 May 2018 - 12:52 AM

Adrian's graph...try again to attach that...

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### #10 Toni Vucic

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Posted 20 May 2018 - 09:02 AM

Thank you Gregg (and Adrian Marsh), with the attached graph that makes sense.

I still wonder about what Transmission Y and Absorption means in this context.

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