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Author Topic: Taming a Cheaply-acquired Light Booth  (Read 868 times)

lhodaniel

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Taming a Cheaply-acquired Light Booth
« on: January 29, 2018, 07:10:12 AM »

Hi guys,

I just got a GTI PDV2 light booth for a very low price. Unfortunately, it doesn't have a dimmer. According to my i1pro, I'm getting around 3000 lux ambient inside the box. My first attempt at matching required a monitor brightness of 200 cd/m2 and it really might need to go a tad brighter. I have a friend who is a retired EE (I'm Met E) and I will explore adding a dimmer. The lamps are the same between the two models, so there is a chance the ballast etc in this one is compatible. Another meatball cure I thought of is to use t8 tube ND filters available from Rosco and others.

Do any of you have experience on how much ND might be needed to get my monitor down in the 120-160 range? Is lux linear with respect to EV or stops? Has someone come up with a formula or ROT correlating monitor brightness to booth brightness? TIA.

Lloyd
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aaron125

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Re: Taming a Cheaply-acquired Light Booth
« Reply #1 on: January 29, 2018, 07:42:58 AM »

Just a quick answer about stops/EV values and lux, etc.

It's really very simple in that each stop is exactly 1/2 or double the light output.

For example: if you've got a reading of 200lux or cdm, one stop reduction would equal 100lux, as in 1/2 the original. One stop increase is therefore 400lux. So, f2.8 is allowing exactly 1/2 the light to get to a sensor/film as compared with f2.0.

That's why it's a very easy system to get one's head around. And therefore, f4.0 is exactly 1/4 the amount of light as compared to f2.0.

Hope this helps somewhat. Btw, what CCT (correlated colour temp) is your light booth reading as per your i1pro evaluation? And what program did you use to obtain said readings/evaluation? Did you use i1share or say, BabelColor PatchTool/CT&A or something else altogether?

Also, what Rev. letter is your i1pro, as in Rev. B, Rev. D, ect?


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aaron125

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Re: Taming a Cheaply-acquired Light Booth
« Reply #2 on: January 29, 2018, 07:44:21 AM »

Just FYI: the revision of your i1pro is printed on the underside of your i1pro device.


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lhodaniel

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Re: Taming a Cheaply-acquired Light Booth
« Reply #3 on: January 29, 2018, 09:06:09 AM »

Thanks, Aaron. I know about f-stops and exposure. (I've been doing this for 40+ years, though this might be shaping up to be a senior day. ;D) I thought that 1 EV down would halve the lux but I wanted confirmation. What I don't know is if there is a correlation or rule of thumb equating display brightness in cd/m2 to ambient light measurements. Since 200 cd/m2 (lux) is a near perfect match for 3000 lux in the booth, it clearly isn't 1:1. I doubt that halving that to 1500 lux would drop the monitor requirement all the way to 100. If it did, I would be surprised.

I used Spectraview II for the measurement because I already have it on my system. The booth checked out at exactly CCT 5000K and is a visual match to the monitor at D65. i1Pro Rev. D with ambient light diffuser

Lloyd
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Doug Gray

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Re: Taming a Cheaply-acquired Light Booth
« Reply #4 on: January 29, 2018, 09:37:02 AM »

Thanks, Aaron. I know about f-stops and exposure. (I've been doing this for 40+ years, though this might be shaping up to be a senior day. ;D) I thought that 1 EV down would halve the lux but I wanted confirmation. What I don't know is if there is a correlation or rule of thumb equating display brightness in cd/m2 to ambient light measurements. Since 200 cd/m2 (lux) is a near perfect match for 3000 lux in the booth, it clearly isn't 1:1. I doubt that halving that to 1500 lux would drop the monitor requirement all the way to 100. If it did, I would be surprised.

I used Spectraview II for the measurement because I already have it on my system. The booth checked out at exactly CCT 5000K and is a visual match to the monitor at D65. i1Pro Rev. D with ambient light diffuser

Lloyd

On a perfect, diffusively reflective white surface (Lambertian), the cd/m^2 level of luminance is exactly the Lux level of illuminance divided by Pi. This is where the 160 cd/m^2 comes from using a 500 Lux viewing booth.


A few notes:

The best white matte, and you should use a matte, paper I've found is Canson Rag Photo. It has a L* of about 97 and is evenly reflective at nearly all angles. It reflects about 91% of the light that hits it and quite evenly at all angles which is required for the formula above to apply.

When illuminated with 500 Lux, it will have an illuminance of about 140 cd/m^2. This is what you get when you use a monitor set up to 160 cd/m^2 and use soft proofing with "show paper white" using any Canson Rag Photo profile. The monitor's white will be adjusted downward to match the degree the paper's white is not perfectly 100% as this is part of the profile.

However, spectrophotometers are not very accurate when sensing illuminance. The calibration that applies to measuring reflective light against their white patch is very good because the reflectance characteristics are known and stable. When using a spectro to measure illuminance there is no well defined reference. Colorimeters and spectros can and do easily vary +/- 20% in light level or so from device to device and you have to spend serious money to get one that measures more accurately. Especially as they get older. They are more consistent measuring CCT and CIEXY coords.
« Last Edit: January 29, 2018, 10:37:26 AM by Doug Gray »
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TonyW

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Re: Taming a Cheaply-acquired Light Booth
« Reply #5 on: January 29, 2018, 01:13:57 PM »

On a perfect, diffusively reflective white surface (Lambertian), the cd/m^2 level of luminance is exactly the Lux level of illuminance divided by Pi. This is where the 160 cd/m^2 comes from using a 500 Lux viewing booth.


A few notes:

The best white matte, and you should use a matte, paper I've found is Canson Rag Photo. It has a L* of about 97 and is evenly reflective at nearly all angles. It reflects about 91% of the light that hits it and quite evenly at all angles which is required for the formula above to apply.

When illuminated with 500 Lux, it will have an illuminance of about 140 cd/m^2. This is what you get when you use a monitor set up to 160 cd/m^2 and use soft proofing with "show paper white" using any Canson Rag Photo profile. The monitor's white will be adjusted downward to match the degree the paper's white is not perfectly 100% as this is part of the profile.

However, spectrophotometers are not very accurate when sensing illuminance. The calibration that applies to measuring reflective light against their white patch is very good because the reflectance characteristics are known and stable. When using a spectro to measure illuminance there is no well defined reference. Colorimeters and spectros can and do easily vary +/- 20% in light level or so from device to device and you have to spend serious money to get one that measures more accurately. Especially as they get older. They are more consistent measuring CCT and CIEXY coords.
Useful info, thanks Doug
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lhodaniel

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Re: Taming a Cheaply-acquired Light Booth
« Reply #6 on: January 29, 2018, 01:29:49 PM »

Ditto on the very useful, Doug. Thanks!

Now that I know to use reflected measurements instead of incident, I re-measured. The paper I used was Epson Enhanced Matte as it was on hand. I might get some of the Canon Photo Rag to play with. With the i1 using SV, I'm getting 500 lux on the bottom shelf, but about 750 lux midway on the back wall. I get similar readings using a Spyder 5 with both SV and the Spyder software. So, it looks like 1/2-stop filter to drop to about 160 and full stop for about 120. I'm ordering both.

If the spectro and the colorimeter aren't the way to measure this, what is? A spot light meter?

Lloyd
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