GH4 GH4 as a color temp meter?

W

wwjd

Well-known member
tell me if this idea is all off whack:

Finding my lights KELVIN color using GH4's white balance.

I have about 12 different lights: CFL, Flourescent, LEDs, fresnel HMI... without buying a $1500 color meter device, I think I can get "in the ballpark" using the GH4 White Balance?

point the light and camera at a nice white or gray wall.
set GH4 to AUTO WHITE BALANCE, note the color mentally in LCD
switch to WB K scale and move it up or down to best match the AUTO'd WB
Usually, I have to switch back and forth K to AUTO and make sure I am close: not blue, not yellowish
Then the K Scale reads what the light is putting out.

Close enough?

Alternatively, I could manually set the WB - but that is kinda the same as AUTO, just fixed, right?

I bought a light that said 3200-5600, but a review mentioned he tested the light and 3200 was really around 3650. He had the proper testing gear.
Anyway, I did my trick with GH4 and mine was around 36-3700 area, so I feel pretty good about the result.

Thinking I could do this with all my lights and label them so I know where they REALLY sit in the spectrum.

Crazy, or COOL?
 
I know this won't show me any color DISTRIBUTION from the light, just ballpark K color.
 
Paul F said:
White walls are often not white.

https://www.elledecor.com/design-decorate/color/advice/a3448/the-right-white-a-70889/
Click to expand...

true. this is just an example to get only a "BALLPARK" where I have no other gear to accomplish this.
I actually used a white plastic trash can, and got similar results to what the guy's proper measure gave him.

Also noticed that setting color via KELVIN scale did not "fix" the bad coloring of some LEDs, where as manual setting WB (and autoWB) did correct those curves.
 
White balance and color temperature only coincide for true black body radiators, I.E. tungsten filament lights or plasma sources like the HIVE. It is a two axis curve: red to blue x axis (color temp)and magenta to green (tint). The tint component will be off for non-black body sources.
 
Read it several times, but not sure what you are saying there, Razz.

We're stuck using GH4, Kelvin and White Balance, how does black body radaitors fit into that in real world usage? (I'm not all that technical, so maybe I am over looking something obvious?)
 
Light a grey card

Shoot a raw still

Take the image into a raw developer

Click white balance, the kelvin and mag-green shift will then be revealed.

Simples.
 
I wonder if the iOS / Android Wifi Apps you can get to control your Lumix cameras would provide the current Auto-WB Kelvin values?

That would be a solution?
 
Razz16mm said:
White balance and color temperature only coincide for true black body radiators, I.E. tungsten filament lights or plasma sources like the HIVE. It is a two axis curve: red to blue x axis (color temp)and magenta to green (tint). The tint component will be off for non-black body sources.
Click to expand...

Saying that it will not be accurate. Kelvin is the temperature of a black body radiator in degree K. When you heat that black body radiator to 5600 degrees K, you will get the spectrum of light that we associate with the 5600K settings in our cameras.

But when you take a light that may throw out some other bits of spectrum, or mixed lighting, that Kelvin number is no longer accurate.
 
wwjd said:
Also noticed that setting color via KELVIN scale did not "fix" the bad coloring of some LEDs, where as manual setting WB (and autoWB) did correct those curves.
Click to expand...

That's because there is no correction for any possible "tint" variation or bias, like old fluorescent lights that were really green. Setting WB with the Kelvin setting on cameras is just basically a variable preset, there is no way for the camera to correct for tint bias. Sometimes it can work, sometimes it doesn't. It just depends on the light source(sources).
 
wwjd said:
Read it several times, but not sure what you are saying there, Razz.

We're stuck using GH4, Kelvin and White Balance, how does black body radaitors fit into that in real world usage? (I'm not all that technical, so maybe I am over looking something obvious?)
Click to expand...

Kelvin temperature reference is based on the color output of a black block of tungsten carbide heated to specific temperatures. The same material used in tungsten filament lights. The color at any temperature is a property of the material. On cameras the Kelvin setting simulates the Red to blue axis color shift with increasing temperature but not the green-magenta tint shift that is part of the equation. This is a basic tutorial I recorded recently for some new Bolex users that illustrates the WB process in Resolve. You do the same thing when you adjust white balance on a video camera.

 
That is very cool, and I appreciate it, just not sure how it relates to using GH4 White Balance setting comparison to K setting to figure out light's color. :)
 
It is quite beyond me why there is no comment on turning the camera to raw photo mode for examining lights.

(in fact examining the full colour cast path including glass and the camera!)

A raw developer will give an excellent reading of temp and tint.
 
wwjd,
You're not too far off from a useful workflow.

The real absolute color temperature doesn't really matter to you, what matters is, at what setting will your camera render grey lit by your light as neutral? THIS is the setting that will save you time and increase your color quality.

So, how do you determine that?

1. While the advice to shoot a RAW still sounds good, the reality is that because of how the camera processes video versus RAW stills, the comparison isn't very useful. (I know, I've tried.) You really need to shoot your tests as video, with your preferred profile.
2. I could find no short cut other than to take my best guess, shoot, drop into my NLE, balance the shot noting how far off and in what direction. Adjust the in camera setting and repeat. Repeat as many times as required to dial it in perfectly.
3. Once I did get it right, I used a silver marker to note the color temperature and color bias right on the light itself. I did this for all my lights that I use as keys. Now I never have to guess at settings, and if I use a second shooter who is unfamiliar with my lights, all they have to do is look.
4. A vectorscope is your friend. Using it and Resolve, I was able to dial in a light in 4 or 5 tries.
 
PS. As others have noted, it's not just color temperature you want to account for, but also the color bias. For example, I have some powerful LED's that have a meaningful green bias. I now know exactly how to dial it out in camera.
 
I get the RAW idea, but I never use it as a still photo camera - just video mode.

RAW sounds like the excellent way to map out the lights though. Yes, I love to also mark out which ones have green tint and which magenta - so I better lean to deal with RAW :)
 
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