Can I Use a Digital Camera To Find 6500K? (1 Viewer)

[Martin Rendall]

The idea is to use a quality digital camera and expensive image editing software, along with Avia, to find an approximation of 6500K. Specifically, for displays which have a temperature adjustment, or access to the appropriate parameters for adjusting temperature.

First I'll describe my proposed technique, and second I'll put some caveats on what I really expect to achieve. Then you lot (especially the ISF techies here) tell me whether it will work.

1) put a 70IRE uniform test screen up (Avia has such a test, if I recall correctly).

2) take a raw picture with Digital SLR camera. Import into Photoshop (or in my case, Phase One), set the white balance to 6500K, and eyeball true white on my 6500K calibrated screen. Or just use the eyedropper idea to see what temperature the result is

3) adjust temperature up or down on the display device and repeat step 2) until happy.


What I hope to achieve:

A poor mans temperature calibration. An ISF tech can build a 6500K result under a variety of "IRE"s. This method will not. But, it will at least get the image somewhere closer than before.

Thoughts?
Martin.

 

[Shawn Perron]

Well, I'm not sure about the rest of your plan, but the avia ire windows can't be used for grayscale adjustment without unplugging the blue and red component cables. There was an authoring error, and color information crept into the window patterns.

 

[Martin Rendall]

Shawn,

Great info! I can do that.

Martin.

 

[Seth_L]

You can do it with a DSLR.

I did it with mine and now it looks great.

You probably need to make adjustments in the service menu though.

 

[ChrisWiggles]

No you cannot do this. I believe this was discussed thoroughly in one of the forums, I will try to find the discussion for more explanation.

As for the Avia error, it is a very very minor error, and certainly steps to remove the error won't hurt, but I'm not sure it will affect anything anyway.

 

[ChrisWiggles]

I hesitate to link to an outside forum, however the discussion is very long, and fairly thorough, so for those who are interested:

http://www.avsforum.com/avs-vb/showt...0&pagenumber=2

This is Guy Kuo's response in that thread, for those who are a bit lazy to read it all:

"1. The gray card which Kodak produces does not have the same color independent of the light illuminating it. What is special about the card is that the pigments used are such that it usually does not alter the color of the light reflecting off of it. In other words, its color varies with the color of the incident light. You cannot assume it is always glows at a magical D65, 5500K or other color of white. You must also illuminate it with a D65 light source.

2. Outdoor sunlight is not D65. Certainly NOT direct sunlight. D65 is roughly a mix of northern skylight + direct sunlight, but isn't that exactly. It is defined as a specific mix of red, green, and blue stimuli to the eye.

3. Every digital camera includes color filters which are used to separate out the three colors in a scene. Unfortunately, those three color separation filters are not necessarily the same from camera to camera. The differences in filter response are especially troublesome when you deal the the spikey spectral output of digital projection. CRT's aren't as bad but the red phosphor is also spikey in spectral output. This means that the same ratio of RGB won't be seen if you change camera brands.

4. Most cameras white balance the scene. This will also vary the ratios you read of red, green, blue. You may be able to store a manual white balance. That WOULD be useful to teach the camera the ratio which is D65 if you also have an accurate D65 light source to serve as your standard illuminant.

5. 6500K is not D65. D65 is slightly off the Plankian curve but near 6500K. The correlated color temperature indicated on a fluorescent bulb as 6500K doesn't mean it is the right color. It's possible to have a CCT of 6500K and be considerably too green or too magenta. You'll get quite close to the right color if you have a bulb which has a phosphor set designed to match D65. The GretagMacbeth bulb in the IdealLume lamp is probabaly the closest to the right color you will find. Your local pet store's bulbs even though claimed to be 6500K with a high CRI is likely still off the mark. Then again, accuracy may not be that important to you if you are actually pursuing doing grayscale this way.

6. Digital cameras which are "calibrated" at the factory aren't set for shooting under D65 lighting. They are set up to work under daylight, flourescent, etc. so you can't expect RGB to be equal when the photographed image is D65.

It all boils down to needing at least one accurate source of light which is the right color so you can obtain the initial ratios which indicate D65 in your camera. Without that reference, you're defining things in a circular manner.


__________________
Guy Kuo"

***

"the camera idea can work, but you HAVE to get a reference light and shine on a Kodak gray card and VISUALLY make gray on your display match the color you see on the card. Then teach the camera that ratio which is now coming off your display as being the desired color. The spectral selectivity of the color filters in the camera would then be compensated for by the selectivity of your vision (which is what the CIE sensitivities in a color meter mimic)

You can probably demonstrate for yourself the problem of simply letting the camera learn a light as the right color of white. Make your laptop, your projector and D65 illuminated gray card all show grays which are visually identical. Then take a picture which includes all three. I'll bet you that the camera won't see all three as having the same color even though it looks right to your vision. By the inverse token, things which the camera sees at the same color won't necesarrily look the same color to a human.
--Guy"

 

[ChrisWiggles]

I'm a little bit confused by Martin's method, but the proper way to do this is to have a proper D65K light source and gray card, and by eye match this on your set. THEN utilize the camera to get that specific camera's reading from your set. Then you can use this to get your set to track correctly across the spectrum. Camera's don't see the way we see. They will see some things that look the same to us, as different, and some things that look different to us, as the same, due to the distribution of the light spectra (spectral power distribution). If you take different displays, that were previously calibrated to D65 correctly, your camera will likely read differently on them, due to differences in the SPD, and the camera's response. Different cameras will also respond quite differently in the way they "see."

That's the laymans version, if Guy's paste is a little out of context or whatnot. But read through that whole thread carefully to understand best.

 

[Martin Rendall]

ChrisWiggles,

Thanks for the great store of information. I will read it through when I get the spare hours. In the meantime, there is one critical difference between Guy Kuo's comments and my own: by using the raw mode of the camera, it won't try to colour correct. That is, there is no need to "teach" the camera 6500K. Instead I would be relying on the raw conversion software of choice (in my case Phase One's, C1 Rebel) to tell me what temperature the white in the raw picture is. This software has a colour profile for the camera which I assume corrects for any specific aberations of the camera implementation.

But much of his other comments are definitely noted. Not necessarily understood, but noted.

But the point is to find some way to get close to 6500K, not ensure accuracy. If the choice was between doing this which could get close, or adjusting it until you think the white look red enough....

Martin.

 

[Seth_L]

Chris,

You can do it with a DSLR because you can control the white balance, (and overide it if you want).

I can take my Canon and take a picture and force it to 6500K and then look at the spectral content and see which color is dominant or lacking.

With care you can certainly get a much better image out of the TV after tweaking it with a DSLR than before and ultimately that's the whole idea of calibration. My set had a nasty blue cast at low IRE in the blacks. That's gone and now I have much more consistent color across all IRE values. It might not be exactly D65, but it's close.

 

[ChrisWiggles]

The key here, is that you're using an approximation that does not respond to light the ways our eyes do. I'm not capable of explaining this in the way that Guy has in that thread, but if you read through all his posts very carefully (many of these ideas were brought up), I think you'll understand that you could get accurate D65 calibration with a camera, but you need to have YOUR set already set to D65 at one point, to figure out how the camera itself responds to that particular SPD, even in raw data.

 

[ChrisWiggles]

Without a D65 calibration of your set with a reference to match by eye, you can really only make your set have linear grayscale, but cannot place that at D65 accurately. You can get close by eye I'm sure, but you can't precisely hit D65.

 

[Martin Rendall]

ChrisWiggles,

Granted. But I still think that it will be better than my eyeballing it. I guess I'll do two things: first, I'll read the thread over. Second, I have an ISF calibrated RPTV, so I'll take a raw picture of the screen and see what the software thinks the temperature is. If nothing else, it should disprove the methology if the temperature reported (by the software) is way off.

Also, I may be wrong, but I think the colour profile for the camera is specifically used to "correct" what a camera sees to what a person sees. However, I will need to see it working to believe it.

Martin.

 

[ChrisWiggles]

There's but one minute and counting til the daily show, so I'll only point you back into that thread for some descriptions as to why camera's dont respond like our eyes. and why it would need some sort of calibration file for a particular display type as well to track correctly, which is what colorimetry equipment usually has. off to the tv!

 

[Seth_L]

Sorry but I just don't agree with you. The cameras are set up to interpret color the same way out eye does or you'd never get prints that are the correct color.

I can take a picture of something and print it on a calibrated printer and then compare the picture to the real scene and the color is extremely close or dead on. If the camera wasn't compensated to see color the same way out eyes do the color results wouldn't match the real scene.

I think the bottom line here is that a DSLR will get a TV much closer to D65 than trying to eyeball adjustments on a set without a baseline.

 

[ChrisWiggles]

Seth: you're missing the issues of how the camera will respond to different spectra of light, which will very unlikely match our own response especially when it comes to matching our interpretation of the specific wavelengths of light output by various displays. If you're talking about ACCURATE grayscale, you can't use the camera as the baseline. I'll bet (with no experience trying it) that you can get close to D65, but whether or not you'll be better there than by eye I don't know. You should,however, be able to judge how linear your grayscale is, but not precisely where in color temperature it is.

"that wouldn't allow the camera to set your projector to the same color. It could only do so if the color filtering in the camera matched the CIE observer curves exactly. They don't or at the very least would be very unlikely to be an exact match. That means colors which look identical to the camera won't necessarily look identical to a human. While doing the above would be perfect for adjusting your projector so your camera maximally enjoys the picture, the human observers will find the grayscale and color a bit off."

"We'd still have a problem with the spectral power distributions of my phosphors not exactly matching the ones in your projector. The SPD interacts with the filter set in the camera to produce the energy levels seen by the camera. Unless those filters in the camera are unusually good matches to the CIE observer sensitivity curves reading my display won't solve the problem. It's a problem with two unknowns, both of which vary (the SPD of the display and the camera filter responses). Unless the filter response is a perfect match to the CIE observer curves or you have correction matrices for that filter set for each type and age of display you can't use the camera to establish your standard color. You CAN probably use it to verify tracking after you fix the standard color white visually against a known, correctly colored D65 light source."

Those two quotes are more from Guy.

And this is from the makers of Colorfacts:

"We actually went down that road...all the way.

We made a prototype software tool that used a tripod-mounted digital camera, which was automated via the USB port to take digital photos of known reference test signals. The photos were downloaded to the computer (automatically), and then were analyzed/averaged pixel-by-pixel to determine how the actual output correlated to the expected output.

When we corrected the software for the camera, the prototype worked reasonably well...for that single camera. It was a tremendous amount of work to get a system that worked with a single digital camera. However, when we tried the same system with 2 other cameras of different brands, it failed horribly...and it wasn't just the automation, but the digital photos themselves varying substantially with regards to color values with the same exact image presented in the same exact environment.

There are CCD-based standard-observer-balanced accurate colorimeters from Radiant Imaging, but those start at $40K and go up from there. Not exactly a typical digital camera. Other than that, a digital camera's CCD filter set would really need to be calibrated to the spectral output of the bulb (or electron gun) of the display device (just like any colorimeter) to work. There isn't a shortcut for this step if accurate colorimetry is the goal.

I would suggest that calibrating with Avia and using just your eyeballs would be preferable to trying to incorporate a digital camera into the mix. Plus, it is much less expensive and will probably get you "close enough" to make you happy."

 

[Leo Kerr]

There is another possibility on a 'home DIY color correction scheme.

I'm thinking this out as I type, so realize that there may be a better way of doing this.

This will get you into the ballpark of your target color temperature. It won't be exact, but realize that one of the best things you can do with your eye is color discrimination.

Do you have a slide projector? One of those old clunky Kodak Ektagraphic IIIs or anything like that? How about a way that you can illuminate a part of of your screen with a halogen light source?

Here's the game:

Take your projector, feed it a suitable gray or white signal onto part of the screen. How you mask off the other part of the screen is relatively unimportant; a reasonably 'crisp' shadow line is most desirable.

Now, take your halogen light source, give it a shadow that matches the one on your projector, so that you have the two different light sources butting right up against one another.

Now, the projector (on the left) should read as much 'bluer' than the halogen light (on the right.) This is normal. Depending on your line voltage and the specifics of the lamp, your halogen lamp is putting out something in the range of 3000-3200K.

Now comes the tricky part. Lee makes a variety of color correction filters that are suitable for film-use. Using a chart from Lee, you can find out what sort of Mired shift you need; I believe a Lee 204 will get you to ~5000K. (It's either 204 or 201; I think it's the 204.) Add in a touch of Lee 208 or so, and you should get close to 6500K. (In case you didn't notice, I'm holding the filters up in front of the halogen lamp.)

Now the projector might look a little green, and the lamp look a little... something. Reposition the lamp to get the brightness of the two 'images' to match again. Those color correction filters will knock a bunch of light out.

Now you go into your projector's color controls and begin to adjust. When the edge between the color-corrected halogen gets close to indistinguishable from the projector, then you're close.

How close?

Well, this is tricky. I'd ballpark estimate that you can get to your target D6500K +/- 500K. Maybe closer; maybe not. Depends a bit on what your lamp ultimately puts out.

For the purposes of this test, consider that Lee has a 'Cinematographers Swatch Book' - it has the technical filters (including other color correction axies,) and the swatches are roughly 3"x3". Convenient for putting in front of the slide projector. If you can't get that size, and can only get the 1"x3" book, well, those can be cut down and dropped into a slide mount fairly easily.

I recommend Lee Filters. I've done this sort of color game work with them before. Rosco, Apollo, GAM, and numerous other people also make color correction filters; your milage may vary.

Leo Kerr

 

[Martin Rendall]

Leo Kerr,

That's nuts! There's clearly more to all this that I would ever have imagined.

Martin.

 

[Leo Kerr]

The thing is, if you can see two things side-by-side, simultaniously, you can distinguish a very subtle variation.

This isn't a perfect solution; in fact, I haven't done it myself, either, but two things could come of it:

1: you might get fairly close (+/-500K) to your target color, and
2: even if you don't make 6500K, you'll be a whole lot closer to being on the black-body curve.

Leo Kerr