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New technique for DIY greyscale calibration using digital camera.  

post #1 of 15
10/1/05 at 7:23am
Thread Starter 
Hi all,

If you happen to have a digital camera and you can set its colour temperature like on Canon (and most other SLR type) digital cameras; then you have a tool which you can use to roughly calibrate your plasma (or other type) screen.

Essentially you set your camera white (colour temp) to 6500K (or other temperature for that matter if you find 6500 too yellow).

You then take a picture of your screen when it has a completely white screen. If the picture comes out with a bluish white then your plasma's white point (colour temp) is too high. If it comes out reddish or yellowish then the plasmas colour temp setting is too low. The amount of blue or yellow bias will give you an idea of how far away you are.

If it is in the ballpark the white should reproduce very close to a true white. This is crude in comparison to the next step which refines things much more.

If you own Photoshop CS2 as well as a (SLR) digital camera which can shoot in RAW mode (most do) you can do even better! Photoshop is able to open raw picture files in a utility called "Camera Raw". With this utility you can actually click on the white on the plasma screen in the picture you took and it will measure the colour temp of the white in question and give you a figure to work with.

If you don't have RAW mode on your camera you can still use the EYEDROPPER tool in Photoshop to help you get the right adjustments real quick. Simply use the dropper tool to analyse the white screen area of the JPEG picture you have just taken. You can examine the R,G,B values of the white area in question. Try several areas in the screen image and note the R,G,B levels that Photoshop shows you. When you calibrate the plasma you simply need to adjust the R,G,B levels relative to your findings with the eyedropper tool. Aim to make the R,G,B levels that are read by the eyedropper tool equal in the white area.

Eg, If the eyedropper tool in Photoshop shows your white screen area to have R: 230 G: 220 B:200. Then it means you'll have to increase the green and blue on your plasma settings.

Take another photo with the new adjusted settings on the plasma and examine the result with the eyedropper tool and adjust accordingly.

SO there you are. You can do your own calibration at home without anything more than your digital camera and Photoshop!

As far as I know this is the first time this has been described to calibrate a plasma or other AV display (for which it would work too).

Hope that gives others some ideas to extend this. Any questions welcome.

Regards,
Ben





It's not a substitute for a colorimeter; but the cameras white balance should be very close to the mark. I've confirmed this with my EOS 20D and its 6500K setting. My monitor is calibrated but the technique does not require a calibrated monitor as the eyedropper tool will analyse the jpeg file independent of how it appears on your monitor.

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post #2 of 15
10/1/05 at 8:20am
This has been tried by numerous people with very limited success. I have had much better success using an optical comparator along with the following technique. Turn off blue and target a lemon yellow with green and red using a gray ramp. Then add blue until the ramp looks gray while being careful not to add to much blue. You can also try and match the comparator across the entire ramp. I am typically able to achieve a significant improvement in gray scale accuracy using this technique. It is not equal to a spectroradiometer, but it is frequently better than a filter based color analyzer. I have not had similar results using a camera.

I believe the major problem with the camera method is the non-uniform light spectrum found in these displays. Similar problems are found when photographing using various light sources with similar properties (i.e. florescent, mercury vapour...).
post #3 of 15
10/1/05 at 9:57am
No! Baaad idea. This is NOT an accurate way to measure grayscale off a display. A camera is NOT likely to have the proper filter curves to measure a display!
post #4 of 15
10/1/05 at 10:45am
Bill Cushman used to recommend using a Polaroid disposable, but I agree with Chris, I would not trust a digital camera without it having been calibrated first.
post #5 of 15
10/1/05 at 11:45am
Even with a calibrated camer, you could not use it on anything but one display. To be able to use a camera accurately to track a display's greyscale you would need to

1) accurately calibrate the display's greyscale using proper colorimetry equipment.
2) measure the greyscale off the display using the *specific* camera

Only then would you be able to use that specific camera on that specific display. It would likely be off on other displays unless they had the same spectra response, which is unlikely.
post #6 of 15
10/1/05 at 4:10pm
Thread Starter 
This is obviously NOT a substitute for a colorimeter as pointed out; but it is far better than by eye alone.

The yellow/blue crude assessment is obviously not as good as using Photoshop to analyse the jpg file from the camera.

Here are some more ideas regarding calibration of the camera.

If your camera does not have a 6500K setting; you may still be able to get it set to 6500K. This requires the ability to set a CUSTOM WB. In this case one would need a reference 6500K source - I would suggest a Daylight (6500K) balanced fluorescent tube as a cheap way to obtain this.

A photograph taken of a white sheet of Letter paper under a 6500K fluoro tube would give you the reference photo for the custom WB if your camera supports it (many do).


Once the camera is set then exposure control is the next real issue. What I would suggest as the best way is to take a photo of a window pattern 100IRE white (definitely available on AVIA and perhaps DVE). To meter the exposure make sure the camera has both the white area and the black area within the metering area. This should ensure a usable exposure; if the whites are blown out then the technique is useless.

It doesn't matter much what the exact exposure level is as long as the white you are photographing is not blown out.

The eyedropper tool then will analyse the white area in the image and give you the relative R,G,B levels that you can calibrate against.

This is a relatively simple technique. It's not going to be like a colorimeter but it's *much* better than by eye.

I may get some pictures together and prepare a step by step procedure once I can post pictures. The principles above should steer a reasonably competent and knowledgable person in the right direction.

Regards,
Ben
post #7 of 15
10/1/05 at 4:46pm
Quote:
The yellow/blue crude assessment is obviously not as good as using Photoshop to analyse the jpg file from the camera.
I do not necessarily agree. You're missing the point that the camera will not at all provide relevant RGB values based on observer curves related to the chromaticity of the display primaries. A camera is not a valid measurer of greyscale from a display unless you FIRST have measured that display's chromaticity AND the camera's response curves, which is FAR more complicated and expensive to do than get ahold of colorimetry equipment. As I said before, the *ONLY way to do this with a camera is to have calibrated a D65 point FIRST, and taken measurements off that screen with a specific camera.

If you take two photos with two different cameras off the same display screen, you will see that the photoshop RGB values will be completely different. Further, if you take pictures of two different, previously correctly calibrated displays with the SAME camera, you will STILL get two completely different RGB readings. You can use a camera as an optical comparator to get a linear grayscale value across the board, but you *cannot* use a camera to get to D65 without doing what I described first.
post #8 of 15
10/1/05 at 4:51pm
Quote:
A photograph taken of a white sheet of Letter paper under a 6500K fluoro tube would give you the reference photo for the custom WB if your camera supports it (many do).
GAH!!!!! First, this is NOT the same as D65. Second, the camera will respond very differently to the chromaticity of that light compared to a display!!!

Quote:
The eyedropper tool then will analyse the white area in the image and give you the relative R,G,B levels that you can calibrate against.
This is NOT correct. You will NOT be approaching D65 using this method. I mean, you may get as close as you can by eye, maybe more or less depending on your eye, but a camera will not give you relevant values for approaching D65.

Quote:
but it's *much* better than by eye.
No it really isn't, because you still have to use your eye at some point along the greyscale. You can ONLY use a digital camera as an optical comparator to get linear greyscale you CANNOT use a camera to find D65 on a display, unless you go through all the hassle I mentioned before, which requires using colorimetry equipment to calibrate your display to D65 first, in which case you've gone around the whole point of trying to use the camera in the first place!
post #9 of 15
10/1/05 at 4:54pm
Please do NOT continue putting this idea forth as a way of calibrating to D65 because it isn't. A camera can ONLY be used as an optical comparator.

Please see this relevant thread, postings by Guy Kuo:

http://archive.avsforum.com/avs-vb/s...0&pagenumber=2

Relevant Quotes:

Quote:
I'm cringing in my corner watching this go spinning off the road and into the drainage ditch.

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.
Quote:
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.
Quote:
You really should set the projector's gray to match the card and lamp VISUALLY first. Then use the reading from the screen and projector to establish the camera's reading for the correct color. Doing it off the card + lamp doesn't correct for the camera's tendency to see slighlty differently from human vision.
Quote:
quote:
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Originally posted by Lifter
.....
With a color corrected monitor, you could then use that to calibrate the digital camera, check it for RGB discrepancies, and adjust as necessary or at least recognize it's problems and compensate. Throw up a full-screen solid 155:66:141 (or whatever) image on your PC monitor, take a picture w/ the camera and check to see how close the numbers match.
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Unfortunately, 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.

Your best, low cost solution to grayscale is to obtain a kodak gray card and a very good reference light. By varying the distance between the card and lamp you can adjust the intensity. Provided you have normal color vision, your eyes are extremely good at picking out differences in two simultaneously viewed colors if they are at the same intensity. Hold the card so it obscures half of a grayscale window and allow the lamp light to strike the card but not the screen. You'll need something to shade the screen from the lamp. Adjust lamp to card distance until the light intensities match. Leave one channel alone (usually green) and adjust the other two channels to make the color of the screen and the card look identical. With practice, you'll find that you can pick up an error of single click of the gain controls. The darker end, visually setting cuts is tougher. Your color vision isn't quite as good at the dark end. Then again, that also suggests (gasp) that the very bottom end grayscale isn't quite as critical since you can't perceive problems there that well any way.

The camera idea seems a good one for looking at tracking. If you can establish the top end accurately using your makeshift "optical comparator," then the camera may allow you to get the dark end more accurately than by eye. It's a thought worthy of experimentation. Just make sure you get at least one good, correct ratio. That you'll have to do by visually color matching one high intensity window to a gray card illuminated with an accurate D65 source.
Quote:
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.
(I pasted a lot redundantly, because it was difficult to dig up that thread in the archives, and this should be a handy spot for these things anyway.)

This is also a relevant quote from Mark Hunter:

Quote:
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.


__________________
Mark Hunter
post #10 of 15
10/1/05 at 5:38pm
Thread Starter 
Chris,

Thank you for pointing out that this will not give one D65 which requires more complex measurements at different levels etc.

The technique I have described is not to get to D65 and never says so.

The technique is outlined to allow someone with modest equipment and existing software to get a better greyscale than what comes standard with the display.

My own experiment proceeded down the following line.

I have a Fujitsu 40US plasma panel. I set the Colour Temp to the +3500K setting which sets the plasma to Red: 194, Green: 217, Blue: 255.

I took a photo of a 100 IRE with exposure settings that don't blow out the "white", and with the camera set at its inbuilt 6500K setting.

This image when analysed with the eyedropper tool in Photoshop shows the white to consist of Red: 127, Green: 140, Blue: 158.

Here is the image which you can test yourself in Photoshop with the eyedropper tool...

http://img.photobucket.com/albums/v431/zuiko/Bluish.jpg

The next image is of my panel showing a 100 IRE white with the following settings for Colour Temp (obtained with Spyder2); Red:255, Green:228, Blue:167.

A photo taken with the same settings as above ie. f4.0 1/60s 6500K WB yielded a photograph that when analysed with Photoshop gave the following figures for the white area; Red: 137, Green: 132, Blue: 138.

Here is the photo which you can again test yourself...

http://img.photobucket.com/albums/v4...Calibrated.jpg

Note how the RGB figures with Photoshop in the second calibrated case show relatively even RGB figures indicating a purer white/grey. The first figures are showing way too much blue which would lead one to reduce the blue bias on their plasma settings and repeat the process.

Through iteration one can get as close as one likes to the ideal which is like in photo 2. I hope this is clear now.


Note that in the photos above there was an incandescent light present which is almost certainly affecting the results to a degree ie. pushing up the measured red. The best results would be with the lights turned off. I wanted to just demonstrate that the colour temp on both photos is at 6500K (camera setting) and not fudged and kept the light on to demonstrate this as seen through the bezel of the plasma. These are out of the camera JPGs scaled to 25% (for internet purposes).

It's not rocket science and is designed to get a better greyscale, not D65, or substitute for a colorimeter. Oh, and there's no Kodak gray card which is mentioned as an integral part of the quotes above.

Regards,
Ben
post #11 of 15
10/1/05 at 6:31pm
Quote:
Through iteration one can get as close as one likes to the ideal which is like in photo 2. I hope this is clear now.
I don't mean to nitpick semantics here, but there is a really important, and key point here that will lead people down the wrong path. You are *NOT* approaching an ideal using this method. You are no better or worse off than doing this by eye.

The SPD of the display, and the camera filters interact depending on different cameras and different displays.

Quote:
Oh, and there's no Kodak gray card which is mentioned as an integral part of the quotes above.
Which is something that is missing in your method. You CANNOT use a digital camera to approximate D65 alone, as I've said. Again, I don't know what color temperature you will be approaching, it may be roughly around D65, but it will probably be with about the same precision as doing it by eye. You can ONLY use a camera as an optical comparator, you CANNOT do as you suggest to approach D65 more accurately than you can by eye.

It is, in my opinion, dangerous for you to suggest this as people will read your statements and think that they can approach D65 using a camera to measure relative values or R, G, and B. This is not the case! The relative R, G, and B values you read from the combination of a specific camera filter set, and the SPD of a specific display do NOT relate to greyscale in the way that you want them to. If you are looking at RGB value in photoshop and B is a bit higher, that may or may not mean that there is too much blue. Take the same shot with a different camera, and your B value may be too low! There is no continuity between cameras or displays, and so you're right back to setting things by eye, because again THE fundamental basic point is that a digital camera can only be used as an optical comparator.
post #12 of 15
10/1/05 at 7:20pm
Quote:
Originally Posted by ChrisWiggles
...It is, in my opinion, dangerous for you to suggest this as people will read your statements and think that they can approach D65 using a camera to measure relative values or R, G, and B. ...
Even though the camera method described is flawed there is no "danger" in this. It is only a display for home use. It is possible that he is making things better than what he started with, but describing grays as being too yellow is not encouraging.
post #13 of 15
10/1/05 at 7:32pm
Jeff,
Wouldn't it be better just to use your technique and the description in the "How to eyeball" thread?

Eric
post #14 of 15
10/1/05 at 8:07pm
Quote:
Originally Posted by Ericglo
Jeff,
Wouldn't it be better just to use your technique and the description in the "How to eyeball" thread?

Eric
It is already listed in that thread by someone else. This is not a procedure I invented. It is just one that I have used many times that works well. Bill Cushman originally suggested it to me and I was able to come very close to D65 without any instrumentation. I was just trying to offer some assistance on gray scale methods. Since I have access to multiple spectroradiometers and color analyzers I thought someone might like to hear how I would do it if I did not have those tools available. I am also able to check what I can do visually with highly accurate tools (Lightspex, EyeOne Pro, Sony PVM-96), but if someone wants to venture out on their own it is fine with me.

I was just trying to point out that the camera method is not a technique that I have found useful or one that is particularly new either. I also own a Canon 20D so I have some experience with that device as a color instrument in particular.

It is not really in my best interest to instruct people on how to do an instrument free gray scale calibration since I sell these services, but I thought Ben might benefit from my experience.
post #15 of 15
10/4/05 at 12:15pm
Geez, I won't even use the Sencore CP288 professional colour analyser I have on anything other than a CRT-based display, and you suggest that using a plain ol' digital camera will be somewhat accurate?!

It might HELP, but with a colour analyser giving nearly instant readouts, there's tons of back and forth between hight/low levels to get a balanced grey scale. There's just no way you can do this with your technique.

It is, however, an interesting concept and I give you full credit for creativity!
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