From: Kent T. <ke...@sp...> - 2003-07-26 23:07:09
|
Howdy, Would it be possible to profile a monitor using a digital camera to measure the monitor's output? Thanks, Kent |
From: Gerhard F. <nos...@gm...> - 2003-07-27 22:23:35
|
Kent Tenney schrieb: > Would it be possible to profile a monitor > using a digital camera to measure the > monitor's output? Hi Kent, basically yes, if a) if the camera permits you to turn off all automatic functions b) if the camera is able to reproduce measurements with reasonable accuracy c) you first profile the camera for the spectral power distribution of the monitor phosphors you want to measure. However, in order to achieve (c) you need a colorimeter - and if you have a colorimeter aynway, there is no need to perform the monitor measurements with the camera. Here is some background: The major problem with using a scanner/camera as colorimeter is, that these devices are usually not "colorimetric", i.e. the spectral sensitivities of their sensors are not linear transformation of the CIE color matching functions. This means, a scanner/camera "sees" colors different than the human eye. It may happen that a scanner returns the same RGB values for two colors which look different for the human eye, and a scanner may even return different RGB values for two metamer colors which look equal for the human eye. This phenomenon is also called "scanner metamerism". Basically this implies, that it is actually *impossible* to establish a unique RGB <-> XYZ mapping, which is valid for arbitrary spectral power distributions. So, how is it possible at all to get scans with reasonable color reproduction? The answer is, it is only possible if we restrict the scans to a specific media type. This usually solves the ambiguity mentioned above, because for a particular media type the set of spectra which can be represented on the medium is very limited, and each CIE color represented on the meduim is usually associated with exactly one spectral power distribution. However this means, if you use an IT8 target on Kodak paper to profile your scanner, then the profile is only *only* valid for scanning Photos on Kodak paper. If you scan e.g. an inkjet print with the same profile, then you will unfortunately get incorrect colors. The amount of the error will depend on the scanner and the media used. (I don't have an instrument to measure it, but according to the large, clearly visible difference, I guess that my scanner gives me even differences of up to 10 dE units between photo paper (-> IT8 target) and an inkjet print with canon inks) And the same applies, if you want to measure the your monitor with a camera. It is only possible to do it accurately, if you characterize the camera for the spectral characteristics of the monitor phosphors. Regards, Gerhard |
From: Kent T. <ke...@sp...> - 2003-07-28 11:29:07
|
Gerhard, This is a very good explaination of the factors involved. Could someone with a colorimeter create a profile for a specific camera allowing it to be used for this purpose? Thanks, Kent Gerhard Fuernkranz wrote: > > Kent Tenney schrieb: > >> Would it be possible to profile a monitor >> using a digital camera to measure the >> monitor's output? > > > Hi Kent, > > basically yes, if > > a) if the camera permits you to turn off all automatic functions > b) if the camera is able to reproduce measurements with reasonable accuracy > c) you first profile the camera for the spectral power distribution of > the monitor phosphors you want to measure. > > However, in order to achieve (c) you need a colorimeter - and if you > have a colorimeter aynway, there is no need to perform the monitor > measurements with the camera. > > > Here is some background: > > The major problem with using a scanner/camera as colorimeter is, that > these devices are usually not "colorimetric", i.e. the spectral > sensitivities of their sensors are not linear transformation of the CIE > color matching functions. This means, a scanner/camera "sees" colors > different than the human eye. It may happen that a scanner returns the > same RGB values for two colors which look different for the human eye, > and a scanner may even return different RGB values for two metamer > colors which look equal for the human eye. This phenomenon is also > called "scanner metamerism". Basically this implies, that it is actually > *impossible* to establish a unique RGB <-> XYZ mapping, which is valid > for arbitrary spectral power distributions. > > So, how is it possible at all to get scans with reasonable color > reproduction? The answer is, it is only possible if we restrict the > scans to a specific media type. This usually solves the ambiguity > mentioned above, because for a particular media type the set of spectra > which can be represented on the medium is very limited, and each CIE > color represented on the meduim is usually associated with exactly one > spectral power distribution. > > However this means, if you use an IT8 target on Kodak paper to profile > your scanner, then the profile is only *only* valid for scanning Photos > on Kodak paper. If you scan e.g. an inkjet print with the same profile, > then you will unfortunately get incorrect colors. The amount of the > error will depend on the scanner and the media used. > > (I don't have an instrument to measure it, but according to the large, > clearly visible difference, I guess that my scanner gives me even > differences of up to 10 dE units between photo paper (-> IT8 target) and > an inkjet print with canon inks) > > And the same applies, if you want to measure the your monitor with a > camera. It is only possible to do it accurately, if you characterize the > camera for the spectral characteristics of the monitor phosphors. > > Regards, > Gerhard > > > > > ------------------------------------------------------- > This SF.Net email sponsored by: Free pre-built ASP.NET sites including > Data Reports, E-commerce, Portals, and Forums are available now. > Download today and enter to win an XBOX or Visual Studio .NET. > http://aspnet.click-url.com/go/psa00100003ave/direct;at.aspnet_072303_01/01 > _______________________________________________ > Lcms-user mailing list > Lcm...@li... > https://lists.sourceforge.net/lists/listinfo/lcms-user > > |
From: Gerhard F. <nos...@gm...> - 2003-07-28 16:49:46
|
> Gerhard, > > This is a very good explaination > of the factors involved. > > Could someone with a colorimeter > create a profile for a specific > camera allowing it to be used for > this purpose? Kent, Actually you need *all three* items, the camera (which you want to use as measuring instrument), the monitor (which you want to measure) and the monitor colorimeter, in order to create the desired profile. The basic idea is to display color patches with well known monitor RGB values on the monitor and measure their XYZ values with the colorimeter. Additionally you use the camera to take a photo of these color patches on the monitor to obtain the camera RGB values for the patches (with all automatic controls turned off, using well defined manual camera settings). From these measurements you can now create 1) the monitor profile which establishes the mapping between monitor RGB values and XYZ values and 2) the camera profile which maps between the captured camera RGB values and the XYZ values. As you see (and as I mentioned in my previous mail), actually the camera is *superfluous* in this procedure, if your only desire is to obtain the monitor profile. So what we're discussing here is probably more theoretic than of practical use. Subsequent measurements (of course with limited accuracy) of *THIS* monitor could however now be performed with *THIS* camera, by applying the camera profile to the captured camera RGB values in order to obtain the XYZ values of the patches. Please notice again, the camera profile created in (2) is basically not valid for measuring any arbitrary monitor, but only for measuring *THIS* monitor (or a monitor which uses the same set of phosphors with the same spectral power distributions) with *THIS* camera. (And there are even more factors I haven't mentioned yet, which don't make life easier and reduce accuracy ...) Gerhard -- COMPUTERBILD 15/03: Premium-e-mail-Dienste im Test -------------------------------------------------- 1. GMX TopMail - Platz 1 und Testsieger! 2. GMX ProMail - Platz 2 und Preis-Qualitätssieger! 3. Arcor - 4. web.de - 5. T-Online - 6. freenet.de - 7. daybyday - 8. e-Post |