Nathan nathan-at-amai.co.uk (Nathan Gaydhani)::22/7/04::2:54 pm:: GMT+0100 >Neil, > >I bow down to your experience in this area, we all learn in this >cascading flow of knowledge. thanks sorry if it sounded like a preach though
>I guess the only problem is where do you draw the line?, I am not sure >if I can justify the expense at the moment! I agree, it can get expensive getting things right. >You say that the eyeOne 'measures the spectral characteristics of any >actual ambient light'. What does that actually mean? you're going to have to do a google websearch for <spectral measurement> I think ;-) << CIE Standard Color Values: In 1931, the CIE defined the spectral value functions of the primary tristimulus values R, G, and B based on the empirically established characteristics of the standard colorimetric observer. In this system, each wavelength of the visible spectrum is associated with a combination of primary tristimulus values. To make these values easier to work with, the CIE specified a system of virtual primary tristimulus values. These are referred to as CIE coordinates X, Y, and Z. X corresponds to an imaginary red tristimulus value (that is, not existing in real life), Y to an imaginary green, and Z to an imaginary blue tristimulus value. The spectral values associated with these standard tristimulus values are referred to as standard spectral values, and the color values calculated on this basis are called standard color values. The standard spectral value curves (relative radiation intensity over wavelength) define the spectral sensitivity of the eye of the CIE standard colorimetric observer.>> Are you saying its looking for >a predominance of a particular colour temperature in much the same way >as our eye/brain? it's not about colour temperature, it's a much more complex description of a lightsource or printed patch than that, One can reach the same colour temperature in different ways. About instruments: (this reference mainly refers to measuring printed colour): << The Tristimulus Color Measurement Process is based on the idea that the spectral sensitivity of the three receptors in the human eye is well described by the CIE specifications. Accordingly, it is possible to measure color by means of three sensors whose spectral sensitivities are similar to those of the receptors of the eye. To illustrate this, imagine three color filters whose transparency corresponds to the standard spectral value curves if the sensor is ideal across the entire spectrum and light conditions are perfect. What makes the concept of the tristimulus process so attractive is the fact that only three measurement values must be established, which then can be directly converted to the CIE standard color values. In practice, the development of color filters corresponding to the exact characteristics of the relevant standard spectral value curves still presents some difficulties. The spectral radiation distribution of the light source used is a critical variable. It should correspond as closely as possible to the specified standard illuminant. Tristimulus colorimeters are only really perfect if all conditions within the device (lighting, color filter, sensor, reproduction geometry) ideally reflect the standardized measurement conditions. Only then will the simplification criteria for the direct derivation of CIE standard color values produce accurate values. Â The color values derived from the measurement are only valid under the prescribed general conditions (illuminant, observation angle, usually D50 at 2 degrees). The device establishes only the integral of the light intensity across the spectrum. By comparison, spectrophotometers establish the light intensity in small intervals, allowing it to be converted to other conditions in accordance with the CIE specifications. Broadly speaking, a tristimulus color measurement device is better suited for establishing color differences than for absolute color measurement. In practice, a spectrophotometer GretagMacbeth Spectrolino is recommended for accurate and more versatile color measuring.>> Quotes above in < > source GretagMacbeth. Once we have spectral measurements of a colour patch we can calculate how it will look under different standard lightsources. If we measure a lightsource (spectrally) we can then apply this lightsouce to the equation. If we have a camera profile (which illustrates how that camera sees known colours in a certain lighting) we can later recalculate the profile when the lighting changes. And that change has to be measured spectrally. Regards, Neil Barstow :: Apple Solutions Expert :: colourmanagement.net Consulting in Imaging & Colour Management, custom profiling, training, seminars implementation :: WE RESELL :- Gretag+eyeOne :: basICColor Squid, Display etc. XRite. :: GTI viewing booths :: Epson :: Proofmaster RIP + ISO Proofing solutions www.colourmanagement.net/ :: www.apple.com/uk/creative/neilbarstow/ =============================================================== GO TO http://www.prodig.org for ~ GUIDELINES ~ un/SUBSCRIBING ~ ITEMS for SALE
