From: dannyrich@aol.com (DannyRich) Subject: Re: contrast of color Date: 04 Oct 1999 00:00:00 GMT Message-ID: <19991003215323.12389.00000407@ng-ff1.aol.com> References: <37F4DD62.3AABA5D2@philabs.research.philips.com> Organization: AOL http://www.aol.com Newsgroups: sci.engr.color X-Admin: news@aol.com There are two problems with these comments to Sally's question. 1) CIELAB is not uniform and thus DE* = 1 does not have the same visual significance for red, yellow, green blue ,white, gray, black. Newer equations, such as CIE94 have attempted to derive local distortions of the CIELAB based on the position of the standard. The British Textile Society did the same thing with the equation known as CMC. CMC units tend to be smaller than CIELAB units. Thus a visual difference of 1 CMC unit might represent a difference of 2 - 3 CIELAB units. 1 CMC unit represents an acceptable commercial tolerance but larger than a just-noticeable-difference. In my own PhD research, observers will able to detect a 0.5 CIELAB unit difference about 70% of the time. Since an MPCD or JND is defined as the the difference that is detectable 50% of the time, by high chroma yellow and blue centers will still above the JND level at 0.5 units. 2) The reason for the large variations in reports of what level constitutes a JND in object colors stems from the fact that visual threshold depends on more than just the color difference. The CIE94 and CMC equations also contain variables for "parametric effects" such as surface texture, illuminance level, separation between color specimens, surrounds, backgrounds, etc. My PhD specimens (and the CMC and CIE94 experiments) were performed on neutral gray backgrounds and surrounds with illuminance levels of more than 1000 lux. The specimens subtended at least a 10 degree visual angle. the CMC specimens were threads wound on a card, my specimens and those of CIE94 were automotive paints on flat, smooth substrates. Work reported by Tennessee Eastman on polypropylene plastic sheeting (pebble grained for automotive interiors) required parametric factors similar to the CMC equation but with a tolerance level of 0.4 instead of the 1.0 used in textile evaluations. That implies that for textured plastics, 0.4 CMC is still above the JND level. If I were to change the separation of the specimens from just touching / side by side to a 10 degree separation, over / under then the tolerances would be much larger. How much larger - I don't really know. There are some guidelines in a CIE report but I do not remember the specifics. Sally needs to define the viewing space more carefully before we can begin to guess a minimum perceptable color difference. >One intention of the L*a*b* formulation is that when you calculate: > > delta-E = sqrt((delta-L*)^2 + (delta-a*)^2 + (delta-b*)^2), > >a delta-E = 1 should be just barely percievable to some viewers. If you >want most people to be able to distinguish between two colors most of the >time, a delta-E = 3 is needed. Delta-E = 3 is called a MPCD (minimum >perceptible color difference) or a JND (just noticable difference) by >some authors. Keep in mind that the L*a*b* space is not a perfectly >uniform space, so the delta-E value that is percievable depends on >location in x-y space and the direction between the two colors. > >Using a delta-E = 1 threshold, M. R. Pointer (The Number of Discernible >Colours, Color Research and Application, 23:1 February 1998) calculated >that there are 2.28 million discernable color/luminance combinations. >Other studies (see references in Pointer) show that printing inks give >1.6 million colors and EBU phosphors give 1.35 million colors. These are >far less than the 16 million colors claimed by 8 bit/color video cards, >never mind 10 or 12 bit video systems. Obviously, some of these >color/luminance combinations are indistiguishable from each other. > >Hope that helps. > >IPLAB wrote: > >> Hallow !! >> I understand that to measure contrast between two objects in color >> picture, I can calculate the vector distance on L a b space (between >> the two objects). >> Does anyone can tell me what is the minimum of the Vector distance >> ,that eye can see ?? >> >> Thanks >> >> from sally > > > DannyRich@aol.com