How many levels to count on may depend primarily on what you expect to do with the medium, and what its capabilities are.
With regard to contrast in the brightness dimension, Ansel Adams based his zone system (http://en.wikipedia.org/wiki/Zone_system) on a comparison between the capabilities of film and print as perceived by the human eye. It's a highly empirical system. What's great about it is that it starts from the desired result and reasons backwards; also it can be used when making recordings in the field. (You might need a similar capability to eyeball a design and determine whether it's going to map well to the media and contexts in which you're going to deliver your interface.) When you look at a scene, the zone method invites you to determine what the brightest aspects and darkest aspects are that are important to bring within the range of your output medium. You identify what zone the brightest aspects of the scene are in, based on the scale applicable to the scene (Adams had a single scale for handling natural reality, which is the context he was observing). Then you identify what zone the dimmest aspects (that matter to the audience) are in. This gives you a range of zones. You map the range of zones to the range available in your output medium. For the system involving the eye of the photographer viewing the scene and the eye of the recipient viewing a photographic print, the range that the eye of the photographer can perceive is about 12 f-stops. The range that the negative and print can represent is smaller, in some cases about 7 f-stops. He observed that you have to control the range and level of the impression that the scene makes on the negative so that you don't lose the detail that you're hoping to capture. One way that Adams mapped zones was to pile up some zones at one end of the spectrum, since that is what image capture (on a negative, in his case) and print development can most easily do. To narrow the range of zones that the print had to represent, he used dodging, in which he waved a small card in front of portions of the scene that he believed were too bright. So to bring this back to your mechanical engineering team, the answer may be that the human eye is sensitive enough (to contrast). The question is what you want to plan for the human eye to perceive when looking at the device. It may be possible to analyze the design vocabulary and the capabilities of the device to represent both the range and resolution of variations in contrast in space and time. You may have to find a more pragmatic way to get a decent working model, but for reference, let's consider briefly an idealized model. Suppose you state the problem in terms of three variables: area of patch to perceive, size of brightness variation, and duration of transition. You'll need to determine how you want to divide this 3-dimensional space into distinguishable tiles. Assuming that you want the variation in a single dimension to be sufficient, you define (for example) the boundaries of a tile in the brightness dimension when, for a patch of the stated area, the variation from one boundary to another in the brightness dimension during the stated time interval is enough to make a significant difference. You are likely to find that time works inversely compared with the other dimensions: shorter times make it easier to distinguish a change. Once you determine the range and define the gradations (patch area, brightness, and duration of transition), your model can start to tell you which levels are sufficiently distinct (or sufficiently indistinct if you need a gradient) for use in your application. That's a good start for monochrome ... if you've got color, the minimum distinguishable tiles might be bigger for some colors than for others. Also, if you're planning to communicate by changing the shape of the indicator, that's yet another dimension. You'd have to determine whether that's relevant to your planning for the mechanical design. Best wishes, Bruce Esrig On Thu, Jun 5, 2008 at 12:29 AM, Tommy Keswick <[EMAIL PROTECTED]> wrote: > I brought this issue up to my dad who is an optometrist. He actually > contacted a colleague of his (a former Dean at the UC Berkeley School > of Optometry, in fact) who says that... > > "the answer is hugely different if you are talking about side by side > light patches where the eye is superb and can detect 1% differences > easily under photopic conditions) VERSUS judging brightness absolutely > (where the eye is notoriously bad at that because it adapts so well to > new environments, and there must go by memory as well). Finally if > talking about changes in time then it depends on whether its > instantaneous (excellent) or over seconds or minutes (very poor) > > So . . . it depends" > > Elise: Do you know if your engineers are... "referring to side by side > lightness (contrast) or judging absolute brightness (from memory)"? > > That apparently very much determines the answer. I might be able to > find out more with that. > > -Tommy > > On Wed, Jun 4, 2008 at 9:11 PM, Chauncey Wilson > <[EMAIL PROTECTED]> wrote: > > Hello Elise, > > > > > > There is a lecture by Psychologist Jeremy Wolfe from MIT about Sensing > > with a focus on visual issues including the collection of light. If > > you have iTunes search for Jeremy Wolfe and then find Lecture 4. The > > podcast is free and the lecture entertaining and informative. > > > > Jeremy has a booked called Sensation and Perception (edited by Jeremy > > and a few other colleagues). It has a section on Dark and Light > > Adaptation which discusses some of the factors that affective > > perceived brightness and intensity. The range of sensitivity is > > really pretty incredible from a small number of photos that can be > > perceived under complete dark adaptation to the light that ones sees > > when looking at white sand at mid-day in the desert. Wolfe discusses > > how the interconnections in the cells of the retina influence > > perceptions of brightness. My memory is a little weak on that. > > > > The sensitivity of the eye is affected by dark adaptation. If I > > remember correctly from my courses in physiological psychology, > > sensitive to light increases to a maximum after about 30 minutes in > > the dark. The sensitivity is related to the regeneration of the > > pigments in the cones and rods. > > > > There is some research showing that light sensitivity during dark > > adaptation increases more slowly with older adults which might > > contribute to night vision problems in older adults. > > > > The brightness of mono-chromatic light varies with wavelength because > > the photopigments in the eye have different sensitivities. This > > causes an effect called the Purkinje effect where different colors at > > equal light levels will appear to be different in brightness. See the > > Wikipedia entry which explains it pretty well: > > http://en.wikipedia.org/wiki/Purkinje_effect > > > > The perception of brightness is influenced by the context -- there are > > some illusions that illustrate this point where the intensity of a > > background influences the perception of the intensity of a stimulus > > embedded in the background. > > > > The best sources of information on perception of brightness and the > > internal and external forces that affect brightness (and a related > > measure called "lightness") would be general human factors texts. > > There will be sections on visual mechanisms that control brightness > > and external factors that influence the perception of brightness. You > > will find that measures of brightness are related to intensity by a > > power function. > > The basic HF texts should be a start for you. > > > > I really enjoyed courses on sensation and perception like this which > > explained why lime green might be a better choice than bright red for > > fire engines and why certain plants appear brighter or more intense in > > the twilight hours of the evening (the Purkinje effect). > > > > This is a good topic. > > > > Chauncey > > > > > > On Wed, Jun 4, 2008 at 3:09 PM, Elise Edson <[EMAIL PROTECTED]> > wrote: > >> Hello IxDAers! > >> > >> My team has received a request from the mechincal engineering team to > help > >> them understand how the human eye perceives relative brightness of > light. > >> Specifically, how sensitive is the human eye to changes in brightness? > >> > >> An initial literature search on ACM & Google did not yield results. Can > >> anyone recommend any articles or other resources, or does anyone know > about > >> related research on this topic? > >> > >> Kind regards, > >> Elise > >> ________________________________________________________________ > >> Welcome to the Interaction Design Association (IxDA)! > >> To post to this list ....... [EMAIL PROTECTED] > >> Unsubscribe ................ http://www.ixda.org/unsubscribe > >> List Guidelines ............ http://www.ixda.org/guidelines > >> List Help .................. http://www.ixda.org/help > >> > > ________________________________________________________________ > > Welcome to the Interaction Design Association (IxDA)! > > To post to this list ....... 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