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Does anyone know how to measure the color of a Post-It note?

  1. Feb 13, 2018 #1
    I'm not sure if this is the right place for this, but since i'm new here i'll post it here until someone tells me it should go somewhere else. Anyway, I am currently wondering how to measure the color of some post it notes. It started as a friendly argument as to which post it pad was more red, a hot pink, or a pinkish salmon color. After causing a riot in the AP art class at my high school, we could not determine which was more red. I've tried an online histogram of the post it notes, the problem is that it is giving me the same graph for both sticky note pads. Does anyone have an idea on how to scientifically determine which pad is more red?
     
  2. jcsd
  3. Feb 13, 2018 #2

    Borek

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    "More red" is a can of worms, unless you define first what you really mean by "red" and what you mean by "more".

    Even then measurements will be quite difficult. It is not something unusual to be not able to reproduce a color in print just because precisely calibrating scanner/monitor/printer is not easy.
     
  4. Feb 13, 2018 #3

    .Scott

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    First off, color is in the eye of the beholder - and not all human beholders agree on the fine distinctions of hue.

    The surface of the post-it note reflects different amounts of light across the whole spectrum. The human eye summarizes that response into a hue - which is not only dependent on the retina, but on the lighting and how close the note is to other contrasting colors.

    That said, try this:
    Agree on a lighting condition. Take a photo with both pads. Pop it up in MS Paint or Gimp, or whatever. Sample the color from the photo and view that color in terms of hue, saturation, and intensity. Red shows up as a hue of zero (right next to 1 and 255).
     
  5. Feb 13, 2018 #4
    So assuming that the actual color of red is the wavelength of 620-750 nm, the average wavelength of red, or the "purest" red, would be a wavelength of 685 nm. So if I could graph all of the wavelengths on a graph of x=wavelength and y=intensity (i'm not sure how I would measure intensity), then calculate the area under the curve, and compare the average to the "pure" red of a 685 nm wavelength, would that yield a viable answer? If it did, how would I even go about doing that? To give some background, I'm a high school student, so I have nearly no education in topics such as these.
     
  6. Feb 13, 2018 #5

    .Scott

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    You would be looking for a spectrometer. Such at this:
    http://ikancorp.com/productdetail.php?id=1975&model=mk350d

    This is not something that your high school is likely to have.
    But the right technical college may be willing to demo one for you.
    Some State labs might have one - and they are often open to demo their equipment to high schoolers.
     
  7. Feb 13, 2018 #6

    russ_watters

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    Did you read the replies you got so far? That approach is missing some key parts of the issue. I'll try again:

    1. The reflectivity of the paper will be different for different wavelengths irrespective of light source, BUT:
    2. The color of incident light is different for different light sources.

    So the measured color depends on the color of the light source. Simply put: it will look redder outside in the sun than inside under LED lights.

    So your first task is to agree on the light source.

    Then you can talk about measuring the reflected light. And there are several methods, some already mentioned (like a camera or scanner). But if you want it on a by-wavelength basis, you need a spectrograph (spectrometer).

    [edit] All that said, I don't see how that would be more useful than the Photoshop answer, which would be in the form:
    50% Red
    30% Green
    20% Blue
     
    Last edited: Feb 13, 2018
  8. Feb 13, 2018 #7

    russ_watters

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    Hmm.... putting some thought into this; shouldn't it be possible to make one using a diffraction grating or prism and a monochrome CCD?
     
  9. Feb 13, 2018 #8

    ZapperZ

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    Scope around the universities near your area and see which one of them has a UV-VIS device and is wiling to perform that measurement on your sticky notes. The reflectivity intensity will be rather low, but who knows, you may get a spectrum and compare which spectrum has the highest intensity near the 685 nm value.

    This might make a nice little science project, to compare human perception of color versus what actual measurement says.

    Zz.
     
  10. Feb 13, 2018 #9

    OmCheeto

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    Agreed.

    IMHO, discussing "color" is the closest you can get to "Politics + Religion" in science.

    Color Political Parties:
    Humanists: Color is a human brain construct!
    Scientists: Color is a frequency of the electromagnetic spectrum!
    Moderates: Both are correct!​
    Color Religious Affiliations:
    Evangelical: If you don't believe as I do, then you are wrong!
    Atheists: There is no such thing as color!
    Unitarian: Both are correct!​

    hmmm..... I think most here at PF would fall into the "Moderate Unitarian" camp.

    According to this website and my eyes, 620 nm is Orange.

    that looks like "Brick" to me.

    Not a bad rendition of "Red".

    hmmmm...... High school was also the the highest level of education I completed.

    Ugh...... I would suggest you send an email to the artist Stuart Semple. He claims to have created the worlds "pinkest pink".

    And "pink"? Ehr mehr gerd. According to this guy, pink should be called "not green".



    ps. Strangest things I've discovered this morning thinking about your question:

    1. The red cones in our eyes are most sensitive to spectral yellow light. It's only the lack of input from the green sensitive cones that makes things look red, as we move to longer wavelengths [ref]
    by eyeballing his graph {590 nm}:
    KallColor1.eye.cone.sensitivity.jpg


    2. As far as RGB levels go, 'black' is just a really dimmed down version of 'white'.
    Code (Text):

    name   Hex        RGB             HSV
    White  #FFFFFF    (255,255,255)    (0°,0%,100%)
    Silver #C0C0C0    (192,192,192)    (0°,0%,75%)
    Gray   #808080    (128,128,128)    (0°,0%,50%)
    Black  #000000    (0,0,0)          (0°,0%,0%)
     
    [ref]
     
  11. Feb 13, 2018 #10

    anorlunda

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    Paint stores do that every day. You can take a paint chip (or a postit note) to the paint store. They scan it and it tells you the amount of pigments to add to a white base paint to match the color.

    Obviously, the device is a digital camera. It controls the illumination, and the result is sensitive to the reflectivity, and variations in color throughout the sample. Everything else is a matter of calibration and semantics (the names we associate with colors).

    The OP might be able to bring his postit notes to a paint store and if he or she is charming enough, convince them to scan them in their machine and to tell the results.

    I think the following from Wikipedia is interesting. It says that the machines use two different illumination colors.
    it also says
     
  12. Feb 13, 2018 #11

    sophiecentaur

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    A friend of mine looks at stars that way. The luminance values (after some calibration) tell him the temperature, significant gases etc. etc. when he plugs in some freebee software.
    The paint store idea would be good. I would bet that the CMY / RGB values for all the manufacturers' ranges are available. Many of them are standard colours.
     
  13. Feb 13, 2018 #12

    Merlin3189

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    If you're not likely to have a spectrometer and the time to do that sort of measurement, maybe you can find an old photographer who still has a simple light meter. (Or just make one.) If you then measured the brightness of each post it under identical illumination from some red light sources, you might see which reflected most light. But I expect the difference would not be great enough to be noticed on the log scale of a photographic meter.

    The eye. is not absolutely calibrated nor fixed in sensitivity, so you have do comparisons side by side and simultaneously. Have a look at how people used to compare brightness with wax spots and the like.
    To avoid the question of which is redder, you can ask people to make them look the same. If the two samples are lit by two sources whose colour can be controlled, you should be able to make them appear the same. Then you record the settings of the illumination on each. Use plenty of trials for each subject with the pads being swapped sides pseudorandomly. If there is any systematic difference, you should be able to detect it.
    A big difficulty is training the observers. You can practice yourself and then help the observers. So long as you yourself are also blind as to which way round the pads are (have them set by someone else) I don't think that would introduce bias.
     
  14. Feb 13, 2018 #13

    CWatters

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    In industry the Pantone colour charts are commonly used to specify and identify colours but their sample books are expensive. You could try asking the manufacturer of the post-it if they can tell you the Pantone colour reference then look up the hex value for that colour which I think is in the format RRGGBB. That would tell you how much red Pantone think is in that colour.
     
  15. Feb 13, 2018 #14

    Borek

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    I can be wrong, but I don't think every Pantone color can be converted to RGB, some printable (CMYK) colors are outside of what can be displayed in RGB (and vice versa).
     
  16. Feb 14, 2018 #15

    sophiecentaur

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    But, to be fair, @CWatters was referring to a Post it note, which is well within the gamut of any three primaries that you will find in any real imaging system.
    The only problem with doing the Pantone things is that a set of Pantone Reference Colours will cost you an arm and a leg (I saw £500 to £1500 on the Pantone site). If you want to use your computer monitor with the Pantone on-line system, you would still need to calibrate your monitor; more expense.
    This thread is getting to be typical of PF threads. The OP doesn't specify an accuracy for the measurement so everyone is posting opinions based on their own assumption about requirements. Good fun but we have reached the end of the line until we can agree on how accurate we want to do this measurement.

    Personally, I reckon going to the local paint supplier and getting a handful of colour charts would do the trick. Especially if you manage to get a repeat match outdoors on sunny and cloudy days and indoors with tungsten, CFL and LED lighting. (I believe in Father Christmas and the Tooth Fairy.) :smile: The same thing would apply to Pantone, too.
     
  17. Feb 14, 2018 #16
    So based on the responses, I've come to the conclusion that I will be doing two-three sets of tests on these notes. I found a teacher at the print shop of our high school who has the Pantone color charts, and has agreed to let me use them. Second, this weekend I will be looking for local Paint shops to see if any of them can scan the notes and would be willing to scan them for me. Finally, I will be asking them if they would be kind enough to send me an email of the images they take, where I can upload them into MS Paint or a similar program, and measure the RGB/CMYK point values of each note and determine which has a higher red composition.
     
  18. Feb 14, 2018 #17

    sophiecentaur

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    Brilliant. Exploit any resource you can access. That's the secret of progress in Science.
     
  19. Feb 14, 2018 #18

    russ_watters

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    "Paint shop"? I don't understand why a "Paint shop" would have a scanner, but there are tons of places to find a scanner - every office will have one or several. Heck, I have 3! Regardles...

    But I need to emphasize a major pitfall you still seem to be ignoring: YOU NEED TO TELL THE SCANNER WHAT COLOR LIGHT TO USE WHEN MAKING THE SCAN*.

    Sorry for yelling, but it still doesnt seem like you recognize that the color of an object depends on the color of the light shined on it.

    *If equipped; if not equipped, you will need to find out the scanner's lamp/scan color and do your own calibration. See here for discussion of the issue:
    https://forums.macrumors.com/threads/need-to-capture-accurate-colors-from-scanner.1903569/
     
    Last edited: Feb 14, 2018
  20. Feb 14, 2018 #19

    CWatters

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    Many DIY paint shops will mix paint to match a sample. They use a scanner for that but I don't know if it will tell you anything.
     
  21. Feb 14, 2018 #20

    sophiecentaur

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    A bit of snake oil, perhaps as it would depend as much on the lighting that's used in your particular room. However, if the paint sample is from the same manufacturer (same basic primaries), then a simple RGB / CMY match would tell them which the original shade was. Near enough for a match where you dumped a bottle of wine on your parents' wall at the party you had when they were away. The carpet is another problem.
    Paint colours are so consistent these days that there's hardly ever any need to mix two tins of paint together to avoid a visible change from one wall to another from a different can.
     
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