Colorimetric Parameters

[ritagoncalves]

Hello everyone,

I've found this program ImageJ just a few days ago, so I have no experience in it and my question should be easy to answer.

I am studying the coloration of fruits and I need to know its exact value of colorimetric parameters, I mean the L*, a* and b*. Is this possible? All that you can tell me about color would be very usefull.

Thank you very much.

Rita

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ImageJ mailing list: http://imagej.nih.gov/ij/list.html

[Michael Schmid]

Hi Rita,

you will need a plugin to convert to L a b colors, e.g. one of these:
 http://imagejdocu.tudor.lu/doku.php?id=plugin:color:conversions_between_rgb_color_space_and_lab_color_space:start
 http://rsb.info.nih.gov/ij/plugins/color-transforms.html
 http://rsb.info.nih.gov/ij/plugins/color-space-converter.html

You have to make sure that the illumination has a color rendering index close to 100% (no fluorescent light, also most LEDs have poor CRI) and the camera's color balance is set correctly, i.e., a standard gray card should appear with neutral color.
  http://en.wikipedia.org/wiki/Color_rendering_index
  http://en.wikipedia.org/wiki/Color_balance

Nevertheless, you should not expect absolute accuracy, only consistent results for one camera type and illumination source. The spectral sensitivities of the R, G, B pixels of different camera types are not the same, so you may get slightly different values with different camera types. You will also get somewhat different results for light sources with different color temperature.

Michael
________________________________________________________________
On Feb 13, 2015, at 12:20, Rita Goncalves wrote:

> Hello everyone,
>
> I've found this program ImageJ just a few days ago, so I have no experience in it and my question should be easy to answer.
>
> I am studying the coloration of fruits and I need to know its exact value of colorimetric parameters, I mean the L*, a* and b*. Is this possible? All that you can tell me about color would be very usefull.
>
> Thank you very much.
>
> Rita
>
> --
> ImageJ mailing list: http://imagej.nih.gov/ij/list.html

--
ImageJ mailing list: http://imagej.nih.gov/ij/list.html

[ritagoncalves]

Hello Michael,

Thank you very much for your answer and advices.

However, i'm still having problems with it. I've made the download of all plugins you mentioned.

As I told before, we are studying the color of fruits. Imagine that I have a picture of a pear and it appears in RGB color type. When I go to the Threshold Color, I adjust the brightness in order to obtain the pear in black and the rest of the picture in white. In the Threshold Color window, it appears the values of Hue, Saturation and Brightness or the values of L*, a* and b*, for example.

First of all, I don't understand this values because L* varies from 0 to 100, and in the Threshold window appears values from 0 to 255.

What I am trying to understand is if there is any plugin that can analyze an image and give me the exact value of L*, a* and b*.

The plugins you showed me don't give this value that I want or probably I am doing some mistake.

Again, thank you very much.

Rita

--
ImageJ mailing list: http://imagej.nih.gov/ij/list.html

[Michael Schmid]

Hi Rita,

the various Lab plugins use different scale for the output. E.g. if the output is 8 bits, it is usual to map the 0-100 range of L to the full 0-255 range of an 8-bit image, for better accuracy. The a and b values are typically mapped to 8 bits such that 128 corresponds to 0.

The Color Space Converter
  http://rsb.info.nih.gov/ij/plugins/color-space-converter.html
creates floating-point output output if you select 'separate iages', so it's output is already in the 0-100 range for L and +/-100 range for a, b.

Michael
________________________________________________________________
On Feb 19, 2015, at 17:59, Rita Goncalves wrote:

> Hello Michael,
>
> Thank you very much for your answer and advices.
>
> However, i'm still having problems with it. I've made the download of all plugins you mentioned.
>
> As I told before, we are studying the color of fruits. Imagine that I have a picture of a pear and it appears in RGB color type. When I go to the Threshold Color, I adjust the brightness in order to obtain the pear in black and the rest of the picture in white. In the Threshold Color window, it appears the values of Hue, Saturation and Brightness or the values of L*, a* and b*, for example.
>
> First of all, I don't understand this values because L* varies from 0 to 100, and in the Threshold window appears values from 0 to 255.
>
> What I am trying to understand is if there is any plugin that can analyze an image and give me the exact value of L*, a* and b*.
>
> The plugins you showed me don't give this value that I want or probably I am doing some mistake.
>
> Again, thank you very much.
>
> Rita

--
ImageJ mailing list: http://imagej.nih.gov/ij/list.html

[ritagoncalves]

Hello Michael,

Thank you very much for your explanation.
I think that I am understanding better now!

Another doubt and I tell you again.

Regards,
Rita

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