how do I assess RGB values

[momweber]

Hello, my dtr wants to do a science fair project determining which detergent gets out stains better. She needs a way to measure the differences on the white fabric. (something a bit more scientific than just eyeballing) Someone suggested measuring the RGB values. When I posted this question on Windows 7 Paint forum, someone suggested that paint.net would be much better for this. I've downloaded it and have searched help and

FAQs etc. on how to do this. Can someone assist please. My dtr would be uploading photos of the different fabrics after washing and comparing these swatches to the control fabric which has no stain. So she would need to compare the RGB values for each stain on the photo (if I am undestanding RGB values correctly) to determine which was "whiter". It would be great if this was something a 5th grader could do. If it is too difficult please let us know so as we can quickly pick another topic.

Thanks for any input, it will be greatly appreciated.

[csm725]

Well, let me explain to you the basics of RGB. 0,0,0 is black (no color, dead) and 255,255,255 is white (total color). I immediately see one flaw with this project: RGB values change extremely easily. So if the lighting is even a bit different, the values can change by 10, 20, or even 50. So I don't know how you could pull off 100% consistent lighting. If you have varying lighting, then there is no way to know which fabric is whiter because you don't know whether it was the lighting or the detergent. You could close all of the windows and have a white light lamp. Just my insight.

To answer your question, the closer the R or G or B value to 255, the whiter. Hope this helps and good luck on your project.

[momweber]

Thanks for the info. I didn't realize the lighting would effect so much. I will definately take that into consideration as I do "my" project. (If you have kids you will understand it is never the student's science project but the parents!!). I appreciate your reply.

[csm725]

Sure. No problem. Happy to help!

[Ego Eram Reputo]

I didn't realize the lighting would effect so much.

It will. The best you can do is duplicate the photo settings as accurately as possible between images. You would be best to photograph them in one session, using a tripod and exactly the same lighting & camera settings.

As another tip: Photograph each piece of fabric alongside a 'control' - a piece of white paper for example. Use the same piece of paper in each image (same location too - just change the fabric between images). This will give you a something to compare the fabric to - a known constant between images. Thus you can compare the results within an image, and don't have to compare images with other images.

I'm not a photographer - someone with more expertise in this field may have other suggestions.