I think you missed what he posted earlier.

He wants to take the RGB readings seperately, which is why he does not want the "white" (ambient) light.

 

that's was at first, we then found agreement dknguyen and me!

we concluded that i would apply filters on 3 different LDR...

__________________
Ibrahim Kamal
check my electronics and robotics page: http://www.ikalogic.com/

 

hi ikalogic,
Just a thought will reading the latest posts, jogged my memory cell.
Used to work on a polychromater system.
It used a prism to diffract the 'white' light source into the spectrum, photo
detectors placed along the spectrum to detect the intensity of the light
at that point of interest in the spectrum.

I still think the optical filters will be best for you experiments.
As suggested by our co-poster, bog standard red,green, blue plastic filters
using a 'white' light illumination source. Check the light/resistance curve
for your LDR, see if it covers the light range of your filters.

Regards
EricG

 

Yes, but now the accuracy of your results will depend on the "purity" of the white light. I suppose you could compensate for it, but that would add useless complexity right? Since you would have to compensate differently with each light source. The advantage of using the RGB light sources is that you only calibrate once. You will know the purity specs of your filters when you buy them, then it is just a matter of making sure the amount of light emerging from the other side of the filters are the same (dimmers? to adjust). I believe measuring brightness is easier than measuring purity, and you always use the same light sources. I assume these are some of the reasons the folks who wrote the magazine article had in mind. Clearly both methods have compromises, I guess it just depends how you will be using it. I agree with everything dknguyen posted earlier, I just wasn't sure if he was clear on what your original intentions were. Good luck and keep us posted!

EDIT: I couldn't see erics post until I hit the submit button on my post! I like the prizm idea.

 

hi sig,
In some of the 'light' measurements systems I have used, I found that a
lookup table in the MCU, [ created in a calibrated set up] worked well.
By using the 'address' calculated from the apparent light intensity as a pointer
to the table, it was possible to give fair measurement repeatabilty.

Nice chatting, looking fwd to your post guys.

EricG

 

I haven't built any of these projects, but a couple at the bottom of the page might help you...

http://www.ullasmann.eu/Tiny13.htm

Seem to be complete and well documented.

 

I think you're confusing things, a red band stop filter will appear cyan, green + blue = cyan, red + green = yellow.

__________________
I also post at the following sites:
http://www.stop-microsoft.org http://www.heated-debates.com
Screen name: Aloone_Jonez

 

Ok Now i AM seriously lost!

If i take a piece of transparent film, put some blue marker on it, so that it is still transparent but everything looks blue when looking through it, Is this a filter that is passing blue light only? (sorry i just got a litle confused, too much of blue+green+yellow+red = ? ....) !

__________________
Ibrahim Kamal
check my electronics and robotics page: http://www.ikalogic.com/

 

There are three different coloured cone cells on the human retina, red, green and blue.

When red and green are equally stimulated we see yellow.

When green and blue are equally stimulated we see cyan.

When red and blue are equally stimulated we see magenta.

When they're all equally stimulated equally we see white.

A blue filter blocks all other colours except blue.

A red filter blocks all other colours except red.

A green filter blocks all colours extepr green.

The above filters are addivite primary colours, they are used for mixing light, if you look at a TV screen closely then you'll see rows of red, green and blue dots.

A yellow filter blocks blue and passes only red and green.

A magenta filter blocks green and passes only red and blue.

A cyan filter blocks red and passes only green and blue.

The above filters are subtractive primary colours, they are used for mixing pigments, you'll often see them printed on the corners of magazines or packaging and if you looked at the pages through a microscope you'll see that all the images are made of these three colours.

Putting a magenta filter on top of a yellow filter gives red, because the magenta filter blocks green and the yellow fliter block blue leaving only red because the only colour that they both pass is red.

Putting a cyan filter on top of a yellow filter gives green because the yellow filter blocks blue and the cyan filter blocks red, leaving only green because the only colour the both pass is green.

Putting a magenta filter on top of a cyan filter gives blue because the magenta filter blocks green and the cyan filter blocks red leaving only blue because the only colour they both pass is blue.

http://en.wikipedia.org/wiki/Color_theory

__________________
I also post at the following sites:
http://www.stop-microsoft.org http://www.heated-debates.com
Screen name: Aloone_Jonez

 

Ah, yeah. I was sitting here trying to figure out what blue + green makes and the closeset thing I could come up with was yellow. Even though I'm aware of CYM and RGB, my preschool knowledge of colours always seems to take control so I keep on thinking of just RGBY since I've never had anything to use it for anything and solidify it in my head. I then end up thinking RGB is additive and RBY is subtractive. As a result, I end up mistaking cyan for green and forgetting that red and green can mix! I always forget about CYM until someone points it out. Colour mixing seems to come up so rarely in control and power systems.

I also forgot about light being additive but filters being subtractive.

 

Yes but once you know the science it all becomes logical, especially when you consider that mixing red and green paint makes brown which is a dim shade of yellow; this is due to the small amount of red and green light reflected. I also remember mixing blue and yellow in equal quantities which resulted in a very dissapointing grey-green.

Some colours are very hard if not even damn near impossible to mix using the preschool RYB system which I suspect exists from before they developed proper pigments. Some would also say that printing cyan is a shade of blue and printing magenta is a shade of red.

I think, I've stumbled on the fact that you can make red from magenta when I was playing around with paints in art class at school.

__________________
I also post at the following sites:
http://www.stop-microsoft.org http://www.heated-debates.com
Screen name: Aloone_Jonez

 

Isn't brown actually a dark shade of red? Where does brown go on the colour spectrum anyways? It doesn't make sense if it's a yellow since it's smackdab inthe middle of the transition between yellow-> red or yellow->green

 

Well, back when I use to paint... Red+green = Brown
Blue+Orange = Brown
Yellow + Violet = Brown

 

Brown is more a dark shade of orange, but some might say yellow.

Anyway aren't you colourblind or am I confusing you with someone else?

I am slightly red-green colourblind, so I might get subtle shades of brown, orange, yellow, red and green confused but I never get distinct colours confused, i.e. I can tell the difference between red, orange, yellow, green and brown bands on resistors.

Attached Images

Brown.PNG (8.8 KB, 7 views)

__________________
I also post at the following sites:
http://www.stop-microsoft.org http://www.heated-debates.com
Screen name: Aloone_Jonez

 

I am, that's why I'm not sure if brown is a dark shade of red or not. Because under some conditions I see it as a really dark red...then again, I can also imagine why green looks like red sometimes.