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Demos: 7C-03 Color Filters

Magenta, Cyan and Yellow (MCY) filters are used to illustrate the subtractive (filter) rules for color. It is shown that these filter combinations produce the RGB primaries to which the eye responds.

Directions: Dim the room lights. Begin by placing the RGB filters one at a time on the overhead to remind the students what these colors look like. Then overlap these filters two at a time to show that light cannot pass through because these are “one-color” filters. Next place the MCY filters one at a time on the overhead to show the colors. Remind them that these are the secondary colors developed in the color addition Demo 7C - 01. Next, overlay the MCY filters two at a time to show that this produces R, G and B.

Suggestions for Presentation: It is important to point out that filters subtract light from the spectrum. To illustrate this nicely, place the slot on the overhead (see Demo 7C - 01), tape the holographic grating in front of the projection lens and place each of the MCY filters over the slot, one at a time. The spectrum on the screen will show that, for example, when the magenta filter is placed over the slot, the green portion of the spectrum is blocked out. Indeed, these filters are sometimes called minus green (magenta), minus red (cyan) and minus blue (yellow). Emphasize that these filters (MCY) are essentially two-color filters. This allows the common color to pass through, something that can’t happen with RGB filters. That’s why MCY filters (inks) are used in color printing.

If you place the three (MCY) filters on the overhead carefully, you can get the MCY and RGB to show simultaneously--a very dramatic effect. Also, where the three overlap, you get black (actually, more of a muddy brown because the filters have some bleed over).

There should be available with this demo a “color-key,” used in color printing. There are separate M, C and Y records and when they are completely overlapped, the full color appears. We recommend you show this. It is very dramatic! Because printing inks are not perfect filters, the colors are often a bit muddy. Therefore, a black record is printed also to sharpen highlights and shadows. Thus, color printing is often referred to as a “4-color” process.

Tell the students to look at packaging materials to see the color “bars” that show the inks used. They will be mostly MCY, but other colors are often added to insure product integrity.

Note: If you have time, show that color pictures in magazines are composed of tiny dots. Why? To get varying intensities of color, small dots give a light color, larger dots give a darker color. They are so small that at normal viewing distances we fuse them on our retinas and we see variations in color intensity.

Applications: Color printing

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Last Updated: May 9, 2016 11:44 AM

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