Introduction

This activity demonstrates additive and subtractive color mixing with color filters—semi-opaque pieces of film that block certain wavelengths of light and let other wavelengths pass through.

Instructions

Click on one of the colored circles at top or bottom to choose the filters you want to use.

Click on the addition/subtraction sign in the center to toggle between additive and subtractive color mixing. The result of the mixture will appear to the right of the equal sign and a brief description of why that color is produced will be shown here.

The Transmission Graph

When you choose a color, you will also see a “transmission graph” to the left, showing the percentage of light from each point on the spectrum that passes through the filter. For example, the blue filter allows 60%–80% of the light in the 400–500 nm range to pass through, completely blocks light in the range of 550–650 nm, and allows some long-wavelength (> 650 nm) light through.

Additive Color Mixing

Imagine that we have two spotlights, each emitting bright white light and both aimed at the same point on a projection screen. We could then place one filter in front of each spotlight (as in textbook Figure 5.10). What we would see on the screen would be an additive color mixture: the light wavelengths that pass through the two filters would be added together as they reflect off the screen and into our eyes. In fact, this is exactly how many projection televisions work, with red, green, and blue filters placed in front of three bright lights that project onto a screen. Alternatively, an RGB computer monitor is so named because it has three types of colored pixels: red, green, and blue. Any color that the computer displays is created by illuminating combinations of these three colored pixels at different intensities.

You can predict what the additive mixture will look like by adding the two transmission graphs. As you try different additive mixtures, click on the mixture colors to read about them. The red + green mixture shows what the additive combination of this particular pair of filters looks like (the result might surprise you!).

In textbook Figure 5.10, a light with a blue filter and a light with a yellow filter are both shone onto a projection screen. The resulting color appears white because it contains the wavelengths for red, green, and blue.

Subtractive Color Mixing

Imagine that we take a single spotlight and aim it at a projection screen. We then place one filter so it blocks some of the light from the spotlight. Finally, we place another filter in front of the first filter. Now, only wavelengths that can pass through both filters will bounce off the screen and onto our eyes (as depicted in textbook Figure 5.9). This is one form of subtractive color mixing: the wavelengths blocked by the second filter are “subtracted” from the wavelengths that made it through the first filter. Paint mixing is a more familiar form of subtractive color mixing (and probably the form of color mixing you are most familiar with). Here, each of the paints in the mixture subtracts out the light wavelengths that are absorbed by the paint and therefore, only wavelengths that are reflected by all of the paints arrive at your retinae.

You can predict what the subtractive mixture will look like by subtracting the two transmission graphs. As you try different subtractive mixtures, click on the mixture colors to read about them. (Remember to click the + or – sign on the left to switch between additive or subtractive color mixing, respectively.) The yellow–blue mixture shows what the subtractive combination of this particular pair of filters looks like.

Another way to visualize the yellow–blue combination is depicted in textbook Figure 5.9. Starting with white light, which contains all wavelengths, first a yellow filter is put in front of the light, which blocks out most of the short wavelengths. Next, a blue filter is put in front of the yellow filter, blocking out most of the medium and high wavelengths. The only wavelengths remaining from the original white light are in the green part of the spectrum. Therefore, the resulting spotlight will appear green.

Both filter colors are the same, so the mixture is also the same color. Choose a different color for one of the filters to create a more interesting mixture.

An additive mixture of blue and red produces magenta (light purple).

Between them, these two filters let through nearly all the short and long wavelengths, but few of the medium wavelengths. Thus, the S- and L-cones will be greatly stimulated, but the M-cone will be less stimulated. This produces the perception of magenta, a light purple color. Note that these two colors produce similar hues when added or subtracted. However, the additive mixture is much brighter than the subtractive mixture. As should be clear if you understand the nature of the two types of mixing, the latter statement is true for all mixtures.

An additive mixture of blue and yellow produces light gray or white.

As you can see from the transmission graphs of these filters, almost all wavelengths will pass through one or the other of them. When you put all the wavelengths together, you get white. This mixture is similar to sunlight, which is a combination of all the wavelengths in the visual spectrum.

An additive mixture of blue and green produces cyan (light blue).

All the wavelengths below about 550 nm pass through one or the other of these two filters, stimulating both the S- and M-cones and producing the color cyan.

An additive mixture of red and yellow produces beige.

All the wavelengths above about 550 nm get through this combination of filters, producing high firing rates in the M- and L-cones and the perception of off-white or beige.

An additive mixture of red and green produces yellow.

This may surprise you: When you add red and green lights together, you get a wavelength mixture that looks nothing like either of the two components. But compare the combination of the red and green transmission graphs, shown at left, to the yellow transmission graph (which you can see by clicking on a yellow filter) and you should realize that red and green together let through almost exactly the same wavelengths as yellow alone. Therefore, the cones will be stimulated almost identically for red + green as for yellow alone, and you therefore see them both as the same color.

An additive mixture of yellow and green produces light greenish yellow.

This is one of those combinations where the two colors combine to produce similar hues regardless of whether the color mixing is additive or subtractive. However, the subtractive mix is darker than the additive mix because less light is allowed through. In fact, the additive mix is brighter than either of the two components alone, and the subtractive mix is darker than either of the two components alone. As should be clear if you understand the nature of the two types of mixing, the latter statement is true for all mixtures.

A subtractive mixture of blue and red produces dark purple.

Both filters block out the middle wavelengths, with the blue filter blocking most of the long wavelengths and the red filter blocking most of the short wavelengths. However, some short and long wavelengths make it through, leading the S- and L-cones to be somewhat stimulated. This pattern of activity is interpreted as purple. Since the luminance is low due to the subtractive color mixing, the result is a dark purple.

A subtractive mixture of blue and yellow produces green.

Only the wavelengths in the green portion of the spectrum (and a few very long wavelengths beyond the visible range) make it through both of these filters. You can visualize the combination of the two filters by looking at the transmission graphs in the figure to the left. Since the subtracted amplitude is lower than the input amplitudes, the resulting mixture looks dark green rather than bright green.

A subtractive mixture of blue and green produces aqua (greenish blue).

The transmission patterns of these two filters are fairly similar to each other, so the subtractive mixture looks similar to both of its components.

A subtractive mixture of red and yellow produces orange.

The wavelengths that make it through both of these filters excite the M- and L-cones in the same way that a single wavelength in the orange portion of the spectrum would. Consequently, this mixture looks orange.

A subtractive mixture of red and green produces dark brown.

This combination of filters effectively blocks almost all wavelengths except a few in the yellow–orange–red part of the spectrum, producing a dark brown color. As discussed in the textbook, brown is a related color, which means that it only appears to be brown in the context of other colors. In isolation, this color would appear as very dark orange or very dark red.

A subtractive mixture of yellow and green produces olive green.

The yellow filter lets through most of the wavelengths that the green filter lets through, but the green filter blocks the long wavelengths that pass through the yellow filter. The resulting mixture looks greenish, but is pushed a little towards the long-wavelength end of the spectrum, making it appear olive green.