Absorption curves show filter effect.
a. No light filter will absorb one specific color, but rather it operates over a range of colors.
Even within the range of colors, the effect of the filter may vary. Charts called absorption curves show
the effect of filters. Figure 1-6 shows absorption curves for some of the most common filters.
b. Since the color of light depends on its wavelength, the illustration shows the effect of the
filter on each wavelength. The relationship of color to wavelength is as follows:
Below 400 nanometers (nm)
400 to 500 nm
500 to 600 nm
600 to 700 nm
Over 700 nm
A nanometer is one billionth of a millimeter
c. The white part of each illustration indicates the light that passes through the filter, and the
gray part shows what is absorbed. Note in the illustration that the red (No. 25) filter passes almost 80
percent of the wavelength (600 to 700 nm), but almost none of the lower wavelength. The green (No.
11) filter passes 70 percent of the green (520nm), 40 percent of the red (650nm), and a small amount of
d. As you can see in the illustration, all filters reduce the intensity of the light that reaches the
film. The amount of overall light a filter absorbs is indicated by a number called the filter factor.