EM 1110-2-2907
1 October 2003
minimum and maximum of this category is based on the ability of the human eye to sense
radiation energy within the 0.4- to 0.7-m wavelength range.
(1) Though the spectrum is divided up for convenience, it is truly a continuum of
increasing wavelengths with no inherent differences among the radiations of varying
wavelengths. For instance, the scale in Figure 2-8 shows the color blue to be approxi-
mately in the range of 435 to 520 nm (on other scales it is divided out at 446 to 520 nm).
As the wavelengths proceed in the direction of green they become increasingly less blue
and more green; the boundary is somewhat arbitrarily fixed at 520 nm to indicate this
gradual change from blue to green.
Figure 2-8. Visible spectrum illustrated here in color.
(2) Be aware of differences in the manner in which spectrum scales are drawn.
Some authors place the long wavelengths to the right (such as those shown in this man-
ual), while others place the longer wavelengths to the left. The scale can also be drawn on
a vertical axis (Figure 2-9). Units can be depicted in meters, nanometers, micrometers, or
a combination of these units. For clarity some authors add color in the visible spectrum to
correspond to the appropriate wavelength.
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