Seeing the Components of Light and Color with a Spectrophotometer

An object absorbs part of the light from a source and reflects the rest. This part of the reflected light enters the human eye, and the resulting stimulus to the retina is recognized by the brain as the color of the object. Each object absorbs and reflects light in different regions of the spectrum in varying amounts; this difference in absorptance and reflectance gives different objects different colors.

If we measure an apple, we get the spectral graph shown in Figure 1a. On the graph we can see that the reflectance (the amount of reflected light) is high in the red wavelength region, but the reflectance (the amount of reflected light) is low in other wavelength regions. Figure 1b shows that an apple reflects light in the orange and red wavelength regions, while absorbing light in the green, blue, indigo, and violet wavelength regions. Therefore, by measuring with a spectrophotometer and displaying the results on a spectral graph, we can see the nature of the apple's color.

Each composite sensor of the spectrophotometer (there are 16 in the 3nh spectrocolorimeter NS408) measures light in a strictly defined wavelength region within the visible wavelength range. Because of this, spectrophotometers can measure differences between pigments that are invisible to the human eye.

If we measure a lemon, we can get the spectrum graph shown in Figure 2a. On this graph we can see that the reflectance (amount of reflected light) is high in the red and yellow wavelength regions, but low in the indigo and violet wavelength regions. Figure 2b shows that the lemon reflects light in the green, yellow, and red wavelength regions, while absorbing light in the indigo and violet wavelength regions. Such is the nature of the lemon color. Such a high accuracy cannot be achieved by the human eye or even by a photoelectric integrating colorimeter, but can only be achieved by a spectroscopic colorimeter.