Image Processing Reference
sunlight refracted through the prism to the thermometers, which had been placed
in the path of the refracted rays of sunlight (the blackening of the thermometers
increases the absorption of energy from the incident light). The thermometers
registered a temperature change even when placed in the dark region just beyond
the red part of the rainbow pattern. Herschel concluded that there were invisible
light rays heating the thermometers, and he wrote about this discovery in a paper
“It being now evident that there was a refraction of rays coming from
the sun, which, though not fit for vision, were yet highly invested with a
power of occasioning heat. . . ” 1
Figure 1.3 shows the best illustration of the apparatus from this seminal paper.
Sir Isaac Newton had discovered in the17 th century that a ray of sunlight
could be dissociated into the familiar colors of the rainbow with a prism.
Herschel expanded on Newton's research by postulating the existence of an
additional prismatic component of light that was invisible, which he named the
“thermometrical spectrum” (the term “infrared” was coined later). He postulated
that the “construction of the organs of sight” allowed for the entry of visible
light, and that light in the thermometrical spectrum was stopped in the “coats
and humours of the eye,” which accounted for our inability to see it. The idea of
invisible light was met with a great deal of resistance in the scientific community,
but Herschel was eventually proved right—invisible light did exist.
Sir William's son, Sir John Herschel, continued his father's experiments with
infrared light and published a paper in 1840 that described an apparatus he used for
recording infrared images of the solar spectrum—the first invisible-light imaging
technology. The apparatus consisted of a prism that imaged sunlight onto very
thin black paper soaked in an alcohol/colored dye solution he called “rectified
spirit of wine.” 2 The lightwaves absorbed onto the paper produced different rates
of evaporation of the dye solution, which resulted in varying film thickness with
attendant colors (like the colors of a thin film of oil in a puddle). This technique
was refined in the 1930s by Marianus Czerny (1896-1985), who invented the
“Evaporograph”—an infrared imaging system that used a thin film of volatile oil
applied to a thin absorbing membrane. Practical infrared imaging did not really
start until the invention of infrared-sensitive photographic film based on the light-
sensitive dye kryptocyanine in the 1930s.
Returning to the sunbeam and prism experiment, consider the other edge of the
rainbow pattern projected onto the white screen. When human eyes look at the
screen just beyond the deepest violet light, they no longer perceive any light at all.
Therein lies the near-ultraviolet (near-UV) region of the spectrum, which begins
1 W. Herschel, “Experiments on the refrangibility of the invisible rays of the Sun,”Phil.Tran.Roy.
Soc.90, 284-347 (1800).
2 J.F.W. Herschel, “On the chemical action of the rays of the solar spectrum on preparations of
silver and other substances, both metallic and non-metallic, and on some photographic processes,”
Phil.Trans.Roy.Soc.130, 1-59 (1840).