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HANDBOOK OF PHOTOGRAPHY 



A~ Continuous spectrum 



B~ Lr'ne spe ctrum 



length or frequency, energy sources frequently produce spurious or undesired fre- 

 quencies as well as those which are desired. For example, sunlight is responsible for 

 the highly desirable phenomenon of daylight, but because at the earth's surface sun- 

 light also contains radiations as short as 290 m^, it produces undesirable sunburn. 

 A group of waves constitutes a spectrum. If waves of all possible wavelengths 

 between two limits are present the spectrum between these limits is said to be con- 

 tinuous; if some of the possible waves between the wavelength limits of the spectrum 

 are absent, the spectrum is said to be discontinuous or may be referred to as a line or 



band spectrum. The energy-wavelength charac- 

 teristics of a spectrum may be represented graph- 

 ically by means of a spectroradiometric diagram 

 in which the wavelength (or frequency) of the 

 waves is represented by the abscissas and the 

 energy is represented by the ordinates in rectan- 

 gular coordinates. The spectroradiometric 

 diagram for a continuous spectrum is a continuous 

 curve (Fig. 2>A) whereas that for a discontinuous 

 or line spectrum is a series of lines, each of which 

 represents the energy at the corresponding wave- 

 length (Fig. 3B). The energy- wavelength (or 

 spectroradiometric) characteristic of radiated 

 energy is of primary importance in studying the 

 effect of light and other radiations upon photo- 

 graphically sensitive materials and the charac- 

 teristics of filters. 



The alteration of the characteristics of the 

 light emitted by a light source before it impinges 

 upon a photographically sensitive material can 

 ^ be accomplished by means of reflecting or trans- 



FiG. 3.— Continuous (A) and line j^itting filters. A reflecting filter is one which 

 \B) spectrums for two hypothetical n . .^ ■,■ , en- •, ,i 



light sources giving very nearly the I'eAects the radiant energy falhng upon it, the 

 same visual and photographic ef- change in the spectroradiometric characteristics 

 fects. The intensity, J, is plotted of the light occurring during reflection and being 

 against wavelength, X, in arbitrary ^^g ^^ selective reflection of this type of filter. 



A transmitting filter is one in which the spectral- 

 energy distribution of the light is altered by passing light through the filter, which 

 must obviously be transparent although it may be (and usually is) colored. 



In this connection it may be useful to distinguish color mediums having different 

 properties. For convenience, these mediums may be grouped into pigments, dyes, and 

 colored glass. Although differing considerably in their physical form, the two latter 

 mediums are optically similar and consequently may be considered together. The 

 pigments are opaque to light and materials containing pigments can only act as filters 

 through the use of reflected light. Dyes and colored glasses, on the other hand, are 

 partially or largely transparent for a substantial part of the visible spectrum and 

 may therefore be used as filters by transmitting light through them. Thus the pig- 

 ments act because of selective reflection, whereas dyes and colored glasses make use of 

 their property of selective transmission; both have selective absorption. 



All filters, whether operating by reflected or transmitted light, are effective because 

 of selective absorption, i.e., the manner in which energy of some wavelengths is more 

 greatly absorbed than energy of other wavelengths, the portion of the energy not 

 absorbed being reflected by, or transmitted through, the filter. Thus selective absorp- 

 tion might be said to be the fundamental characteristic of all filters. However, since 



