obviously not the v.'ay to use a color filir. for'^ spectral measuring 

 device, although it has been tried. A color film does not need 

 to produce a spectral match to the objects it records. Color 

 films do attempt to produce an im.age v;hich looks like the 

 original object. 



Let's take a closer look at this film image. The film 

 image is produced by combining three dyes . This image of an 

 algae sample consisted of the dye amounts shov/n in Figure 2. 

 All the images produced by this film, are produced v;ith various 

 amounts of these three dyes . When I v;ant to measure the spectral 

 density curve of a color-film image, I do not need to use a 

 spectrophotometer. I know that the film imiage contains only 

 three dyes. I can calculate the amounts of the dyes in an im.age 

 from three measurements--the red, green, and blue densities of 

 the film sample. These analytical densitometry techniques are 

 familiar to m^ost photographic engineers . From the dye amounts 

 and the spectral density curves of the individual dyes, I can 

 compute the spectral density curve of the film sample. Thus I 

 can construct the entire snectral density curve from three measurements 



J 



31-6 



