Trichromy

Humans are trichromats.  That means we see in three basic colors: red, green, and blue (RGB). These are the additive primary colors.  Our brain knits these three colors together to create all the different colors we see.  Trichromy is the three-color process used in photography that generates the colors that our eyes can see.

Some history. The first photograph was taken in 1827. In the 1860s the first color images were created. In 1935, Kodak invented Kodachrome.  Kodachrome had three layers of emulsion coated on a single base, each layer recording one of the three additive primaries.  Kodachrome was fundamentally three black and white films that had color dyes added to them in processing. During processing the black and white imagery was also removed.  When all layers were properly aligned and viewed together, you saw color. 

Image created by using "Trichromy", combining three B&W images into one color image.
Nikon F100 using Kodak Tri-X Pan 400 film.
Click for an enlargement

While Kodachrome is unfortunately no longer made nor processed, the basic idea is still valid and usable.  By creating three black and white images, one for each primary color, and then combining them, we can make color imagery out of black and white film.

In this article, we’ll be taking black and white photos using a faithful old Nikon F100, some color separation filters, and combined with Photoshop we will create color with trichromy.

To generate a color image from black and white film, we need to create three separate images that each contains only the color information for one of the three primaries.  This is accomplished through the use of color separation filters.  These filters are designed to block out most of the other two colors, while letting in the color for the filter itself.  For RGB, we need red, green, and blue filters.

The set of filters I am using is a red #25A, a green #58, and a blue #47B.  You will see this set listed in some places as the filters to use, and are very easy to locate.  Looking online, you might see other filters mentioned.  For example, you might see a red #29, a green #61, or a blue #47.  That will work as well, but while the blue and red filters are easily found, the green #61 is not and you will have to special order it. You might also see a mix and match of these six, or a couple of other ones thrown in as possibilities as well.  I picked the #25A, #58, and #47B for three basic reasons: