Creating a Panchromatic Background

Seeing as the coloured filters used in autochrome photography were plated on a panchromatic film, we are going to start by creating this film.

We are going to use the Lab colour mode lightness channel to obtain an image with more accurate grey colours than we would using the Greyscale mode. As a result we can simulate a real panchromatic layer, that is to say, one capable of converting the entire colour spectrum into greyscale values.

But before we start, save a copy of the photo below on your hard disk, then open it in Photoshop.



Figure 1: This photo is ideal for an autochrome.

Now proceed as follows:

1. Select Image > Mode > RGB Colour.
   
   
This process converts the image from the Indexed Colour mode (that of the GIF format) to RGB Colour mode (Red, Green, Blue). Consequently, we will be able to take advantage of the image's chromatic layers.


2. Select Image > Copy.
   
   
A copy of the image has been created.


3. Highlight the original image, then select Image > Mode > Lab Colour.
   
   
4. Select Window > Show Channels. In the palette, select the L channel (that is, the Lightness channel). It contains the image without the colour information.
   
   
5. Select Image > Mode > Greyscale. Then click on Yes in the warning dialog box that appears.
   
   
The original image is reduced to an 8 bit coded grey channel (256 greys). The autochrome's panchromatic background is ready.