Collimated vs. diffuse light – a cyanotype example

On a forum, someone asked how to get their cyanotypes made with 3D objects instead of a negative crisper. The fundamental issue at work here is how collimated or diffuse the light source is. Here’s a quick example of the difference between both.

Collimated light simply means that all the light rays are parallel to each other. An example is direct sunlight from a cloudless sky, especially when the sun is high in the sky and the least amount of atmospheric haze gets in the way of the light. Note how during these conditions (think a bright summer noon), the sunlight projects sharp shadows.

The opposite is diffuse light, which means that the rays go off in all directions. Overcast skies will refract the light in all directions, creating diffuse light. Note how on an overcast day, there are no shadows to speak of at all.

But how to harness this for alt. process printing? Nothing wrong with using sunlight, of course, but it’s not very predictable or dependable – at least not where I live! There are generally to ways to use collimated light:

1: Use a point-source light source. Typical examples include high-pressure discharge lamps, but also a single high-power LED. The ideal collimated light source is a laser, but lasers are generally not practical for alt. process darkroom printmaking due to the small spot they illuminate.

2: Use a collimator to turn (semi-)diffuse light into collimated light. This can be done through lenses or tubes. The set of condensors in a condensor enlarger is in fact a collimator, but generally not very useful for alt. process printing due to the attenuation of UV light that occurs in them – although with high-power LEDs, we are approaching the point where such optics become a viable option.

An intermediate solution of semi-collimated light is an array of LEDs. Each LED is (virtually) a point source, but having lots of them in an array, creates some diffusion. By contrast, a bank of UV tubes behaves more like a diffuse light source. The difference between them is aptly illustrated by these two prints:

Two cyanotypes of some nuts & bolts placed directly onto the paper. Diffuse light source (UV tubes) on the left, quasi-collimated (array of LEDs) on the right.

Both prints were made with classic cyanotype chemistry onto scraps of etching paper I had lying around. The images were made by placing a collection of nuts & bolts from the parts bin directly onto the paper. The 3D-nature of the objects emphasizes the difference between both light sources used.

For the print on the left side, I used a bank of Philips UV-BL tubes, which is a very diffuse light source. Note how only objects that make contact with the paper print sharply, such as the ring in the lower right corner. Objects like screws leave only a narrow shadow, since a lot of light creeps past the cylindrical shape, leaving only the strip unexposed where the object touches the paper.

The right side print was made with an array of UV LEDs and an otherwise identical approach. Note how the screw threads are fairly sharply depicted here – as are all objects, even if they don’t directly touch the paper. The distance between the print and the light source was about 40cm for this print. Increasing the distance will make the image sharper still.

For prints from a negative, the difference is generally less striking, as long as the contact between the negative and the paper is good. There are processes where collimation does play an important role, though. For instance, photopolymer intaglio printing relies on a collimated light source to accurately image the individual dots (intaglio pattern) that become the ink wells during printing. Likewise, carbon transfer using halftone screen negatives requires collimation due to the thickness of the carbon tissue, which can result in dot gain especially when using very high screen resolutions.

Leave a Reply

Your email address will not be published. Required fields are marked *