Cottage Tip: Densitometry Simplified

Densitometry and optical density seem to be a very abstract topic to too many photographers, but they're not. Optical density is in fact a very handy way to express the loss of light through a medium; in our case photographic film or paper. It is a valuable tool for image analysis.

Optical density is defined as:

OD = Log₁₀(Io / I), where Io is the non-attenuated light intensity (e.g. light reflection from blank paper base) and I is the attenuated light intensity (e.g. light reflection from an image area of the same paper)

In case when Io / I = 2 (the light intensity halves, i.e. is reduced by one stop), the density value equals to 0.30 which is a very handy number to deal with.

Very unfortunately, dedicated densitometers are quite expensive devices, even used ones. A new one can easily cost you about 1000 € or even more. If you're lucky, you can find a used one for a few 100 €. (And you still spend the same amount of money as for a SLR in good condition). Here I show an example how to make densitometric measurements of reflected light (i.e. from prints). Everyone who has ever dealt with alternative printing techniques has noticed the expression »image density«, be it for a cyanotype, platinum printing, and of course gum bichromate printing. Or any other technique. You can easily live without densitometry, but for getting consistent and predictable results in your printing, it is better to use it, especially in color or multi-layer printing. And you don't even need a densitometer. For densitometric measurements of acceptable precision, you need just a decent SLR or DSLR with spot metering capability-it is better to have a camera with 1/3 stop setting increments, but you can get along with a camera with 1/2 stop settings (like mine). For total (visual) densitometry, this is all you need. But for measuring the density of all three image-forming colors in color printing (yellow, magenta, cyan), you will also need a set of RGB filters. They need not to be the expensive optical quality filters-a set of RGB lighting filter gels (like Lee) are just right for the job, provided they faithfully represent the three primary colors (red, green, blue). You can buy a set of them online for a few €. I cut them in 75x75 mm squares and I hold them in front of the lens when I take the measurements.

The measurements were made with my Canon EOS 5, set in Av and spot mode. Aperture f/2.8 and ISO 100. Illuminated by window light.

Measuring is easy, just set your camera in aperture-priority mode at the aperture and ISO setting at your convenience, set the spot-metering mode, and place you print on an evenly illuminated surface. And you don't even need to have a focused image (actually it is better not to). First take the reading of the paper base (in secs) and then on the spot on the image you want to measure.

Say, you got these two readings for paper base and your spot of interest, 1/45 s and 1/15 s, respectively. Now you calculate the logarithm of their inverse values (actually their camera readings):

OD = Log₁₀(45 /15)= 0.48

This value tells you that the reflected light on that spot is attenuated by about 1.6 stop.

For the sake of illustration, I prepared a sheet of drawing paper (see above) with spots of different colors approximating the black and the complementary colors (yellow, magenta and cyan) differing in intensity (density). They are made with pastels and are by far not ideal, but they show the basic principle anyway.

When you want to measure the density of the yellow color, you measure it with the blue filter in front of the lens, since blue is complementary to yellow. For magenta, you use the green filter. And for cyan, the red filter. This is because you want to block the other two colors during your readings. Write down your readings and the calculate the logarithm values. These are the densities of selected image spots. Easy, isn't it?

Of course, these measurements are not super-precise, but they can help you a lot when you engage yourself in alternative printing.

The tools I used to measure the densities: my trusty SLR, and for YMC colors, the 3 Lee filter gels-red, green and blue. Pocket calculator not shown :)