Science Buddies: "Ask an Expert"

I am trying to find out if there is a standard scale to measure darkness of a filter that has filtered particulate matter. The filter did not catch enough particulate matter to change the mass of the filter but there is a definite difference in color. The filter was used to catch smoke.
Thanks!

Hello lcsains,

Many years ago I used a set of cards with different, calibrated shades of gray on them. I could estimate the darkness of some unknown gray level by comparing it to the cards. A web search just now didn't turn up where to get such a set. I found sets with three cards, but that wouldn't give you a very accurate comparison.

You can make some cards of your own by using a computer application (most any drawing or photo-editing program) that allows you to make a rectangle and set the gray level, and then print a set of pages or even a single page with multiple rectangles of different gray shades. Then use these as a comparison to your filter. This is quick and easy, but the gray levels won't be calibrated. If you have or could borrow a photometer (light meter), you could use it to calibrate your gray rectangles.

Good luck, WW

If you have a digital camera with manual exposure controls that will display a histogram of the RGB information, you could take a picture of an unused filter and one of the used filter using the same lighting, framing, and manual exposure controls and then compare the histogram data.