Wednesday, September 10, 2014

A Guide to reflectance measurement devices, part 1

You wouldn't think it would be all that hard. You go into McSpectros, and ask the guy behind the counter to show you a reflectance measurement device. You expect the guy to ask whether you want to measure the specular or the bulk reflectance. You read my blog on specular and gloss, so you know that those are the two critical parts to look at.

When you get to the front of the line, you expect to be shown a couple different models. Maybe one will be pimped out with an LED light show synced up with Beyonce for the Millennial crowd. The deluxe model (eligible for senior discount) will have a cup holder for a Venti sized Starbucks, and will play Michael Buble as measurements are made.

Oh... are you in for a rude awakening! There is a bewildering array of choices. I started counting up the different possibilities for configurations of an instrument, and came up with 18 different types that are in use and officially blessed in the standards. Some of them are interchangeable -- measurements made with one device should match those of another, at least in theory. But  there are still 12 different types of non-interchangeable measurements that can be made. The wonderful thing about standards is that there are so many to choose from.

"I'd like a grande, two pump, d/8 spectro tea latte with SPEX, please."

In reality, the big question is "gloss or bulk". You want to measure one or the other, or both. But rather than ask that simple question a wise reflectance measurement device salesperson will ask what line of business you are in.

Are you in the graphic arts?

If you are in the graphic arts, then you got it easy. All the instrumental decisions have been made for you. Thou shalt use either a 45/0 instrument or a 0/45 instrument. A 45/0 instrument is one where the light hits the sample at 45 degrees (preferably in a cone, all around the sample) and measures the reflectance at 0 degrees, which is to say, perpendicular to the surface of the sample. A 0/45 instrument simply interchanges the illumination and detection angles. The cool thing is that a wise old fellow named Helmholtz once said that 45/0 and 0/45 are interchangeable (usually). Most everyone I know believes him, so it must be true.

Illustration of either 45/0 geometry or an upside-down umbrella

One of the rationales for picking the 45/0 or 0/45 geometry is that it emulates the way one would normally read a Victoria's Secret catalog. Perhaps this may not have been apparent to everyone, but whenever I take a sidelong glance at this catalog that has been discretely addressed to my wife, the first thing I notice is that the magazine is printed on a high quality glossy stock. If I should happen to pick it up (which rarely happens, of course) I will naturally orient the prurient magazine so as to avoid seeing the specular reflection. One could argue that this natural viewing condition is something like 45/0.

Another rationale for 45/0 is that, when you convert these reflectance measurements to density, you have a number that is almost kinda sorta proportional to ink film thickness. One of the weaknesses in the correlation between the two is that darn specular reflectance. Even though 45/0 was designed to get rid of specular, a little bit or a lot of the specular will show up, depending on the smoothness of the surface.

This leads me to the topic of next week's blog post, polarized spectrophotometers.


  1. What? You only came up with 18 'certified' geometries? Did you include 30:T (or is it D:30 - I forgot) used to measure ceiling tile. And yes, there is (or used to be) a spectrophotometer that did that measurement. And don't get me started on all the little quirks and peccadillos (which is a cross between a pileated woodpecker and a nine-banded armadillo) of each of the 'approved' geometries.