Seeing Color in a New Light: The Significance of Color Measurement Devices

Color management is about having a target, a process and a way to verify you have hit your target, and the spectrophotometer plays a vital role both in the process and the verification stages. If you are serious about color, then you need to be serious about your color measurement device. You need to know what is available, what they do and how they vary from one to another.

Measurement Devices

For the purpose of this article — and for the majority of applications you will encounter in the graphic arts arena — I’ll be looking at the X-Rite i1 Pro2 and eXact; the Barbieri SpectroPad and Spectro LFP Series; and finally, the TECHKON SpectroDens. Each of these devices has pros and cons that would lead you to consider one over another. Some of those variables that I’ll look at are lamp temperature, aperture size, optical brightening agents (OBA) and fluorescence (measurement conditions), measurement geometry and inter-instrument agreement. Before going too far, I want to make sure that we’re all on the same page about what is going on with these devices and what they do. A spectrophotometer uses a light source that shines (or is reflected) onto a substrate and the reflected light is then measured by a detector. This is a bit of an oversimplification, but it does capture what the device is doing, and as we cover the variables it will help to understand what difference they are making. If you recall how we see or how color is perceived, then you remember that you need a light source, an object and an observer.

Lamp Temps and Aperture

Of the variables, lamp temperature is probably the smallest one to consider. The X-Rite i1 Pro2 and the eXact both use a tungsten bulb for the main light source. A tungsten bulb generates heat when it is used and even though this is a small factor, it is one to be considered. The heat of the tungsten bulb can impact the color being measured (it’s known as thermochromic and though small, it can affect some inks). The other devices that I’ve covered here use LED bulbs as their light source, so the heat is even more minimal.

The next variable and one that plays a much larger role in selecting a device is the aperture size. The X-Rite i1 Pro2 has a 4.5-millimeter aperture and this is the only size available for this device. The X-Rite eXact has four aperture sizes available and you select which size you want when you purchase the device — 1.5-, 2-, 4- or 6- millimeter. The Barbieri SpectroPad comes with one aperture size available, which is 6-millimeter. The Barbieri Spectro LFP qb has a switchable aperture so that you can select 2-, 6- or 8-millimeter — and with the new unit, the head is detachable for spot color measurements. And finally, the TECHKON SpectroDens has a 3-millimeter aperture standard, and an optional 1.5-millimeter aperture.

So what difference does all this make? Remember when I noted that the spectrophotometer works by shining a light down onto the substrate and the reflected light is picked up by a detector? The larger the aperture, the more light that is available to be picked up by the detector and the more repeatable the measurements will be. If you think about all the different materials that can be printed, from smooth vinyl to textured canvas, and if you think about light being reflected off the surface, I’m sure you can imagine that some smooth materials will do just fine with a very small aperture because there is very little variability in the surface of the material. However, other textured materials (think textiles) might benefit from a larger aperture that lets in more light to shine onto the substrate. This avoids misreadings due to shadows that are cast by a single light source. And let’s not stop with just textured materials but think about the different droplet sizes that printers use. Proofing printers use very fine droplets whereas grand-format printers can use much larger droplet sizes. If you are using a printer that uses large droplet sizes or measuring a substrate that has a lot of texture, you will want a spectrophotometer with a larger aperture to reduce the variability in your measurements. Lastly, if you are using a polarization filter, which cuts down dramatically on the light that it allows in, then a larger aperture is a must.

It’s important to understand what is going on with OBAs and the role they play when looking at spectrophotometers. This was an issue with the earlier X-Rite i1 Pro models (gray exterior) that are still in use in many graphic shops across the industry. The difference in the i1 Pro and the i1 Pro2 (black exterior) is how they handle UV light. UV light (in the 340-380 nanometer range of the spectrum) can excite OBAs, which make the substrate appear whiter/brighter. With the older i1 Pro device you had two choices: order a device with or without a UV cut filter. There were plenty of proponents on both sides of this issue and you could start a good discussion by advocating one over the other. If you have an i1 Pro, you should make sure you know which one you have, and you can tell by looking at the device as it will have an indicator to show if it is a UV cut device. One thing I would say, I’d really want to know that UV content and how it is affecting my measurements. I’ll talk about measurement conditions next and will cover how the newer devices manage OBAs and UV light.

Measurement Conditions

Measurement conditions are based on ISO standards, specifically 13655:2009. M0 is for legacy measurements and is basically considered the “wild west.” There is no standardization of UV content in the illuminant, whereas M1 dictates that the spectral power distribution of the illuminant should match CIE illuminant D50. M1 takes OBAs into account and therefore controls the variability that you don’t have with the M0 condition. The M1 measurement condition is considered the standard for North America. Also note that there is an M1 part 1, which means the device measures once and gets the data needed for UV content without having to measure the line a second time and calculate mathematically. The M2 measurement condition is one where UV is excluded (either filtered or mathematically excluded). And M3 adds a definition of polarization that is based on M2. (Typically polarization is used to cut down on gloss.) The M3 polarization cuts out 90% of the light, making it necessary to use a larger aperture to let in enough light. With a small aperture and a polarization filter, there is higher variability in the readings. Some instruments with a polarizing filter do not offer the option of a small aperture size because the noise level will be unacceptable for precise work. M3 is typically used by offset printers so that they can measure “wet” sheets, which have a greater gloss to them.

The X-Rite i1 Pro2 has M0, M1 and M2 measurement conditions available. The X-Rite eXact offers M0, M1, M2 and M3 measurement conditions. The Barbieri SpectroPad offers M0, M1 and M2 measurement conditions. The Barbieri Spectro LFP qb offers M0, M1 part 1, M2 and M3 measurement conditions and also offers the ability to measure reflective and transmissive (backlit) materials. The Spectro LFP qb is the only device (in digital printing) that can measure fluorescent inks. The TECHKON SpectroDens uses LED lamps and can provide measurement conditions M0, M1, M2 and M3.

Measurement Geometry

ISO 13655:2009 references ISO 5-4, which defines the following four geometries that may be used in a spectrophotometer:

• 45 degree annular: 45° a:0°
• 45 degree circumferential: normal 45° c:0°
• Normal: 45 degrees annular 0° :45° a
• Normal: 45 degrees circumferential 0° :45° c

The takeaway here is to make sure any device you purchase is rated for one of these standards — and all of the devices in this article meet one of these standards. And as a quick aside, do not buy a device that states it conforms to DIN 5033 because that does not necessarily mean it conforms to ISO 13655:2009.

Simply stated, what this standard refers to is the angle of the light source and the angle of the detector — or thinking back to what I mentioned earlier about the perception of color, this is the “observer.” For instance, an X-Rite i1 Pro2 is a 45-degree/a:0-degree ring illumination optics (the a is for annular), which means the single tungsten bulb shines its light into a mirror which is then directed down to the substrate at a 45-degree angle and the detector (or pickup) is at 0 degrees (or located directly above the aperture). The X-Rite eXact uses multiple lights set in a ring to get 45-degree circumferential light with the detector/pickup at 0. The Barbieri SpectroPad is also a 45-degree/ c:0-degree circumferential device which uses three banks of seven LED bulbs for its light source. (See SpectroPad drawing, page 40).

An important point to note is when you are working with a single light source unit such as the i1 Pro2 and you are trying to avoid variables, always measure the same direction and don’t allow prints to be rotated so that you are measuring some patches at a 90-degree angle from other patches. Even on a relatively smooth surface you can have a 1-3 Delta E deviation in measurements creating an unacceptable variable in your color management process. And another quick aside, if you are measuring spot colors or single patches, always use the guide that comes with your spectrophotometer (for those that use one, such as the i1 Pro2). The device needs to be a prescribed distance from the patch being measured. By not using the guide you are again introducing a variable into your color management process.

As you can see, if I was working with fabrics or other challenging substrates I would want the multiple light sources and the larger aperture of a SpectroPad or the Spectro LFP qb (or an X-Rite eXact with the larger aperture and the scan ruler). However, if I was doing mostly graphic work then the i1 Pro2 or Techkon SpectroDens will serve me well. Spot color libraries can be loaded onto the X-Rite eXact, the TECHKON SpectroDens and the Barbieri devices. Having more light sources and the ability to either select a larger aperture (such as the X-Rite eXact when you purchase it) or on the fly (as with the Barbieri Spectro LFP qb) has a cost associated with it. Also, the ability to wirelessly connect or have verification (such as the SpectroPad with the DOC software) makes for a more useful tool. I always say you get what you pay for, and you pay for what you get. Knowing why you’d select a device with a larger aperture or with multiple light sources is important to allow you to make a good decision when you select your next color measurement device.

About the Author

Ray Weiss, digital imaging specialist for SGIA, joined the association in 2014. He provides solutions and technical information on digital printing as well as digital equipment, materials and vendor referrals. He oversees several workshops at SGIA along with the association’s digital equipment evaluation program. Weiss is the Project Manager for both the PDAA Certification program and SGIA’s Digital Color Professional certification program and is an instructor for the Color Management Boot Camps. His 25-plus-year career in the graphics industry has spanned owning his own prepress and offset business to digital wide-format sales, training, support and service. Weiss has extensive experience in color management and worked closely with the Smithsonian Institution to implement a color managed workflow in their Exhibits department.