Consistent Color Measurement

Color measurement devices work by measuring the amount of light reflected at different wavelengths. Measurement consistency is affected by factors such as ambient temperature, calibration, paper opacity, optical/sensor design of measurement device. An understanding of these factors can help ensure more reliable print color control by measurement.
Software settings are another practical issue in achieving consistent results. Spectral measurements are mathematically converted to color and density values. These conversions depend on settings and options selected through software. It is important to check software settings that pertain to color measurement and conversion.

Settings

Spectrophotometers have a number of settings which all have an effect on the reporting of color values. For measurements to agree, all the settings should first agree.

In the US, the most common settings are:

• Status T
• No Polarization
• Absolute White Calibration
• D50/2°
• M0
• ∆E76 (∆Ecmc & ∆E00 are coming into use, you really need to ask your customer)

Effect of illuminant and observer angle on CIE a*-b* values

Three papers viewed with ultra-violet (UV) light shown on left and without UV light on right. The presence of FWA (fluorescent whitening agents) can change the color drastically when viewed in light that contains some UV. In the absence of UV, the impact of FWA on appearance is minimal.

The color difference between two yellows, using different color difference formulas

Environmental Conditions

Spectrophotometers are not nearly as delicate as they were years ago, but they are still precision instruments. Extremes of temperature and dropping are not good for them, needless to say. But even temperatures ten degrees out of the normal operating range can temporarily have an effect on the measured values. Where possible, measurements should be carried out in an area where the temperature is controlled.
Dust or other contaminants are not friends of your spectrophotometer. A dirty lens can effect measurements. Dry compressed air can be used to clean dust from any external glass, but internal dust or smears must be taken care of by a service technician.

Calibration and Certification

There is one critical – and often under-appreciated – item that often comes along with a spectrophotometer: the calibration plaque. This plaque serves as a reference so the device knows what “white” is. If the plaque gets dirty, or if the plaque from a different unit is used, error will be introduced. (All spectrophotometers have such a plaque. Some plaques are internal to the unit.)

Is the certification of the instrument out of date? While spectrophotometers are very reliable instruments, they will drift, or be damaged in non-obvious ways. Instruments should be periodically recertified according to manufacturer’s specifications.

Backing

In the picture below, a page from a catalog was placed on top of a white backing (on the left), and a black backing (on the right). The backing material not only has an effect on color, but also can enhance or prevent show-through from the other side. This is true for light-weight stocks and can be huge for translucent or transparent films.

To numerically demonstrate the effect of backing, two areas on this sheet were measured with white and with black backing. The change in density due to ink on the other side is about three times as large with white backing, so you should measure over black backing when there is print on the other side. But ISO standards favor measurement over white backing. Regardless, for color measurements to agree, one must use the same backing material each time.

To get consistent color measurements pay attention to measurement settings, room temperature, presence of dust, white tile cleanliness, periodic device service and certification, use of consistent backing.

More articles about illuminant and observer angle, about measurement conditions, about color difference formulas, and about the use of polarization filters for measuring reflectance.