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.
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.
|Illuminant||D50, D65, A, F1 – F12 …||The ISO standards for the graphic arts require a D50 illuminant and the 2° observer, but various other combinations, such as D65/10°, are commonly specified in the packaging industry.|
|Observer||2° or 10°||The ISO standards for the graphic arts require a D50 illuminant and the 2° observer, but various other combinations, such as D65/10°, are commonly specified in the packaging industry.|
|Measurement condition||M0, M1, M2, or M3||If the substrate that you are measuring does not have fluorescent whitening agents, then it doesn’t matter a great deal whether you choose M0, M1, or M2. But if you are printing on paper, and the paper is white, then it is likely that you have some fluorescence, and the preferred measurement condition is M2.|
|Color difference formula||ΔEab, ΔECMC, ΔE94, ΔE00 …||In comparing using ΔE, you have another array of choices. In 1976, the ΔEab (“delta E a b”, also known as ΔE76) formula was introduced. To improve These color differences did not agree as well with our perception as one would hope, so a number of alternative formulas have been suggested, including ΔECMC, ΔE94, and ΔE00 (also known as CIEDE2000). The choice that is currently preferred in the standards committees is ΔE00, but the official recommendation is relatively new.|
|Density filter||Status T, Status E, Status I …||If you are measuring density, then you have to choose the filter set for measuring densities. Status T is commonly used in the US, and Status E in Europe.|
|Density Reference||Absolute or paper relative||Another option is whether you want the measurement to be relative to the substrate you are printing on or relative to an absolute white.|
|Polarization||On or off (on is M3)||Another choice when measuring density is whether or not to use a polarization filter. This is enabled by selecting the M3 condition. In situations, where there are variations in glossiness and you don’t want the color measurements to be impacted by gloss changes, the M3 mode is used. For example, polarization is useful if you are measuring the density of ink before it has completely dried.|
In the US, the most common settings are:
- Status T
- No Polarization
- Absolute White Calibration
- ∆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
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.
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.
|White Backing||Black Backing|
|#1||0.132 D||0.183 D|
|#2||0.203 D||0.206 D|
of ink on
|0.071 D||0.023 D|
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.