Selecting a Densitometer Aperture

Selecting an Aperture Size for your Color Reflection Densitometer:

Many users ask: what aperture size should I use with my color reflection densitometer?  The answer here is generally it is best to use an aperture that is as large as possible for the printing method employed. What this means is that if the printing process permits a 4mm color bar, than a 3-4mm aperture would be recommended. The reason for this recommendation is simple: a larger aperture reads from a larger sample, and therefore accounts for more of the variance within that sample in a single reading.

However, there are compelling benefits for using a smaller aperture even when the majority of work can accommodate a larger color bar. One example here is the ability to read "around" a possible imperfection in the patch (i.e.: a "pinhole"). In this example, the "pinhole" could skew the density reading if incorporated into the result; however with a smaller aperture it may be possible to carefully position the instrument so that the imperfection is not included in the reading.

In addition, it is important to recognize that typically instruments with smaller apertures can be positioned more quickly than those whose target windows are equal to or slightly smaller than the target being measured. This speed of positioning facilitates ease of use: research and common sense tells us that ease of use is a major factor in explaining systems use. Simply put, individuals are more likely to use something if it is easy to use: if a smaller aperture contributes to this than it may be a wise choice for many shops.

Furthermore, it is more likely that instruments with smaller apertures can be used to measure areas within the image, when necessary and permitted by the work being done. One example here is headline type: for this reason newspapers often require smaller apertures to potentially read the density of headline type. Another example is when there is a special solid color in the work that is not especially well represented by the color bar. Here, the job may dictate that readings be taken within the image, and it is more likely that the measureable area could be read by a smaller aperture.

Another factor to consider has to do with the color bar: when evaluating densitometer apertures maybe it is time to evaluate the color bar being used, as well. This is because for many users it is desirable to have certain patches represented by the color bar in each key zone. These desired patches include solid density for each printing unit, and more and more users want three color gray patches: these three color grays are typically printed next to a black tint for visual as well as instrumental evaluation. In addition to those patches that are desired for each ink key zone, other patches need to be inserted at various points in the color bar: these include overprints (traps), tone value patches (normally highlight, midtone and shadow), slur targets, etc. If the color bar for each ink key zone needs to have a solid for each printing unit together with one or more three color gray patches and corresponding black tints for visual evaluation, as well as its share of the other patches like overprints, tone values, and slur targets, the size of the patch could be critical. Obviously, presses with more printing units will potentially need to have smaller color bar patches than those presses with less printing units if solids in each key zone are required. These factors may dictate a smaller color bar patch subsequently a densitometer with a smaller aperture is required.

It is also important here to recognize that certain line screen rulings are just too coarse to measure with a single tone value reading from a small aperture. A reference chart is provide below and should be used as a guideline when an instrument aperture is selected.

ANSI and ISO standards recommend the following minimum aperture sizes be used when measuring tone value with the corresponding screen ruling:

Nominal Screen Frequency	Round Aperture	Non-round Aperture*
Lines per Inch	Minimum	Minimum
65	3.8 mm	11.3 mm
85	3.0 mm	7.1 mm
100	2.6 mm	5.3 mm
120	2.1 mm	3.5 mm
133	1.9 mm	2.8 mm
150	1.7 mm	2.3 mm
175	1.4 mm	1.5 mm
200	1.3 mm	1.3 mm
*Area can be achieved using either a single measurement or by averaging multiple measurements in random or adjacent locations to the sample. This includes the use of scanning type instruments that make continuous measurements from the sample.

Another factor to consider with instrument aperture size selection is the need for common apertures among instruments used for comparison: whenever readings from different instruments are to be compared, the apertures should be identical, if possible. This is imperative if an organization is to embark on an inter-instrument agreement program and expects to drive as much variance as possible out of the readings.

In addition, the need for sufficient signal for accurate readings frequently dictates aperture size. One example here is realized with the use of polarization filters: such filters may limit the choice of smaller apertures. For example, with X-Rite 500 series instruments polarization filters are available for apertures 3.4 mm or larger. The reason in this case is that the polarization filter cuts down on the "signal" required to ensure accurate readings when paired with smaller apertures, where the increased signal enabled by the larger apertures permits the use of polarization filters without compromising instrument accuracy. Another example is with the X-Rite 939: the specified accuracy of this particular instrument dictates that the smallest aperture available for this unit is 4 mm.

It is important to note that some instruments allow users to change the apertures, while others require that the instrument be sent to a service location for aperture changes. It is important to research this before a new instrument aperture is selected, as users may find it costly to change apertures once they have the instrument.

A related concept to instrument aperture selection is averaging capability. Some instruments support "on board" averaging of readings. This means that a user can take multiple readings and the instrument will average these into a single result. In this manner instruments that have smaller apertures will replicate the statistical sampling of a larger instrument, even when reading tone value. Of course, this adds steps to the process and makes the process a bit more difficult, but may represent the best way to work in some applications. An example here would be if a smaller aperture were required for 95% of the work, but a small percentage of the jobs required a coarse line screen ruling that differed from the recommendations of the smaller aperture. Here, when the coarse line screen ruling is utilized, averaged readings of the tone value patch could yield a result as accurate as the larger aperture. Instruments that do not support on board averaging typically have the ability interface to a computer, where a simple spreadsheet could be used to achieve the averaging necessary. The X-Rite 500 series does not support on-board averaging for the densitometric and colorimetric attributes.

What are the aperture sizes available for popular X-Rite Instruments? Please see the chart below:

X-Rite 500 Series
5XX (no suffix)	"Standard"	3.4 mm
5XX-S (Most Popular Option)	"Small"	2.0 mm
5XX-L	"Large"	6.0 mm
5XX-P*	"Publication Microspot"	1.6 x 3.2 mm
*Publication Microspot carries a premium on new instrument purchases, and requires that the instrument be returned to an authorized X-Rite service center if a user desires an existing instrument be converted to a Publication Microspot aperture. For users with "Standard," "Small" or "Large" unpolarized 500 series instruments, an aperture kit (part #518-80) can be purchased that enables switching among these particular apertures. Users of polarized aperture 500 series instruments can purchase an aperture kit that enables users to change between the "Standard" and "Large" apertures (#518-80LP)

X-Rite 939
4mm	8mm	16mm
User changeable apertures, ships with instrument

X-Rite SpectroEye Aperture Factory Installed
SpectroEye LT		4.5 mm
SpectroEye		4.5 mm
SpectroEye LT SA		3.2 mm (for 3.5 mm targets)
SpectroEye SA		3.2 mm (for 3.5 mm targets)

X-Rite DensiEye Aperture Factory Installed
DensiEye	Large Aperture	3.0 mm
DensiEye	Small Aperture	1.6 mm

X-Rite QuikDens Aperture Factory Installed
QuikDens	Large Aperture	3.0 mm
QuikDens	Small Aperture	1.6 mm

^©Copyright Bruce Leigh Myers, Ph.D. July 19, 2008

Comments

[HP LaserJet P2055dn]

Its amazing to see someone put so much passion into a subject.