7 Considerations in Aperture Size for Print Measurement

Print color and density measurement can be done with different aperture sizes on a spectrophotometer. Here are 7 things to consider when pondering aperture size for color measurement of prints:

1. What size are the color patches on the color control bar?
   Typically small aperture is used where smaller color patches need to be measured. This is often the case for presses with many printing units and where extra patches are being used for each ink zone. Space constraints require the use of smaller patches.
2. Which aperture is more convenient?
   Small apertures can be pretty convenient to use. Easier to position quickly, measure around small print defects, measure within image, measure density of large type. This makes a compelling case for trying to use small aperture across the board even if you are using larger color patches.
3. Which one is more accurate?
   Larger aperture lets in more light, making the measurements a bit more repeatable. A 3 mm aperture lets in 4X as much light as a 1.5 mm aperture. More light means less sensor noise. It is also important to note that by averaging 4 measurements on a 1.5mm aperture you can get about the same precision as 3mm aperture. SpectroDENS features a sensor with extremely low noise making it acceptable to use 1.5 mm aperture in most situations.
4. Are you using coarse line screen rulings?
   Nominal screen frequency also impacts the selection of aperture size. You can use 3 mm aperture at or above 85 lines/inch. You can use a 1.5 mm round aperture at or above 175 lines/inch. Coarse screens preclude the use of small apertures.
5. Do you need to compare two measurements?
   There can be some difference in the readings between small and large apertures. If measurements between a standard and a print or between two instruments need to be compared, it is best to use the same aperture size where possible. SpectroDENS is designed and calibrated for tight agreement between 1.5 mm and 3 mm apertures. Density measurements vary by less than 0.01D between the two aperture sizes and color measurements by CIE DE of about 0.30 on print samples.
6. Is a polarization filter needed?
   Polarization filters are a clever way to reduce the impact of gloss differences on color measurement. However, these filters cut out 90% of the light, making it necessary to use larger aperture to get enough light. With a small aperture and polarization filter, there is higher variability in the readings. Some instruments with a polarizing filter do not even offer the option of a small aperture size because the noise level may be unacceptable for precise work.
7. Will you need to change apertures for different needs?
   Some devices require you to pick the best aperture for your application when buying one. So you make the choice up front and deal with it. Others devices, like SpectroDENS provide swappable aperture sizes that you can change out based on need, giving you greater flexibility.