Free introductory course
Press-Side Measurement Essentials
From manual densitometer to inline scanning.
Course syllabus
- Handheld vs strip-reader vs inline scanner: pros, cons, cost
- The geometry that matters: 45°/0°, polarization, UV filtering
- Color bars decoded: what to put on the sheet and why
- Repeatability vs reproducibility: the trap that costs you hours
Course content
The full lesson, module by module
The video is the introduction. The complete written course is below, structured to match the syllabus. Read it in one sitting or come back module by module.
Handheld spectrophotometers — X-Rite eXact 2, eXact 2 Plus, eXact 2 Xp — are the swiss-army knives of color measurement. Single-patch reads, mobile, low entry cost, no host computer required. They are the right device when you measure infrequently, when you need to verify a specific patch, or when you check a job at the customer site.
Strip readers (scanning tables) sit beside the press and scan color bars across the full sheet width. They take a sheet from the operator, scan it in a few seconds, and feed the result back to a console or to ink-key correction software. They strike the balance between speed and capital cost, and are common in mid-volume sheetfed shops.
Inline scanners like X-Rite IntelliTrax2 (model 2900) and IntelliTrax2 Pro (model 2900PRO) sit at the delivery end of the press and scan every sheet that needs measuring. Scan time is under 10 seconds. They are the right answer when you run multiple jobs per shift and cannot afford to interrupt the press for measurement.
The choice is not "which is best" — it is "which fits your volume and your tolerance". A 50-makeready-per-week shop running tight ΔE specs needs inline. A 5-makeready-per-week shop printing forgiving work can live with a handheld. Most real shops own at least two device classes and use them for different jobs.
A 45°/0° geometry illuminates the sample at 45° and reads it at 0° (normal). This is the standard for graphic arts because it most closely simulates how the human eye views a print under typical lighting. ISO 5-4:2009 specifies the geometry for spectro measurement of printed surfaces. IntelliTrax2 uses 45°/0° ring illumination (the sample is lit from all 360° around the 45° cone) to minimize directional artifacts.
Polarization is an optical filter that removes the specular component — the surface gloss — from the measurement. Polarized readings give you the "wet" ink density even on a dry sheet, which is useful when you compare wet press readings to dry proofs. Polarization typically raises density values by 0.05–0.20 D and increases inter-instrument agreement on glossy stocks.
UV filtering matters because most modern papers contain optical brighteners that fluoresce under UV. A measurement that includes UV (M1) reads the paper as your customer sees it under D50 viewing booths. A measurement that excludes UV (M2) reads the substrate "as if" it had no brighteners. M1 is the safer default for new contracts; M2 is useful when you need to match a non-OBA reference.
The geometry, polarization, and UV choices are not independent — they are stacked on each measurement. Every spectro reports them in the metadata. When two readings disagree, the first place to look is the measurement condition, not the ink.
A color bar is the strip of test patches placed on every press sheet, typically along the gripper edge or tail. It contains solids for each ink, percentage tints for tonal value, gray balance patches, overprints, and sometimes special control patches for spot colors. Without a color bar, automated measurement cannot tell what to read.
Common color bar systems include System Brunner, GMI / Bestcolor, X-Rite Standard, and customer-specific bars. They differ in patch sequence, patch size, and inclusion of special tests. The right choice depends on your customer requirements and your measurement device capabilities.
Patch height matters. IntelliTrax2 can read patches as small as 2 mm high — significantly tighter than the 4–5 mm minimum for older inline systems. Smaller bars steal less printable area, but they require the scan head to position the sheet precisely. Patch width is typically 3–4 mm, sized to match an ink zone (most presses have 32 mm ink zones).
The edge zone — the distance between the sheet edge and the start of the color bar — matters because it determines where the scan head can position. IntelliTrax2 needs to engage the bar within 38 mm of the sheet edge. Plan the imposition accordingly: a color bar pushed too close to the edge will not scan reliably; a color bar pushed too far in steals printable area.
Spot colors deserve their own discussion. If you run Pantone or custom brand colors, add dedicated solid and tint patches for each, not just the four-color overprint. A spot-color bar should mirror the structure of the process-color bar but for the extra inks in the job.
Repeatability is the variation when the same instrument reads the same sample multiple times in a row, by the same operator, under the same conditions. It is an intrinsic property of the instrument. A modern handheld spectro has repeatability around 0.05 ΔE on white; a modern inline scanner like IntelliTrax2 reports 0.15 ΔE max on white. These are tight enough for most production work.
Reproducibility is the variation when different instruments — or different operators, different times, different temperatures — read the same sample. Reproducibility is always worse than repeatability. The relevant industry metric is Inter-Instrument Agreement (IIA), and a good modern spectro reports IIA in the 0.3 ΔE average / 0.45 ΔE maximum range.
Why this matters: a brand owner with a handheld in their lab will read a different number than your inline scanner reads on the press, even on the same sample. If you do not understand the gap, you will spend hours chasing a phantom drift that is actually just inter-instrument disagreement.
The fix is procedural, not technical. Establish which instrument is the reference for each customer relationship, calibrate the others against it on known samples, and document the offsets. When the brand owner says "we measured 1.8 ΔE on your latest delivery", you should be able to say "our inline measured 1.3 ΔE; our handheld measured 1.6 ΔE; both agree with you within IIA tolerance".
The other fix is hardware: scheduled annual recertification of every measurement device through the manufacturer (or a certified service partner). UV LED sources drift, sensors age, and tile references wear. Without recertification, you are slowly walking away from your own reference.
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