Introduction

Over the years, a question has frequently arisen: Can the SpectroDens be used to measure offset printing plates? The answer depends on the specific platemaking process and the types of print products involved.

Mechanism of Operation

The SpectroDens is a reflective spectrophotometer that analyzes reflected light from measured specimens (such as ink, paint, and substrates), closely simulating human color perception. Upon activation of the measurement button, the following sequence of events occurs:

• The device activates its internally defined LED light source to illuminate the specimen positioned beneath its aperture.
• The specimen absorbs, reflects, and transmits light at various wavelengths based on its physical properties.
• A portion of the reflected light is captured through the SpectroDens optics at the aperture.
• The captured light passes through internal diffraction gratings, separating it into different spectral components at the sensor's resolution interval. This data is interpolated to form a spectral reflectance curve from 400 nm to 700 nm at 10 nm intervals.
• Based on the spectral reflectance curve, the device employs predefined equations to calculate values such as density, CIE Lab*, and ΔE.

Human vision operates in a comparable manner:

• The Sun provides illumination, with light encompassing both visible and invisible electromagnetic waves. Without light, objects exhibit no visible color (e.g., the ocean appears pitch black at night).
• Each object interacts uniquely with light, influencing its visual appearance.
• Light reflected or transmitted by objects reaches the human eye.
• Upon striking the retina, light is converted into electrical signals by photoreceptors.
• These signals travel through the optic nerve to the brain, where they are processed into the images we perceive.

(Reference: National Eye Institute)

In contrast, the SpectroPlate employs a significantly simpler method. It is equipped with a high-precision optical lens system and a high-resolution CMOS color matrix sensor with a high dynamic range. Each measurement is taken through a 1 × 1 mm aperture under homogeneous, spectrally broadband LED illumination. The resulting image, a 1024 × 1024 pixel capture capable of displaying 16 million uncompressed RGB colors, is processed by a powerful graphics signal processor using sophisticated imaging algorithms.

Test Case Comparison

Given the differences in operational mechanisms, a comparative test was conducted using a Fujifilm LH-PJ linear plate, imaged at 2400 dpi and 175 lpi RCS using AM screening.

SpectroPlate

Measurement with the SpectroPlate is straightforward. After configuring appropriate settings (such as Plate Type, Positive/Negative imaging, and AM/FM screening selection), the user may immediately proceed with measuring tint patches. In this test, the SpectroPlate recorded an exact 50% dot on a 50% tint patch.

SpectroDens

For those unfamiliar with using SpectroDens on printing plates, it includes a dedicated Printing Plate function. This function appears nearly identical to the Dot Area function based on the Murray-Davis equation, with the distinction of incorporating a Yule-Nielsen n coefficient, defaulted at n = 1.

The measurement procedure is as follows:

1. Calibrate the device using the non-image (paper white) area of the printing plate and the integrated white ceramic tile on the charging base.
2. Measure the solid patch on the plate.
3. Measure the selected tint/screen patch.
4. The LCD displays the dot measurement as a percentage.

Following this procedure, the SpectroDens reported a measurement of 52.8% for the 50% patch. If the print product can tolerate a ±3% dot gain, the SpectroDens serves as a suitable tool for relative quality control in the plate room.

The Yule-Nielsen n Coefficient

There are two primary types of dot gain: geometrical (mechanical) and optical (visual). From a platemaking perspective, measuring geometrical dot gain is critical to ensuring a stable and repeatable baseline before print production, thereby preventing unexpected tonal shifts during press runs.

When only a reflective spectrophotometer is available, optical dot gain as described by the Murray-Davis equation must be minimized during measurement. The Yule-Nielsen n coefficient facilitates this adjustment, allowing measurement to more accurately reflect geometrical dot gain.

Given that the Fujifilm LH-PJ plate is a linear offset plate, the 50% patch is expected to measure precisely at 50%. By increasing the n coefficient to 1.29, the SpectroDens measurement aligned with the SpectroPlate reading at 50%. This coefficient should remain applicable across other tint levels, provided the parameters remain consistent (i.e., Fujifilm LH-PJ, Positive imaging, AM screening).

It is noteworthy that because the SpectroDens calculates Dot Area based on both the paper white (plate base) and solid patch measurements, the consistency of these two reference patches is crucial. Should noticeable variation occur among plate batches, or during annual plate recertification performed by plate vendors using dedicated plate reading devices, it is prudent to adjust the n coefficient accordingly.

Conclusion

In summary, while the SpectroDens can be adapted for offset plate measurement with careful calibration and adjustment of the Yule-Nielsen n coefficient, the SpectroPlate remains the more straightforward and inherently precise tool for this application. Understanding the underlying mechanisms and ensuring consistent calibration practices are key to achieving reliable measurement results across different platemaking conditions.