Measuring the Color of Black: Precision in the Deepest Shades

Since prehistoric times, black, particularly charcoal-based black, has been used as one of the earliest colors. Despite its long history, accurately measuring and quantifying black remains a challenge due to its minimal light reflection. This article explores how advancements in technology and color science have enabled more precise control and evaluation of deep black shades, enhancing reliability in quality control.

1. The Role of Black in Daily Life

Black has been a crucial material since the dawn of human civilization, with carbon black production traced to ancient China, Egypt, and India. By the 15th century, the demand for carbon black surged with the invention of the printing press. Today, it is found in everyday applications, including inkjet printers, rubber reinforcement, and electrically conductive plastics. Its most notable use is as a pigment in paints, coatings, and plastics, where its higher tinting strength makes it superior to iron black or organic pigments. However, not all blacks are the same, as their undertones and reflective properties vary significantly.

2. Defining Blackness: Jetness Indices

Black is typically defined by its complete absorption of visible light. Carbon black pigments can absorb up to 99.98% of light. The blackness of a material, such as a black coating, is determined by its absorption coefficient. However, even deep black coatings can have undertones, either bluish or brownish, that affect their visual perception. For instance, black with a bluish undertone appears richer and darker than one with a brownish undertone.

This hue-dependent perception of black is quantified by the term "Jetness" (Mc). Jetness is determined by calculating the blackness value (MY), which is derived from the Tristimulus Value Y using a 10° Observer. However, the MY value alone does not account for undertones, so the Jetness value Mc is calculated to describe the depth of black. A bluish undertone leads to a higher Mc value, indicating deeper blackness, while a brownish undertone results in a lower Mc value.

The difference between MY and Mc defines the hue of a black color, known as the undertone (dM). (Fig. 1) Positive dM values indicate a bluish shade, while negative values suggest a brownish shade (Fig.2 ) These measurements of blackness, Jetness, and undertone are standardized in international norms such as ISO/DIS 18314-3 and DIN 55979.

3. Challenges in Measuring Black Colors

3.1 Signal-to-Noise Ratio

Measuring black with a spectrophotometer is challenging due to the minimal light reflected by black samples. While a white sample reflects nearly all light, darker colors absorb most of it. As black absorbs more light, the signal captured by the spectrophotometer decreases, while electronic noise remains constant. This changes the "signal-to-noise ratio," making it harder to achieve accurate and repeatable measurements of dark colors. For innovative black coatings with lightness values below L* < 1 (Fig.3), this is an even greater challenge, pushing the limits of spectrophotometer performance.

3.2 Pro-Model Spectrophotometers

Gardco carries the spectro2guide Pro and the color2view Pro advanced handheld and bench-top spectrophotometers capable of measuring deep black colors and their undertones. Working with carbon black pigment specialists, BYK-Instruments ensured that their devices could assess the accuracy of the deepest blacks. The success of these devices is largely due to the use of high-powered LEDs that offer both short- and long-term stability, combined with precise calibration and operation modes tailored to black color measurement.

To overcome the signal-to-noise ratio challenge, these instruments use higher energy LED lamps and extended illumination times, especially in a specialized Jetness mode. In this mode, if the blackness value falls too low, the instrument automatically switches to a regular mode to measure Grayness values (G-values) instead of Jetness values (M-values), in line with the DIN ISO 18314-3 standard. These advancements enable the spectro2guide Pro and color2view Pro models to measure deep black samples with exceptional precision.

4. Color Measurement of Deep Black Colors

4.1 Sample Preparation

The spectro2guide Pro was tested on six high-gloss black samples labeled “Test 1” through “Test 6,” each with different MY and dM values. It's critical that black samples are perfectly clean and glossy before measurement, as even small contaminants can significantly affect results. To ensure a pristine surface, samples were cleaned with distilled water and lint-free wipes before testing. The lamp can be used to assess the quality of the sample surface under strong direct light. A 15 - 45-degree angle of illumination has proven to be most effective.

4.2 Visual Assessment

Samples were visually assessed in the byko-spectra Pro light booth under daylight conditions (D65) at a 45-degree angle. The byko-spectra pro uses a combination of filtered tungsten-halogen lamps and LEDs to simulate D65 lighting resulting in the highest rendering Class A according to CIE 51.2. This setup is ideal for color-critical evaluations and guarantees accurate color matching under standardized lighting, however the samples can be sorted by the degree of blackness by a trained observer. The samples were sorted by blackness level, with “Test 6” being the deepest black (MY), while “Test 1” had a yellow undertone (dM).

4.3 Testing Procedure

Each sample was measured 50 times consecutively at the same spot, with the spectro2guide Pro directly connected to smart-lab Color software to minimize user error. The software analyzed the measurement data in real time.

4.4 Results

The spectro2guide Pro and the color2view Pro successfully measured the blackness and undertones of all six samples, with results closely matching the visual assessments. For the ultimate black sample, “Test 6,” the average blackness value (MY) was 378, with a Tristimulus Y value of 0.0116. Despite this extremely low remission of light, the standard deviation for 50 measurements was only 0.0002, demonstrating the device's precision.

Conclusion

BYK Instruments' Pro-model spectrophotometers have set a new standard for measuring deep black colors, delivering unprecedented accuracy and reliability. These instruments can now be used to ensure color harmony even in the blackest shades, meeting the high standards required for quality control in various industries.

References

ISO 18314-3:2015, Analytical Colorimetry - Part 3: Special Indices.

DIN 55979:2020-12, Pigment Black Measurement Standards