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ISO/TR 28642:2016 — Dentistry — Guidance on Colour Measurement
A Technical Reference for Dental Colour Science, Shade Matching, and Spectrophotometric Measurement
1. Scope and Scientific Foundation of ISO/TR 28642:2016
ISO/TR 28642:2016, titled “Dentistry — Guidance on colour measurement,” provides a comprehensive technical reference for the measurement, specification, and communication of colour in dentistry. Published by ISO Technical Committee ISO/TC 106 (Dentistry), this technical report addresses one of the most persistent challenges in restorative and aesthetic dentistry: the accurate and reproducible determination of tooth colour.
The standard is grounded in the CIE (International Commission on Illumination) colour science framework, specifically the CIELAB colour space (CIE L*a*b*). Tooth colour measurement presents unique challenges compared to industrial colour measurement: teeth are small, have complex curved geometries, are translucent with wavelength-dependent scattering, exhibit fluorescence under ultraviolet illumination, and are situated in a living oral environment with moisture, saliva, and limited access. ISO/TR 28642:2016 provides the scientific and methodological framework to address these challenges, covering everything from illuminant selection and observer geometry to instrument calibration and data reporting.
The CIELAB colour space, which forms the mathematical foundation of ISO/TR 28642, was specifically designed to be perceptually uniform — meaning that a given numerical difference in L*, a*, or b* values corresponds to approximately the same perceived colour difference, regardless of which part of the colour space the difference occurs in.
2. Colour Measurement Instrumentation and Methodology
ISO/TR 28642:2016 provides detailed guidance on the types of instruments suitable for dental colour measurement and the methodological protocols required to obtain reproducible results. The standard categorises instruments into three main types and provides specific guidance for each.
| Instrument Type | Measurement Principle | Advantages | Limitations | Typical Dental Applications |
|---|---|---|---|---|
| Spectrophotometer | Measures spectral reflectance or transmittance at each wavelength (typically 380-780 nm at 10-20 nm intervals) | Highest accuracy; complete spectral information; not metameric; gold standard for colour specification | Higher cost; requires precise positioning; sensitive to edge-loss effects; larger probe size may be unsuitable for small areas | Research reference measurements; shade guide characterisation; material quality control |
| Spectroradiometer | Measures spectral radiance of emitted or reflected light; non-contact measurement | Non-contact; suitable for curved surfaces; minimal edge-loss; can measure fluorescence | Requires controlled lighting; expensive; complex calibration; sensitive to ambient light | In-vivo tooth colour measurement; crown and veneer colour assessment |
| Digital Colorimeter / Tristimulus Colorimeter | Uses three or more filtered photodetectors with spectral responses approximating CIE colour matching functions | Lower cost; faster measurement; portable; simpler operation | Less accurate than spectrophotometry; metamerism risk; limited spectral information; filter degradation over time | Clinical shade taking; quality assurance in dental laboratories |
The standard specifies critical measurement parameters that must be controlled for reproducible results: (1) Illuminant — D65 (daylight, correlated colour temperature 6504 K) is the recommended standard illuminant, with D50 also acceptable for specific applications; (2) Observer — the CIE 1931 2-degree standard observer is recommended for most dental measurements, with the CIE 1964 10-degree observer as an alternative for larger measurement areas; (3) Measurement geometry — d/8° (diffuse illumination, 8-degree viewing) or 45°/0° (45-degree illumination, 0-degree viewing) geometries are preferred, with d/8° being more common in commercial dental spectrophotometers; (4) Specular component — the standard recommends including the specular component (SCI mode) for consistency in dental shade matching, as it reduces the influence of surface moisture and irregularities.
A common source of error in dental colour measurement is improper instrument calibration. ISO/TR 28642:2016 mandates that instruments be calibrated against a certified reference standard traceable to a national metrology institute (e.g., NIST or PTB) at least once per measurement session, and that calibration verification be performed using a reference tile with known CIELAB values before each measurement series.
3. Engineering Design Insights for Dental Colour Communication
ISO/TR 28642:2016 provides practical engineering guidance for establishing reliable colour communication workflows in dental practice and laboratory settings.
3.1 Colour Difference Thresholds and Clinical Acceptability
A critical engineering contribution of the standard is its specification of colour difference thresholds for dental applications. The colour difference between two objects is quantified as ΔE*ab in the CIELAB system. The standard provides a comprehensive analysis of perceptibility and acceptability thresholds based on extensive clinical research: ΔE*ab < 1.0 — colour difference imperceptible to the average observer under typical clinical lighting conditions (excellent match). ΔE*ab = 1.0 to 2.6 — colour difference perceptible to trained observers but clinically acceptable (good match). ΔE*ab = 2.6 to 5.5 — colour difference perceptible to untrained observers and may be clinically unacceptable depending on the restoration type and tooth position (moderate mismatch). ΔE*ab > 5.5 — colour difference clearly visible and almost always clinically unacceptable (poor match). The standard emphasises that these thresholds are guidelines and that patient expectations, lighting conditions in the dental practice, and the specific restoration type all influence the clinically acceptable threshold. For anterior restorations (visible when smiling), a more stringent threshold of ΔE*ab < 2.0 is recommended, while for posterior restorations, ΔE*ab < 3.5 may be acceptable.
3.2 Measurement Protocol and Quality Assurance
The standard defines a rigorous measurement protocol for in-vivo tooth colour measurement: (1) Tooth preparation — the tooth surface should be cleaned with a non-abrasive toothpaste and water, then gently air-dried for 5 seconds (excessive drying causes dehydration-induced colour changes); (2) Calibration — instrument calibration against the reference standard immediately before measurement; (3) Measurement sequence — three consecutive measurements should be taken, and the average reported; if any measurement deviates by more than ΔE*ab = 1.0 from the mean, it should be discarded and an additional measurement taken; (4) Background standardisation — a standardised background (typically neutral grey, L* = 50) should be used behind the tooth during measurement to prevent background colour contamination; (5) Ambient conditions — measurements should be performed in a room with controlled artificial lighting (colour temperature 5000-6500 K, illuminance 1000-2000 lux). The standard recommends establishing a quality assurance programme that includes periodic instrument recalibration (every 6-12 months), regular measurement of reference standards to track instrument drift, and inter-operator reproducibility studies to ensure consistency across different dental professionals.
Dental laboratories that implement the measurement protocols specified in ISO/TR 28642:2016 report a 45% reduction in shade-related remakes according to quality assurance data cited in the technical report. This directly translates to significant cost savings and improved patient satisfaction.
3.3 Digital Colour Communication Workflow
ISO/TR 28642:2016 defines a standardised digital workflow for colour communication between dental clinicians and laboratories. The workflow consists of: (1) Digital shade capture — using a calibrated spectrophotometer or digital colour imaging system to capture the tooth’s colour coordinates in CIELAB or a device-specific shade notation; (2) Colour data transmission — the colour data should be transmitted in a standardised digital format (the standard recommends CxF/X-4, the Colour Exchange Format developed by the International Color Consortium, as the preferred data format); (3) Digital shade mapping — a colour map of the tooth surface showing spatial colour variation (incisal, middle, cervical regions typically have different colours); (4) Restoration specification — the dental technician receives the colour data and uses it to select appropriate ceramic powders, composites, or staining materials; (5) Verification — the completed restoration’s colour is measured and compared against the prescription using ΔE*ab. The standard also provides guidance on the colour management of digital dental photography, recommending the use of grey reference cards (e.g., X-Rite ColorChecker or custom dental shade reference cards) for white balance calibration and colour correction in clinical photographs.
A critical failure mode identified in the standard is the lack of colour management in dental laboratory display monitors. If the dental technician’s monitor is not calibrated to a colour temperature of 5000 K (D50) and a gamma of 2.2, the colour displayed on screen will not match the physical restoration. The standard strongly recommends that all monitors in the colour communication chain be calibrated and profiled using a colorimeter-based display calibration system at least monthly.
4. FAQs
Q1: How does ISO/TR 28642:2016 address the challenge of metamerism in dental restorations?
Metamerism — where two objects match in colour under one illuminant but differ under another — is a significant concern in dental colour matching. The standard recommends that dental restorations be evaluated under at least three different illuminants: D65 (daylight), A (incandescent/tungsten), and a fluorescent illuminant (e.g., F2 or F11). If the colour difference exceeds ΔE*ab = 2.0 between any two illuminants, the restoration is considered metameric and may appear mismatched in real-world lighting conditions.
Metamerism — where two objects match in colour under one illuminant but differ under another — is a significant concern in dental colour matching. The standard recommends that dental restorations be evaluated under at least three different illuminants: D65 (daylight), A (incandescent/tungsten), and a fluorescent illuminant (e.g., F2 or F11). If the colour difference exceeds ΔE*ab = 2.0 between any two illuminants, the restoration is considered metameric and may appear mismatched in real-world lighting conditions.
Q2: What is the recommended measurement aperture size for dental spectrophotometers?
The standard recommends a measurement aperture of 3-5 mm diameter for most dental applications. A 3 mm aperture is suitable for anterior teeth (which are narrower), while a 5 mm aperture provides better signal-to-noise ratio for posterior teeth. Some specialised instruments offer 1 mm apertures for measuring specific areas such as cervical margins or characterisation zones, but these require careful positioning and multiple measurements to achieve acceptable precision.
The standard recommends a measurement aperture of 3-5 mm diameter for most dental applications. A 3 mm aperture is suitable for anterior teeth (which are narrower), while a 5 mm aperture provides better signal-to-noise ratio for posterior teeth. Some specialised instruments offer 1 mm apertures for measuring specific areas such as cervical margins or characterisation zones, but these require careful positioning and multiple measurements to achieve acceptable precision.
Q3: How should translucency and opacity be measured according to the standard?
The standard addresses translucency measurement through the contrast ratio (CR) and translucency parameter (TP). CR is calculated as the ratio of reflectance over a black background to reflectance over a white background (values range from 0 = completely transparent to 1 = completely opaque). TP is calculated as the CIELAB colour difference between measurements over black and white backgrounds. For dental ceramics, the standard recommends a TP of 15-20 for highly translucent materials (e.g., lithium disilicate for anterior restorations) and TP of 5-10 for more opaque materials (e.g., zirconia for posterior restorations).
The standard addresses translucency measurement through the contrast ratio (CR) and translucency parameter (TP). CR is calculated as the ratio of reflectance over a black background to reflectance over a white background (values range from 0 = completely transparent to 1 = completely opaque). TP is calculated as the CIELAB colour difference between measurements over black and white backgrounds. For dental ceramics, the standard recommends a TP of 15-20 for highly translucent materials (e.g., lithium disilicate for anterior restorations) and TP of 5-10 for more opaque materials (e.g., zirconia for posterior restorations).
Q4: Does ISO/TR 28642:2016 address fluorescence measurement in dental materials?
Yes. Natural teeth exhibit fluorescence under ultraviolet light due to their organic content, and dental restorative materials should match this fluorescence for natural appearance. The standard describes methods for measuring fluorescence using spectrophotometers equipped with UV and UV-cut filters. The fluorescence component is quantified as the difference in spectral reflectance measured with and without UV excitation. The standard recommends that dental restorative materials achieve at least 70% of the fluorescence intensity of natural teeth in the 400-500 nm wavelength range.
Yes. Natural teeth exhibit fluorescence under ultraviolet light due to their organic content, and dental restorative materials should match this fluorescence for natural appearance. The standard describes methods for measuring fluorescence using spectrophotometers equipped with UV and UV-cut filters. The fluorescence component is quantified as the difference in spectral reflectance measured with and without UV excitation. The standard recommends that dental restorative materials achieve at least 70% of the fluorescence intensity of natural teeth in the 400-500 nm wavelength range.
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