Colorimetric characterization of gonio-apparent titanium anodized samples

In the following, we present characterizations of the colour changes of gonio-apparent titanium anodized samples with illumination and observation directions.

3.1 Gonio-spectrophotometer used for the chatacterizations

The device used for the measurements is a sequential-scanning laboratory gonio-spectrophotometer built at Laboratoire Georges Friedel. This device, called OptiMines, is fully described by Matsapey et al. [27]. It is composed of three main sub-systems:

- a rotating illumination arm containing a 50 W fibered halogen lamp which illuminates the sample after being collimated by an optical system,

- a rotating and movable sample holder

- a rotating detection arm containing a second optical system which focusses the light beam reflected by the sample onto a fibered Maya2000 Pro spectrometer

The illumination and detection arms have only one rotational degree of freedom and move in the same plane. The sample holder has two rotational degrees of freedom: it may rotate on itself or tilt around the motion plane of the illumination and detection

arms. The sample holder may also be moved back to allow the direct spectrometric measurement of the source.

The key features of this device are its high angular resolution (less than 0.02°) and its low incident light half-divergence and detector’s angular half-acceptance (both are equal to 0.1°), which make it very sensitive to rapid angular variations of the spectral reflectance. The effective wavelength range of the gonio-spectrophotometer is [380 nm – 1100 nm], with a spectral bandwidth of 3 nm and a spectral resolution of 0.5 nm. This device has the particularity to combine both small size (the whole setup operates inside a volume of about 1 m x 1 m x 0.6 m) and high angular resolution.

In the paper describing this setup [27], the ability of OptiMines to measure both the BRDF of diffusing materials as well as the colour evolution of glossy gonio-apparent ChromaFlair samples was demonstrated. In the following, we present colorimetric characterizations of gonio-apparent titanium anodized samples. The colour variations around the specular direction are characterized, more specifically the influence of the direction of the incident light on these colour variations on the one hand, and the influence of the surface roughness of the sample on the other.

3.2 Anodized Ti samples

The samples are 3 cm x 4 cm coupons sampled in commercial-grade pure titanium plates (ASTM grade 1 or 2). Two types of grade 2 plates were used: the first one is 1.3 mm thick and has an initial rather rough surface finish before preparation; the second one is smoother, with a thickness of 2 mm. The grade 1 plate is 2 mm thick, with a relatively smooth surface topography.

Different surface preparations have been carried out on the samples as presented in Tab. 1, where the denomination of the various samples is also mentioned. The average roughness parameter Ra [28] indicated in Table I was measured with an optical profiler Wyko NT9100 (ex-Veeco) Bruker Nanoscope^TM. After degreasing with acetone, the samples are anodized in sulfuric acid 0.5 M, with a galvanostatic regime current of 6 A. The anodizing process is stopped when the cell potential reaches the desired value. This value is taken equal to 10 V, 20 V or 90 V, to generate different oxide layer thicknesses and thus different colours, as the colours are due to an interference phenomenon occurring in the TiO2 layer [2].

Denomination Material Sample

Tab. 1 – Materials, surface preparation and denomination of the samples.

Fig. 5presents digital colour pictures of the anodized samples with an observation angle either equal to or different from the incidence angle of light, which respectively corresponds to the “specular” and “out of specular” geometries.

At first sight, we observe that the colour of the samples, for the same anodizing potential, may vary with the surface finish. This phenomenon is especially noticeable for the samples anodized at 90 V, which appear either pink in series 2 in the specular direction or green in series 4 in the same observation conditions. One may also notice that the colour of the 90 V sample in series 1 appears pink in the specular direction, indicating that the anodizing process yields different oxide layer properties on grade 1 or grade 2 metallic plates.

All the samples do not have the same level of gonio-appearance, i.e. the same variation of colour rendering according to the viewing angle. It is more pronounced for the 90 V samples in series 4, whereas the roughest samples (series 2) are almost not gonio-apparent.

Fig. 5 – Digital colour pictures of the anodized samples of the series 2 and 3 (a) and of the series 1 and 4 (b). The incidence angle of the illuminant source is 45° and the observation angle is either 45° (specular direction) or 70° (out of the specular direction).

3.3 Characterization method for the colour variations

Many parameters may influence the spatial colour variations of the samples. We investigated this influence through many directions and give in this section some examples illustrating its characteristic features. We first study the influence of the surface finishing on the chromatic paths (colour variations) around the specular direction. This is illustrated for an incidence angle of 45°, by two comparisons:

- the first one between the samples anodized at 10 V of series 2 and 3, - the second one between the samples anodized at 90 V of series 2 and 4.

We may observe that the rougher the surface, the weaker the colour variations in the chromaticity diagram. Focusing on the mirror polished samples, we study the influence of the incidence angle on the chromatic paths around the specular direction. This is illustrated on the sample anodized at 90 V of series 1, with four different incidence angles: 30°, 45°, 60° and 70°. We observe that for grazing incidence angles the the colour variations are weaker. This study also highlights the evolution of the colour in the specular direction for the previously cited angles.

3.4 Results and discussion

In this part, we will represent the colour evolution of the samples, as described above, in the CIE 1931 chromaticity diagram. The rationale for this choice as well as the details of the calculation of the (x, y) chromaticity values from the BRDF values are given elsewhere [29]. We summarize here the most significant results.

The chromatic paths of the samples are first investigated around the specular direction, for an incidence angle of 45°, with respect to the influence of the surface roughness, for two different anodizing potentials, 10 V and 90 V. Errore. L'origine riferimento non è stata trovata. shows the colour variations of the samples anodized at 10 V of series 2 and 3 and the samples anodized at 90 V of series 2 and 4, in the CIE-chromaticity diagram around the specular direction. We observe that for smooth or mirror polished surfaces, the saturation of the colour, evaluated here as the distance of the colour to the D65 white point, exhibit higher variations around the specular direction than for “rough” samples. The highest saturation variations occur for the most specular samples (10 V of series 3 and 90 V of series 4).

Moreover, for these samples, the most saturated colour are observed in the specular direction, which is consistent with the interferential origin of the colours. In contrast, non-interferential coated surfaces such as metallic plates with thick varnish coating, would display a less saturated colour due to the white light component reflected by the surface of the varnish in the specular direction.

Finally, the fact that the colours of the 10 V samples vary in the chromaticity diagram along almost straight lines passing through the D65 point means that the hue is almost invariant according to the observation angle for all surface roughness conditions. This is however not the case for the 90 V sample of series 4, which is consistent with the qualitative observation of the high gonio-apparent character of this sample, as illustrated in Fig. 5.

In order to check our goniometric colour measurements, we compared, for the samples of series 2, the (x, y) chromaticity values deduced from the goniometric measurements to those deduced from a colorimetrically calibrated picture of the samples. These components are represented, for the 90 V sample, by a blue triangle on Errore. L'origine riferimento non è stata trovata.. We may consider that the sample colour caught from the digital picture is in good agreement with the colours deduced from goniometric measurements, given the following limiting factors: the illumination and detection solid angles are different for the gonio-spectrophotometer and in the picture acquisition configuration, and the accuracy of the angular positioning of the camera is limited.

We then characterize the chromatic paths around the specular direction for various incidence angles. Errore. L'origine riferimento non è stata trovata. presents this characterization for the series 1 sample anodized at 90 V at incidence angles equal to 30°, 45°, 60° and 70°. For all values, we observe that the colour with the highest saturation is in, or very close to the specular direction. The specular colour evolves from bluish to pinkish when the incidence angle increases, and its saturation decreases. The hue modification reflects the gonio-apparent character of the sample.

The decrease of the saturation may be explained by the increase of the Fresnel reflection coefficient of the air/TiO2 interface which leads to a higher difference of

amplitude between interfering light waves. The contrast of the interference fringes thus decreases, giving a spectral BRDF with less pronounced minima and maxima.

As the color saturation decreases in the specular direction with the incidence angle, we also observe that the saturation variations around the specular direction become weaker with an increase of the incidence angle.

Fig. 6 – (a) Chromatic paths in the CIE chromaticity diagram of the series 2 and 3 samples anodized at 10 V and the serioes 2 and 4 samples anodized at 90 V. The incidence angle is equal to 45°. The central surrounded black dot represents the D65 illuminant. Sub-figures (b) and (c) corresponds to magnifications of the chromatic paths of (b) the samples anodized at 10 V and (c) the samples anodized at 90 V. The values indicated next to the data points on the graphs are the angular positions, in degree, of the detection arm. For the sake of clarity, not all detection arm positions are indicated. The light blue triangle in sub-figures (a) and (c) corresponds to the (x,y) chromaticity coordinates of the 90 V sample of series 2 deduced from a colorimetrically calibrated digital picture of the sample, in the specular direction and for an incidence angle of 45°. The black points in sub-figures (b) and (c) indicate the specular direction.

4. Conclusion

The development of materials with particular visual effects has a wide range of applications in industry. However the characterization of their appearance is challenging, because these effects are linked to sharp variations of the reflectance.

This paper presents first a short review of existing characterization devices. Then colorimetric characterizations of gonio-apparent anodized titanium samples, through a purpose-made gonio-spectrophotometer with high angular resolution are presented. These characterizations constitute a first step for quantifying and ultimately modelling of the chromatic paths that can be obtained with anodized titanium. For a deeper understanding of these observations, a further study is being conducted to link physico-chemical and optical properties of the metal/oxide system.

As titanium exhibits excellent mechanical and physical properties, developing titanium based materials with exciting visual effects is very promising for architects and designers (they already have been demonstrated to be suitable for jewel production [30]). Moreover, as anodized titanium surfaces are very stable, this

metal/oxide system is promising for the development of physical standards of gonio-appearence. Existing standard calibration colorcharts allow the rapid and accurate colour characterization of lambertian surfaces. The development of reference colorcharts, possibly using the Ti/TiO2 system, with a set of well-calibrated gonio-apparent colours is promising for accurately characterizing the visual rendering of gonio-apparent surfaces. It requires further investigations of the structure and optical properties of the oxide layers, complemented by electromagnetic modelling of the interference colours.

Fig. 7 – Variation of the colour of the 90 V sample of series 1 around the specular direction for various incidence angles: 30°, 45°, 60° and 70°. The blackened points correspond to the specular direction. Sub-figure (b) is a magnification of the surrounded area of sub-figure (a). The values indicated next to the data points on the sub-figure (b) are the angular positions, in degree, of the detection arm. For the sake of clarity, not all detection arm positions are indicated.

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