Colorimetric control of leathers for use in leathergoods

The pitfalls of colour in leathergoods Colour is very difficult to pin down: what we see is not always what we think we see; however, what we think we see can be measured. Also, during the manufacture of leather products such as wallets, the management of pieces of the same colour is a delicate stage. Currently, production workers carry out a visual control of the pieces and samples coming from the cut:  the various parts of the neck and body for, say, credit card holders, taking the different substances into consideration. They ensure that perceived colours match, in order to reassemble the pieces by batch. Over a hundred pieces can be treated at the same time. One must recognise the colour differences between them, ensuring that:

• pieces match from one skin to another
• pieces from a single skin can be used together

Colour matching in leathergoods is an essential procedure. If pieces have not been matched exactly, a repair may be necessary after assembling the article. This would involve searching the stock for a piece in the appropriate colour. Furthermore, it can be interesting to control the colour of pieces made in different materials (leather, textile etc) so that once assembled, a multi-material product appears uniform in colour. Limitations of current controls This operation is currently carried out manually and by the same person throughout the working day. This control has several limitations:

• perception of colour varies between individuals
• the human being has poor colour memory
• the difficulty of communication on the subject of colour
• colour varies according to the light
• perception of colour deteriorates with fatigue

Objectives of the project The project, led by CTC, aimed to validate a method of colorimetric analysis, and then an automatic tool capable of matching pieces of leather or other material, which are to be assembled into one single product. This system is capable of analysing colour and brilliance of different materials by generating a ‘harlequin' effect1, as well as controlling the presence of metamerism2. This tool is objective, fast, precise and constantly reliable. Optimal batching for leathergoods In order to correctly analyse the colour of a leather, CTC opted for a spectrocolorimeter equipped with a sphere of diffusion capable of setting itself free of the texture of the leather and only extracting the information which pertains to the characteristics of colour, which allows the control of different types of materials in a single batch. The data supplied by the spectrocolorimeter (reflection curve) are not usable in this state to characterise the perceived colour of the sample, so CTC decided to develop two systems which meet differing objectives and costs. A choice can be made between a validation of manual sorting and an automatic batching. Validation of manual batching: control via CMC sorting This control is carried out with the help of a portable spectrometer with an integrated sorting programme. This control is traced through the provision of an existing visual control which the operator has introduced a priori to the control. The operator chooses a sample from among a ‘population' which will be referenced as ‘standard' from the batch of pieces which will make up a single product. Automatically, the system will define an ellipse of acceptability around a point measured in the 3D HSL space (HSL = hue, saturation, luminosity). This space allows us to more easily describe colour using intelligible data. This ellipse is measureable according to the colorimetric distance tolerated by the client and standardised by the International Commission on Illumination (CIE). Furthermore, it adapts automatically to the analysed colour as human vision is more sensitive to green than black for example. The ellipse will be narrower around the point in the green and the tolerance will be more important in the black. Straight away, the operator analyses a second sample which he deems suitable for the batch, and the system indicates whether the sample is in fact acceptable, ie that it is located within the ellipse of acceptability: if so the system displays ‘Good'. If not, it indicates ‘Poor'. The operator repeats this process until a complete batch is obtained. Furthermore, this method allows us to resolve the problem of a potential repair after the product has been assembled. Even if it is just one piece that has to be replaced, the operator can simply analyse a neighbouring piece from the product and analyse a replacement piece or a skin from the stock which falls within the defined colorimetric distance. Due to the system's portability, this stage is easy to carry out. Automatic control In this method, the operator will measure the set of parts of the same colour and the apparatus will automatically sort batches of parts according to the colour parameter. Thus the operator will be able to assemble parts from the same batch without worrying about the colour. The visual matching should be carried out only according to the grain and the brilliance. a. Optimal method After having been ‘flashed' with the spectrocolorimetry, all the data on the samples desired are transferred to a computer. The treatment is carried out with a colorimetric software programme developed by CTC, allowing batching of samples according to the number of samples per batch, the light, the colorimetric colour distance parameters tolerated by the client and standardised by the CIE with a management of the harlequins.  This method is awkward because it requires handling samples without automating the station. CTC opted instead for the sorting 555 method which allows the user to obtain a code for each sample after each measurement. b. Sorting 555 Sorting 555 is a partition of colour distance in a box whose dimensions correspond closely to the colours tolerated by the company. All the samples present in this box are accepted and receive the same colour code. This method is well suited to the challenges of automatic batching, because the sample's classification code is obtained directly after measurement and thus the colour category of the sample is directly affected. Moreover, by using this method to finish batches we carry out regroupings of boxes, and have a perfect command of the error induced according to the components of bringing together L and a or b. Promising results This system also allows us to take into consideration the phenomenon of metamerism which is present on certain leathers, according to the acceptable colour distance under the primary source of light defined by the user (for example D65 ‘daylight'). The system also calculates the colour distance between two samples under a secondary light source (eg F11 flourescent tube') and defines the metamerism as the colour distance under the secondary light source, having restored the colour distance under the primary light source to zero. The user specifies a variation value of the colour distance between the two defined light sources. A batch may be acceptable under one light source but rejected if the notion of metamerism is taken into account. The system perfected by CTC allows them to improve the reliability (unchanging to visual fatigue, calibrated daily against a reference slide and integrating all types of lighting into one measure) and the productivity of batching in leathergoods (less than one second for the analysis of a piece). Understanding colour A small theoretical detour is essential to understanding what is at stake in the control of colour. In a human being, the notion of colour is intuitive: it's a phenomenon which is interpreted by the brain of the observer, of an image perceived by the eye. Hence the subjectivity of judgements and the difficulties in communicating about colour. In perceiving a colour, one must analyse three parameters:

• The light source
• The object
• The observer/receiver

The light source causes colour to vary considerably: fluorescent light in production, daylight, shop lighting... The object absorbs, diffuses, reflects, diffracts and refracts the light. According to its texture, its relief, its colour and its capacity to reflect, it will resend a spectre of light to the viewer. The viewer or observer picks up this light: it must be calibrated according to the source of light in the scene being observed. And in this case, the calibration of the measuring apparatus will always be easier than that of the human cortex... Colour is deceptive: subject to interpretation, it varies constantly. Furthermore, colorimetry provides elements for analysis with the goal of specifying colour, from the starting point of its three constituent parts. Footnotes 1 The management of harlequins consists in establishing an order of positioning of the samples in a batch according to the increasing colour distance 2 With metamerism, two samples of which the colorimetric distance is acceptable under a given light source (natural light for example) and is not greater under another light source such as a fluorescent tube