The Portable Spectrophotometer

Although we have been colour-matching by eye for over forty-five years, since 1995 we have been using a spectrophotometer to ensure accuracy.

The spectrophotometer can be used for:

• Colour matching

• Reformulation of paint colours

• Quality assurance

• Identification of colours

• Colour surveys

Colour Matching

An existing coloured object or painted surface can be measured, and paint produced to match it. This is typically in the form of wallpaper, fabric, or a flake of paint that is brought to us in the shop. As the spectrophotometer is portable, however, we can also carry out on-site measurements if required. This is especially useful if the object is very large, or is in the form of paint that cannot be removed from a wall.

Software with the spectrophotometer allows matches to be made for all light conditions (daylight, domestic lighting and office lighting being the most commonly specified) and metamerism can be avoided, or reduced. (Metamerism occurs when two colours match in one light, but fail to match in another. Paint matched to a fabric might be perfect within a room, for example, but by the window it might look quite different.)

As well as measuring flat colour, a match to the average can also be made to objects that have uneven colouration. By taking a number of measurements of a Portland stone facade the average colour can be found, and we could reproduce this in the form of a masonry paint for the render of a newly built extension (for example).

In a similar fashion, the background of a wallpaper or fabric can be measured, and a match to the average produced in order to paint a radiator.

Reformulation

Frequently a customer will mix several paints together in order to produce their perfect colour. In the same way, a decorator might mix a colour up in a paint kettle and be concerned that he might not have enough for the job. If the result of such a one-off mix is brushed onto a piece of paper and sent to us in the form of a solid colour, we will be able to match it and produce as much, or as little, as is required. Furthermore, the colour will be given a code number for future use.

Equally; discontinued paint colours, proprietary colours that are not available in the required finish, or colours from ranges that are prone to batch-to-batch variation or long delivery times, can be measured and formulated. You can be sure that each tin will be exactly the same as the last.

Quality Assurance 

A client might have specified a particular colour to his painter and be concerned that only that colour is applied. We can measure the specified colour (the Standard) and compare it with paints supplied by the contractor (the Batch). This can be useful to both parties and prevent apparent differences caused by lighting and finish.

In cases of difficulty between specifier and contractor, paints can be measured and the disputed colour either certified as being a good match, or confirmed as being unacceptable.

Identification of Colours

We have an enormous collection of colour references ranging from 19th century paint colour cards, to obsolete or foreign ones. We also keep a wide range of modern colour standards designed for various industries.

If we can identify the reference, we can frequently match it and supply the paint if it is to be in a conventional modern formulation; or

We can identify the closest proprietary colour to it in a conventional modern formulation; or

We can supply a numerical colour reference for a colour. Typically these are the closest NCS number, the Munsell reference, or the CIE L*a*b* reference. In addition, reflectance values at 10nm intervals can be supplied, to enable reproduction by a competent manufacturer.

A print-out can be generated certifying a match as being within agreed tolerances

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Colour Surveys

Existing colours in a building can be measured and identified. If colours were originally from different ranges of paint, mixed on site, or from obsolete ranges, the nearest approximations from just one proprietary range can be identified and specified. Equally, the colours can be matched in new paint.

Further information can be found in: Patrick Baty, "Getting the Measure of Colour." Historic House, Vol 24 No 4 November 2000: 40-42.

NOTES

The portable spectrophotometer is a handheld unit weighing only 1.3kg, making it possible to take measurements virtually anywhere. Most of the time we use it in the static / desktop mode, but it can easily be taken on site.

Unlike physical samples, which are subject to ageing and fading, precise colour control can be guaranteed using electronically stored colour values.

If an exact match is not required, merely a colour that is fairly close, a search can be made using the software, and a result obtained within minutes.

COSTS

A) On the shop premises

1) Quick Search

If paints are being bought from us - no charges are made for a measurement followed by a quick search for a fairly close match.

2) Colour Match

A charge of £30 + VAT will be made for each colour specially formulated. This price will be added to the cost of the paint, and will be a once-only charge.

Multiple Colours - Where three or more colour matches are required £25 + VAT per sample will be charged.

B) On Site Measurement

Consultancy rates will apply, and reduced charges made for each colour specially formulated. Travel expenses and accommodation will be charged at cost.

BRIEF TECHNICAL DETAILS:

Minolta CM-508i

1) Measurement area must be larger than 8mm. The sample should be of a solid colour unless an average reading is required.

2) Display data

Spectral reflectance: Graph or numerical values

Colorimetric values: L*a*b* or Hunter Lab colour difference graph; Numeric absolute colour and/or colour difference values in the following notations: XYZ, Yxy, L*a*b*, WI (ASTM E313), WI (CIE), YI (ASTM E313), YI (ASTM D1925), ISO Brightness (ISO 2470), Munsell notation, ISO Status A or Status T density.

3) Illuminant: CIE Standard Illuminants A, C, D50, D65; Fluorescent: F2, F6, F7, F8, F10, F11 (TL84), F12 (Ultralume 3000).

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