One of the most important tools in a good sheet and paint shop is known as a photo- spectrometer. Thanks to this valuable tool and the science of spectrophotometry, we can achieve the same color as the rest of the pieces, regardless of the degree of wear.
How a spectrometer works
This versatile tool is based on the science of spectrophotometry to measure the amount of intensity of light absorbed after passing through a model solution. A spectrophotometer is precisely an optical instrument to measure the intensity of light about the wavelength and that can be of several types, mainly of atomic or molecular absorption (which is the one that concerns us) and that should not be confused with mass spectrometers.
A spectrometer is formed by the spectrometer, with which it can generate a range of specific wavelength of light and the photometer, with which the amount of photons that are absorbed by the sample is detected.
This class of instruments has the ability and the need to project a very particular monochromatic beam of light towards a given sample. The electromagnetic energy absorbed by this sample enters the device through the opening and separates into its wavelengths for each of its primary components. This is achieved thanks to the holographic grid.
This light source, used to illuminate the sample, must be very stable and directional. As light sources commonly used tungsten lamps(tungsten), lamps xenon arc based or deuterium, which is typically used in atomic laboratories.
Essentially, this grid acts to separate each color from white light. This separate light is then focused on a CCD sensor, where one pixel of this CCD sensor then measures the intensity of each wavelength (or each color, if it is in the visible spectrum).
The values for these intensities are converted to digital values in a computer and the result is a spectrum that shows the intensity of each wavelength of the light beam.
An example would be a spectral measurement of the visible range that we perceive as color. The white light would enter the monochromatic chamber and separate into a rainbow of colors. This rainbow, with blue light at one end and red at the other, focuses on the CCD. For each pixel or light cell of this CCD sensor, we will be offered an accurate measurement of the intensity of a color.
Not only at hardware level there are very marked differences between the different models of spectrophotometers in the market. The different manufacturers implement different techniques in the processing and treatment of the information obtained, which gives rise to very marked peculiarities among them.
Calibration and maintenance of a spectrophotometer
As it could be expected, a team that requires high precision in its measurements is subject to strict maintenance, cleanings and calibrations. It must be borne in mind that, simply with the generation of an imperfect beam of light, it will be impossible to obtain admissible reliabilities.
On the other hand, cleaning the surface of the instrument, its filters and of course its source of light (which includes lamp and condenser), is essential to carry out periodically.However, there are models with a level of isolation of components such that this cleaning in depth is no longer so critical. However, it is still highly recommended.
Use of the spectrophotometer in sheet metal and paint
When a customer gives us his vehicle to make a partial repair of sheet metal and paint, one of the main problems is to achieve the exact color and tones between the piece that must be repaired and repainted and the rest already existing.
In fact, if there is no wear and tear of our vehicle paint, mainly due to UV rays, weather conditions in terms of cold, rain, extreme heat and a long list of additional factors, it would be as simple as mixing the new paint based on the code of the manufacturer of our vehicle.
However, even without the incidence of this wear and tear on the paint, this method would not work in cases of repainted vehicles with customized colors different from the standard one.
For this reason, in some sheet metal and paint shops, we bet on the use of this kind of tools, with which it is possible to guarantee the exact shade between two paint samples.
How a reflectance spectrophotometer works
Reflectance spectrophotometers measure the proportional amount of light reflected by a surface as a function of wavelengths to produce a reflectance spectrum. The reflectance spectrum of a sample can be used, along with the standard CIE observer function and the relative spectral energy distribution of an illuminant to calculate the CIE XYZ tristimulus values for that sample under that illuminant.
The operation of a spectrophotometer consists of illuminating the sample with white light and calculating the amount of light reflected by said sample in a series of wavelength intervals. The most usual is that the data are collected in 31 intervals of wavelengths (the cuts are 400 nm, 410 nm, 420 nm … 700 nm). This is achieved by passing light through a monochromatic device that breaks the light into different wavelength intervals. The instrument is calibrated with a sample or white tile whose reflectance in each segment of wavelengths is known compared to a perfect diffuse reflection surface.
The reflectance of a sample is expressed as a fraction between 0 and 1, or as a percentage between 0 and 100. It is important to realize that the reflectance values obtained are relative values and, for non-fluorescent samples, they are independent of the quality and amount of light used to illuminate the sample. Thus, although the reflectance factors are measured using a particular light source, it is entirely correct to calculate the colorimetric values for any known illuminant.
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