Nanomania

Nanomania

How important is resolution when selecting a spectrophotometer?

Thanks to development in micro electronics colour meters in recent years have become smaller and smaller, more comfortable to use and more powerful. Whoever needs a colour meter today is faced with a wide choice of models and suppliers – and that can be a real headache. What criteria should the decision be based on? The most obvious would seem to be price and features: the number of displayed colour spaces, the size and ease of handling, the accessories…

But accuracy is even more important – and this is where the confusion begins.

More important than resolution…

A spectrophotometer measures the spectrum of a colour, or rather the spectral reflection of the specimen to be measured in the visible light range. So the accuracy depends on the spectral sensor. But this is only half of the story. The illumination, the geometry (e.g. symmetry and opening ration of the Ulbricht sphere) and the optics have a considerable effect, especially on the absolute accuracy and on agreement between different meters. However, the quality of these factors is difficult to assess and cannot be easily described in a brochure.

In contrast, the sensor performance can be easily expressed in figures. The spectral resolution in particular is often highlighted as the essential quality feature. But this is quite wrong: with otherwise identical meters and sensors a better resolution will certainly also yield more accurate results. But no manufacturer offers the same meter with different sensors. So the comparison must always be made between the meters as a whole.

… is the accuracy of measurement

The spectral reflection of the specimen is defined as the percentage of the reflected radiation at various wavelengths. The accuracy with which this spectral curve is determined reflects the accuracy of the colour measurement. Each point on the curve has two coordinates: its wavelength in nm and its degree of reflection in percent (fig. 1). It is important for both these coordinates to be measured with precision and above all with repeatable accuracy. The spectral resolution (the number of points at which the measurement is made) is less important than the accuracy with which these points are determined. What is the point of a high resolution if the wavelengths shift from one measurement to another or the degree of reflection fluctuates in the tenths of a percent range?

Only if these parameters are determined with high accuracy does the resolution also become important. But the required resolution also depends on the steepness of the curve. A value of 10nm is completely sufficient for measurements of body-colour reflection, and even 20nm are enough as a rule.

For evaluation of light sources Konica Minolta provides a spectro radiometer with a resolution of 0,X nm an, which can measure the discontinuous spectra of fluorescent lamps, other discharging lamps with narrowband emission lines or LEDs. However, such narrowband spectral distribution does not occur in reflecting body colour.

Is the half-intensity bandwidth of the monochromator correct?

The monochromator resolves the light reflected by the specimen into it’s spectral components and the sensor then converts it into electrical signals. So the monochromator is responsible for the spectral resolution, while the sensor determines the accuracy of the signal measurement, i. e. the degree of reflection. The number of sensors behind the monochromator determines the spectral resolution as specified in the brochure. But does the monochromator actually reach this resolution? This can be determined by the half-intensity width which specifies the degree of "smudge" with which a wavelength is resolved by the monochromator. It should be of the same order of magnitude as the sensor resolution. A resolution of 3 nm is of little use if the pre-connected monochromator can only manage 10nm. This would be rather like designing a standard car engine with the power of a Formula-1 engine.

Rustling in the tree top…

…may have a soothing effect on your soul. The noise of photoelectric sensors however may set your nerves on edge. This phenomenon is known from digital photography. Increasing the number of pixels at unchanged size of sensor, will increase the resolution as well, but the size of each CCD element will inevitably decrease. However, the smaller the size of the elements the lower its receptiveness for light particles and the electronic noise will increase. In case of low reflectance – i.e. with dark materials – this background noise can negatively influence both measuring values and repeatability. (In case of digital photography, areas which should be black will show coloured shades). Sometimes it will be helpful to connect several elements with each other. In this case the original resolution of let’s say 2 nm will result in a measuring value of 10 nm – the initially obvious advantage of the high resolution will thus resolve itself!

Other parameters…

…in practice often have far more influence on accuracy and repeatability. When using the latest state of the art and high value equipment technology, the accuracy of measurement depends rather on the colour and surface fluctuations of the specimen or of application problems at small or highly curved surfaces than on the resolution. That's why the quality cannot be expressed in mere figures. The true quality of a spectrophotometer is only brought to light by a practical test with real specimens.