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Reasearch has dedicated itself, as a practical alternative, to the study of spectral reflectancy, keeping in mind a certain number of values or matehematical procedures that allow to evaluate the acceptability of a new light source (P.A.Lovett, A.R.Hill, M.B.Halstead – The effect of clinical judgements of new types of fluorescent lamp – III – Fourier statistical analyses leading to a new specification for lamps.).

New criteria have been proposed for use as a base for luminancy specifications for use in clinics and hospitals. These criteria comprise the specification of the lamp’s chromaticity and its properties. Two groups of acceptable and unacceptable light sources were created considering the results of various experimental observations.

None of these single individual parameters or colour differences could justify the above distinctions.. The only measure that allowed to discriminate light sources has been The Special Index of Colour Yield: according to the CIE method (Method of Measuring and Specifying Colour Rendering Properties of Light Sources – CIE Publ. 13,2 1974).

The application of statistical methods of cluster analysis has allowed us to obtain new representative curves, essential for the discrimination between acceptable and unacceptable light sources and to evaluate narrow-band lamps that are coming to market.

In other words we have been able to define the limits for an acceptable light source and the base for new specifications.

We close reminding of a recent work of Rea te al. (M.S. Rea, A.R. Robertson, W.M. Petrusic – Colour rendering of skin under fluorescent lamp illumination), where the problem has been faced from a different point of view, hypotizing that the most important factor is the manner in which this light source “renders” the skin to the observer.

It follows that the own skin colour stimulates the selection of the light source and also that, if two light sources do not produce different effects, it doesn’t make sense to ask oneself which is the better light source.

Art 5/8 - Related article: The difficulties with reproducing the colour of the skin (part 4)

These instruments are designed to measure the transmittability and reflectancy factors of, respectively, transparent and opaque objects. Although they have much in common with the spectroradiometers, their objectives are different. In fact, the spectrophotometer gives us a comparison between the energy that emanates from the object and its illumination. The geometries regarding the illumination have been standardized by the CIE. Obviously, the specification is less critical for spectrophotometers than for spectroradiometers.

Anyway, the calibration is in any case a delicate operation.

The frequency calibration is performed through sources that emit certain lines, well spaced in the spectrum and of the necessary intensity.

The photometric range is performed through filters of different optical density.

In the past decades, the “Lovibond Tintometer” was perfected which uses red gold-coated, yellow chromium-coated and blue cobalt-coated glass. These filters are used for the spectrophotometric measurements on transparent objects like oil, sugar solutions and beer and reflecting objects like margerine.

The transparent objects are easier to handle than reflecting objects. In fact, the latter are not perfectly homogeneous; furthermore a part of the radiation is absorbed and reflected from beneath the surface in a disorderly manner.

There is no such thing as a universal spectrophotometer. The choice which to use in a particular situation can only be made when one understands how it works, the object to be measured is well defined, the spectrum interval and one has established the required precision.

Although the spectrophotometers are among the best designed instruments and even if they are used with the appropriate precautions, different values may be read on even the same instrument under different conditions. These differences are the result of the combination of many small fluctuations due to variations within the instrument such as temperature, humidity and vibrations.

When these fluctuations are random, one can calculate the so-called index rms (root mean square), on a suffiently large set of data.

For the best spectrophotometers, the index is 0.1%.

In a typical spectrophotometric application a singular reflectancy value is used for all colour measures for each of a number of limited frequencies; however, some instruments produce the average of a certain number of measures.

The mathematicians have developed a formula that indicates us the differences between the spectrophotometric data and the trichromatic component values X, Y and Z.

Art 4/7 - Related article: COLORIMETRIA, The Spectroradiometers

The spectroradiometers are instruments designed to measure radiometric energy according to its frequency. In practice, it measures it based on the comparison between the source to be measured and a reference source, whose spectral repartition is know. The energetic power emitted by the source in a given direction penetrates in the input slot through optics (for example, a sphere). After the optical dispersion the radiation goes through the output slot which, in turn, is coupled with the detector. The photoelectric reaction of the latter is processed by a computer which shows the output signal on a display. The computer also interfaces with several other components in order to automate its operation.

Typically, the data supplied by a spectrradiometer are :

• Spectral distribution of the energy of the source,

• Values of the tricromatic components according to the CIE 1931 o 1964 system,

• Yield index of the colour,

• temperature of the correlated colour,

• photometric quantity (for example, its luminancy).

If the spectrometric test, instead of being a source, is an opaque object illuminated by a given source, the spectral reflectancy of the object is used.

Anyway, the width of the spectral band in which the energy is concentrated, is the datum of primary importance in spectroradiometrics. The bandwidth depends on:

• the complexity of the emitted energy’s spectral distribution,

• the type of the weighing functions being used.

Usually, the bandwidth transmitted has to be about the same as that used for tricromatic component calculations.

Art 3/7 - Related article: COLORIMETRICS: Modern Instruments

Nature has given us a highly sensitive photo-colorimetric reference: our skin. In particular, having our hands always under our eyes, we know their colour exactly and their dependency on the spectral composition of the source which illuminates them.

A problem that had to be faced is the variation of the skin due to pathological conditions and its evaluation by medical personnel.

In the UK, at the beginning of the ’70s, the Medical Research Council (MRC) recommended a certain kind of lamp with a colour temperature of Tc = 4000 K and a yield (on the Crawford scale) of less than 30. This choice emerged after a research on lamps with a temperature between 3000 K e 6500 K, with different yields.

At the end of the ’70s several neon lamps were available with phosphor that emitted on a narrow spectral band. Some of these lamps had good CIE colour yield values, close to 85. A research was started to establish whether these new neon lamps could be exploited in clinics. This research required precise data on the spectral reflectancy of the skin.

In parallel, simultaneously in three dermathological clinics, research was done on the alterations of skin colour due to pathologies of various types (The effect on clinical judgements of new types of fluorescent lamp - I – Experimental arrangement and clinical result. P.A. Lovett, M.B.Halstead, A.R. Hill, D.A.Palmer, T.S.Sonnex, M.R.Pomter ).

The researched lamps had a colour temperature ranging from 51 a 93.

The medical staff, in turn, had to evaluate, supported by diagnostic estimates and the distinctness of skin wounds under various sources, which was the most appropriate illumination and the subjective scale to evaluate the colour.

The result was, as in the studies of the ’60s, that the “best working” lamp is that of 4000 K, even if several other possible alternatives could not be excluded.

The experimental routine used in this work (The effect on clinical judgements of new types of fluorescent lamp - I – Experimental arrangement and clinical result. P.A. Lovett, M.B.Halstead, A.R. Hill, D.A.Palmer, T.S.Sonnex, M.R.Pomter ) resulted very positive, observations were continued and, in particular, the possible relation between the colorimetric measurements and visual observations (P.A.Lovett, M.B.Halstead, A.R.Hills – The effect on clinical judgements of new types of fluorescent lamp - II – Colour measurements and statistical analysis.). The spectroradiometer was therefore applied (with a Pritchard) on patients with various pathologies, of various ages as well as on normal persons. The observations were performed by well-trained personnel with the “Lovibond Flexible Optic Tintometer”.

The general problem manifested itself immediately in all its complexities. For example, using the same source, the relation between the distinctness of the wound and the surrounding area is not that simple. As the wounds with very different colours were always evaluated as “distinct”, also those with a little colour difference were easily identified. There must be other factors beside the colour: the part of the body, the distribution, the surface texture.

The conclusion was that only in a few pathologies the difference in the skin colour can assist in distinguishing between the clinical case and normality. For example, in anaemia and cyanosis. Neither colorimetrics nor the difference in colour were valid elements in distinguishing them from other pathologies, not even with different sources. This has brought to the conclusion that there is no need for the use of a specific type of lamp dermathology.

Art 4/8 - Related article: The difficulties with reproducing the colour of the skin (part 3).

The modern colorimeters for industrial use are designed to supply automatically the component values and the trichromatic coordinates, without the assistence of the human eye as a measuring instrument. They are divided in three groups: spectrumradiometers, spectrophotometers and trichromatic component colorimeters.

Art. 2/7 Related article: Colorimetrics: Measuring Instruments.