The work detailed in this study describes the design of a rapid
scanning multiwavelength reflectance spectrophotometer and its
application to the measurement of cutaneous pigments. In this
instrument the radiation from a 6V, 20 W quartz halogen lamp is
transmitted to the reflectance probe by fibre optics and illuminates
the skin surface perpendicularly. The diffuse reflected light is then
collected and transmitted to a rotating circular variable filter, CVF.
The CVF is driven by a BBC Microcomputer and the spectrum from 356 to
721 nm is measured at 200 equally spaced wavelengths in 2.8 s.
Based upon the spectral differences between the absorbances of
oxy and reduced haemoglobin, two indices were defined for quantifying
the amount of haemoglobin, the "haemoglobin index", and oxygen
saturation, the "oxygenation index", in a blood containing sample.
The haemoglobin index is derived from the spectrum of haemoglobin
as the difference between two gradients calculated between three
isobestic points (527.5,544, and 573 nm). To measure the oxygenation
index, two isobestic points (544 and 573 nm) and a point at 558.5 nm,
where the specific absorption coefficient of oxy and reduced
haemoglobin differ, are used.
Linear calibration curves of the haemoglobin index against
haemoglobin content in-vitro using collimated and diffused light were
obtained. The oxygenation index was converted to percentage oxygen
saturation by comparing the oxygenation index of a sample with that of
reduced and fully oxygenated blood. The validity of the method was
examined by measuring the oxygen dissociation curve of human blood at
pCO2 of 40 mmHg, pH of 7.33, and 37 °C and comparing the results with
between measured and published curves confirms that the oxygen
saturation of blood can be accurately measured by the present method.
The scattering properties of the epidermis were studied by
measuring the angular distribution of light scattered by a sheet of
epidermis. It was found that most of the radiation transmitted through
the epidermis is forwardly orientated; 67% of the incident radiation
at 630 nm remains within 22.5 degrees of the incident beam.
To facilitate the study of the absorption properties of epidermis
a new index called the "melanin index" was introduced and quantified
using a synthetic melanin compound for calibration purposes. The
melanin index is based on the slope of the LIR spectrum of skin
surface between 650 -700 nm, and was used to investigate the melanin
pigmentation levels of different ethnic groups.
Reflectance spectrometry was used to study the amount of
haemoglobin and oxygen saturation of the superficial blood in skin.
Variations in the haemoglobin index of an arm and a finger tip were
studied under different conditions of temperature and height of the
limb relative to the heart. The effects of applying a topical
vasodilating chemical and of the thermal regulatory function of blood
at three different sites on a hand 'were also studied. In all these
measurements the results were in good agreement with the biological
prediction and confirmed the satisfactory operation of the
instrument, the validity of the indices and their usefulness in
quantifying "skin colour" changes.
To find the correlation between in-vivo haemoglobin index and the
true amount of haemoglobin in blood in g/dl, skin reflectance
measurements and blood samples were taken from patients who attended a
blood clinic. The correlation coefficient between haemoglobin content (g/dl) and the haemoglobin index
of the patients were 0.4 and 0.2 respectively. The weak correlation coefficient is attributable to the difficulty of defining the volume of blood observed when measuring the haemoglobin index. The instrument was used to measure changes in the blood content of the skin of animal and human subjects following treatment by photochemotherapy.