Three explanations for the very surprising robustness of 1/R
(390 nm) to predict EMC can be identified. First, intra- and inter-individual variations in oxygenation are of minimal influence on the remittance at 390 nm since this wavelength represents an isosbestic point for Hb and HbO2
. Second, due to high scattering of 390-nm light in human skin, this wavelength probes predominantly the epidermis and is therefore only moderately sensitive for inter- and intra-individual variations in dermal blood volume (see the following). Third, using a skin remittance model,8
we found that the two wavelength methods are sensitive for skin surface melanin concentration (integrated over depth) measuring equal MI regardless of whether the same amount of melanin is distributed over a thick or thin epidermis. This is because the penetration depth of red light is at least an order of magnitude larger than the epidermal thickness, and thus, the remittance is virtually independent of how the melanin is distributed. In contrast, due to its much shallower penetration depth (in the same order of magnitude as the epidermal thickness), R
(390 nm) is more sensitive for volumetric melanin concentration (proportional to absorption coefficient). At short times t
) is also more proportional to the volumetric melanin concentration.
The rationale for relating 1/R
(λ) to ΔT
ignores inter- and intra-individual variations in scattering, which are, obviously, of strong influence on remittance. Our data suggest that among the 23 subjects, variations in scattering are of less concern than variations in blood volume (see the following). A possible explanation is that the main source of scattering at 390 nm is melanin itself; coupling absorption and scattering into a single EMC-dependent parameter.9
The single-wavelength method at 390 nm may be improved in at least two ways. First, the specified FWHM bandwidth of a channel in the spectrometer is approximately 15 nm. A narrower bandwidth would further reduce the perturbing effect of variations in oxygen saturation and narrow the relationship between 1/R
(390 nm) and ΔT
. Second, absorption by Hb or HbO2
relative to that of melanin is approximately 10 times greater at 390 nm  than at 755 nm (used for the PT measurements). Therefore, skin with a relatively high dermal blood volume fraction will have R
(390 nm) decreased by a larger amount than its corresponding 1/ΔT
value. Deviations from the trend line in seem to confirm this: deviations are somewhat skewed toward the area below the fit. We hypothesize that these data points are for skin with relatively high dermal blood volume fractions. We performed additional measurements on four subjects, 15 spots each, in which we compressed the skin for approximately 2 s and measured R
(λ) within approximately 0.5 s after the pressure was released. Compression always increased R
(390 nm) compared to R
(390 nm) measured without compression. PT measurements were also performed. The R
(390 nm) measured after compression correlate better than the R
(390 nm) data obtained without compression (), supporting our hypothesis. In other words, the “noise” in may have been smaller had we used compression. Possibly, a more controlled and reproducible compression could be done with a probe that measures R
(390 nm) through a UV transparent window that simultaneously compresses the skin, also known as diascopy.10
Diascopy may be useful to enhance MI accuracy as well, although the deeper penetration of red wavelengths would require us to empty blood vessels to much greater depths than for the 390-nm method.
Preliminary results of the compression technique. R (390 nm) increases more for lighter skin, rendering a higher correlation with 1/ΔT than presented in and .
Even without the compression technique, we believe that the results are both surprising and promising. In addition to a potentially higher accuracy of estimating EMC, particularly for patients with lighter skin phototypes, the proposed single-wavelength remittance method at 390 nm has a number of potential advantages over existing methods. First, it may be applied on skin with high dermal chromophore concentrations (e.g., tattoos, vascular lesions), where methods using wavelengths with deep penetration into human skin likely give inaccurate results. Second, a probe measuring R
(390 nm) may be miniaturized, as opposed to probes using red light, which have to collect light remitted from a relatively large lateral area. Miniaturization would allow for evaluation of small pigmented lesions (without the need to resort to hyperspectral imaging such as the SIAscope11
) or of hard to access areas such as between the toes or in the ear.
To the best of our knowledge, a single wavelength in the UV at an isosbestic point is a novel method to quantify pigmentation. A single-wavelength or single-band method to estimate EMC was proposed12,13
but referred to red wavelengths. The UV or near UV to quantify EMC has been explored but always involved algorithms using multiple wavelengths,5,14,15
which might provide better correlation than the single-wavelength R
(390 nm) method. To date, however, we have not been able to identify such an algorithm with our current set of data. We evaluated an existing algorithm using two UV wavelengths5
—420 and 400 nm—and found a poor correlation with ΔT
. Similarly, the two-wavelength method was evaluated for 700 to 650 nm,1
720 to 620 nm,16,17
and 710 to 650 nm.18
All revealed much better correlation than the 420- 400-nm method but performed poorer than the R
(390 nm) and MI methods, in particular for lighter skin. We also tried, unsuccessfully, to use the EI (measured simultaneously with the MI), to improve the correlation with ΔT
. Other commercial melanin meters have not been evaluated yet. Last, just like many proposed optical methods to quantify EMC, our method should be calibrated and validated with EMC values obtained with histochemical methods, to express 1/R
(390 nm) in units of melanin mass per skin volume.
With these reservations in mind, and to the extent that ΔT for early times t (e.g., <50 ms) can be assumed a good measure of EMC, we conclude that R (390 nm) is a promising method to quantify EMC and associated IMSRE for the range of phototypes studied.