Shape Analysis of Skin Reflectance Spectra

Author

Junhui He

Published

October 25, 2026

1 Overview of ISSA face and palm sites

ISSA Face and Palm Sites Eumelanin Class Summary Table:

Eumelanin Class EML (<25) EMIL (25-49) EMI (50-74) EMIH (75-99) EMH (>=100)
Site
Cheek 216 1978 227 50 4
Cheek Bone 24 186 18 0 0
Chin 271 1507 211 36 5
Ear lobe 9 394 57 12 3
Forehead 77 1772 190 30 3
Nose tip 29 582 31 1 0
Palm 123 618 27 1 0
Total 749 7037 761 130 15

2 Derivative-based Shape Analysis

We can classify spectral shapes based on the first derivative of skin reflectance. Within the 500–600 nm wavelength range, we apply the following criteria:

  • Monotonically Increasing: The first derivative remains positive.

  • U-shape: The derivative exhibits exactly one zero-crossing from negative to positive, indicating a single valley.

  • W-shape: The derivative exhibits two zero-crossings from negative to positive, indicating two distinct valleys.

2.1 Frequency and proportion tables

Frequency table of shape classifications for face and palm sites:

Shape W-shape U-shape Increasing
Site
Cheek 647 950 878
Cheek Bone 48 110 70
Chin 350 1305 375
Ear lobe 167 225 83
Forehead 253 1030 789
Nose tip 75 488 80
Palm 538 200 31

Frequency table of shape classifications for face and palm sites by eumelanin class:

Shape W-shape U-shape Increasing
Site Eumelanin Class
Cheek EML (<25) 132 73 11
EMIL (25-49) 515 869 594
EMI (50-74) 0 8 219
EMIH (75-99) & EMH (>=100) 0 0 54
Cheek Bone EML (<25) 12 9 3
EMIL (25-49) 36 98 52
EMI (50-74) 0 3 15
Chin EML (<25) 122 139 10
EMIL (25-49) 222 1087 198
EMI (50-74) 6 73 132
EMIH (75-99) & EMH (>=100) 0 6 35
Ear lobe EML (<25) 7 1 1
EMIL (25-49) 149 193 52
EMI (50-74) 8 26 23
EMIH (75-99) & EMH (>=100) 3 5 7
Forehead EML (<25) 33 42 2
EMIL (25-49) 220 977 575
EMI (50-74) 0 11 179
EMIH (75-99) & EMH (>=100) 0 0 33
Nose tip EML (<25) 15 14 0
EMIL (25-49) 60 457 65
EMI (50-74) 0 16 15
EMIH (75-99) & EMH (>=100) 0 1 0
Palm EML (<25) 96 27 0
EMIL (25-49) 432 167 19
EMI (50-74) 10 6 11
EMIH (75-99) & EMH (>=100) 0 0 1

Proportion table of shape classifications for face and palm sites:

Shape W-shape U-shape Increasing
Site
Cheek 0.261 0.384 0.355
Cheek Bone 0.211 0.482 0.307
Chin 0.172 0.643 0.185
Ear lobe 0.352 0.474 0.175
Forehead 0.122 0.497 0.381
Nose tip 0.117 0.759 0.124
Palm 0.700 0.260 0.040

Proportion table of shape classifications for face and palm sites by eumelanin class:

Shape W-shape U-shape Increasing Total Samples
Site Eumelanin Class
Cheek EML (<25) 0.611 0.338 0.051 216
EMIL (25-49) 0.260 0.439 0.300 1978
EMI (50-74) 0.000 0.035 0.965 227
EMIH (75-99) & EMH (>=100) 0.000 0.000 1.000 54
Cheek Bone EML (<25) 0.500 0.375 0.125 24
EMIL (25-49) 0.194 0.527 0.280 186
EMI (50-74) 0.000 0.167 0.833 18
Chin EML (<25) 0.450 0.513 0.037 271
EMIL (25-49) 0.147 0.721 0.131 1507
EMI (50-74) 0.028 0.346 0.626 211
EMIH (75-99) & EMH (>=100) 0.000 0.146 0.854 41
Ear lobe EML (<25) 0.778 0.111 0.111 9
EMIL (25-49) 0.378 0.490 0.132 394
EMI (50-74) 0.140 0.456 0.404 57
EMIH (75-99) & EMH (>=100) 0.200 0.333 0.467 15
Forehead EML (<25) 0.429 0.545 0.026 77
EMIL (25-49) 0.124 0.551 0.324 1772
EMI (50-74) 0.000 0.058 0.942 190
EMIH (75-99) & EMH (>=100) 0.000 0.000 1.000 33
Nose tip EML (<25) 0.517 0.483 0.000 29
EMIL (25-49) 0.103 0.785 0.112 582
EMI (50-74) 0.000 0.516 0.484 31
EMIH (75-99) & EMH (>=100) 0.000 1.000 0.000 1
Palm EML (<25) 0.780 0.220 0.000 123
EMIL (25-49) 0.699 0.270 0.031 618
EMI (50-74) 0.370 0.222 0.407 27
EMIH (75-99) & EMH (>=100) 0.000 0.000 1.000 1

2.2 Frequency and Proportion figures

2.3 Skin reflectance spectra on forehead and palm

2.4 First derivative on forehead and palm

2.5 Comparison of non-facial sites with the face

The manuscript uses the palm as the single non-facial comparison site. To support this choice, we compare every non-facial ISSA site with the pooled facial sites on three criteria:

  1. Spectral shape: proportions of W-shaped, U-shaped, and increasing spectra after derivative-based outlier removal, applied separately to the pooled facial sites and to each non-facial site, as above.
  2. Coupling to facial pigmentation: among participants measured at both a given site and the face, the slope and Pearson correlation of the site’s melanin index on the participant’s mean facial melanin index. This criterion uses all records before outlier removal.
  3. Sample size: the number of spectra after outlier removal and the number of contributing component datasets.

Participants are identified by (dataset, subject) pairs.

Spectra (cleaned) Datasets W-shape U-shape Increasing Paired participants (all records) MI slope on face (all records) MI correlation with face (all records)
Site
Face (pooled) 7923 11 0.194 0.518 0.288 NaN NaN NaN
Back of Hand 1694 10 0.011 0.539 0.450 1715.0 0.917 0.848
Inner arm 1666 9 0.032 0.597 0.370 1690.0 0.653 0.774
Neck 1158 7 0.075 0.383 0.541 876.0 1.008 0.935
Outer arm 1609 9 0.022 0.351 0.626 1630.0 0.892 0.793
Palm 769 6 0.698 0.261 0.040 777.0 0.418 0.680
Ring finger 242 3 0.884 0.103 0.012 243.0 0.619 0.546

The palm differs most clearly from the face among the well-sampled non-facial sites:

  • W-shaped spectra account for 69.8% of palm spectra, compared with 19.4% of the pooled facial spectra and 1.1–7.5% at the back of the hand, arms, and neck. These hair-bearing sites are dominated by U-shaped and increasing spectra.
  • Palm pigmentation is the least coupled to facial pigmentation: the palm melanin index increases by 0.42 units per unit of mean facial melanin index (correlation 0.68, 777 participants), whereas the slopes for the other non-facial sites range from 0.62 to 1.01.
  • The ring finger shows a similar contrast in shape, but it has only 242 cleaned spectra from three component datasets, compared with 769 palm spectra from six.

2.6 Key observations

  • The skin reflectance spectra of lighter skin tones predominantly exhibit a W-shape or U-shape profile, whereas darker skin tones generally display an increasing shape.

  • The proportion of the W-shape decreases as the Melanin Index (MI) rises. In contrast, the proportion of the U-shape peaks within the EMIL class (\(25 \leq \mathrm{MI} < 50\)), and the increasing shape becomes more dominant at higher MI values.

  • Spectral shape distributions vary significantly across different body sites.

  • Among the non-facial sites, the palm differs most clearly from the face in both spectral shape and pigmentation, which motivates its use as the single non-facial comparison site.