Last updated: 2025-12-11

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Knit directory: SkinOptics/

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File	Version	Author	Date	Message
Rmd	e11deee	Junhui He	2025-12-11	calculate EI
html	e11deee	Junhui He	2025-12-11	calculate EI
Rmd	53d4f9c	Junhui He	2025-12-11	create SkinOptics
html	53d4f9c	Junhui He	2025-12-11	create SkinOptics

Clarifications

Melanin index (MI) is calculated based on skin reflectance at 680 nm. Specifically, MI = -100 * log10(R680 / 100), where R680 is the reflectance percentage at 680 nm.
Erythema index (EI) is calculated as EI = 100 * log10(R680 / ((R550 + R560)/2)), where R550 and R560 are the reflectance percentages at 550 nm and 560 nm, respectively.
FACE site refers to Forehead and Cheek, while PALM site refers to the Palm of the hand.
Representative FACE MI:
- For each subject, the FACE MI is given by the Forehead MI if the Forehead MI is available; otherwise, it is the Cheek MI.
- For subjects with multiple measurements at the same FACE site, the average MI across those measurements is used.
First derivatives of skin spectra are estimated using local polynomial regression with bandwidth selected by the dpill function, scaled by a factor of 0.7.
Eumelanin classes are defined based on melanin index ranges as follows:
- Low: MI < 25
- Intermediate low: 25 ≤ MI < 50
- Intermediate: 50 ≤ MI < 75
- Intermediate high: 75 ≤ MI < 100
- High: MI ≥ 100
The pdf files generated in this analysis are saved in the “output” directory.

Visualizations

Distribution of melanin index by site

Version	Author	Date
53d4f9c	Junhui He	2025-12-11

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Unique subjects per site across representative face melanin index bins (Heatmap)

Version	Author	Date
53d4f9c	Junhui He	2025-12-11

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First derivatives of reflectance by eumelanin classes

Number of subjects in each eumelanin class (Forehead/Cheek)
Eumelanin class	Number of subjects
Low	296
Intermediate low	3801
Intermediate	418
Intermediate high	80
High	7

Number of subjects in each eumelanin class (Palm)
Eumelanin class	Number of subjects
Low	127
Intermediate low	623
Intermediate	27
Intermediate high	1

Version	Author	Date
53d4f9c	Junhui He	2025-12-11

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Version	Author	Date
53d4f9c	Junhui He	2025-12-11

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First derivatives of absorbance by eumelanin classes

Version	Author	Date
53d4f9c	Junhui He	2025-12-11

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Version	Author	Date
53d4f9c	Junhui He	2025-12-11

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Correlation between reflectance derivative sd and melanin index

Version	Author	Date
53d4f9c	Junhui He	2025-12-11

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Regression analysis


Call:
lm(formula = sd_ref ~ log_mi + I(log_mi^2), data = sd_face)

Residuals:
     Min       1Q   Median       3Q      Max 
-0.07754 -0.01585 -0.00113  0.01262  0.11441 

Coefficients:
             Estimate Std. Error t value Pr(>|t|)    
(Intercept)  1.608490   0.044784   35.92   <2e-16 ***
log_mi      -0.708782   0.024262  -29.21   <2e-16 ***
I(log_mi^2)  0.079846   0.003271   24.41   <2e-16 ***
---
Signif. codes:  0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1

Residual standard error: 0.02462 on 4599 degrees of freedom
Multiple R-squared:  0.6491,    Adjusted R-squared:  0.6489 
F-statistic:  4253 on 2 and 4599 DF,  p-value: < 2.2e-16


Call:
lm(formula = sd_ref ~ log_mi, data = sd_palm)

Residuals:
      Min        1Q    Median        3Q       Max 
-0.066026 -0.016322  0.000318  0.015338  0.071978 

Coefficients:
             Estimate Std. Error t value Pr(>|t|)    
(Intercept)  0.560294   0.013141   42.64   <2e-16 ***
log_mi      -0.120610   0.003783  -31.88   <2e-16 ***
---
Signif. codes:  0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1

Residual standard error: 0.02377 on 776 degrees of freedom
Multiple R-squared:  0.5671,    Adjusted R-squared:  0.5665 
F-statistic:  1016 on 1 and 776 DF,  p-value: < 2.2e-16

Version	Author	Date
53d4f9c	Junhui He	2025-12-11

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Correlation between absorbance derivative sd and melanin index

Version	Author	Date
53d4f9c	Junhui He	2025-12-11

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Correlation between erythema index and melanin index

Version	Author	Date
e11deee	Junhui He	2025-12-11

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R version 4.3.1 (2023-06-16)
Platform: aarch64-apple-darwin20 (64-bit)
Running under: macOS 15.6.1

Matrix products: default
BLAS:   /Library/Frameworks/R.framework/Versions/4.3-arm64/Resources/lib/libRblas.0.dylib 
LAPACK: /Library/Frameworks/R.framework/Versions/4.3-arm64/Resources/lib/libRlapack.dylib;  LAPACK version 3.11.0

locale:
[1] en_US.UTF-8/en_US.UTF-8/en_US.UTF-8/C/en_US.UTF-8/en_US.UTF-8

time zone: America/Detroit
tzcode source: internal

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base     

other attached packages:
 [1] KernSmooth_2.23-26 openxlsx_4.2.8.1   scales_1.4.0       lubridate_1.9.4   
 [5] forcats_1.0.1      stringr_1.6.0      dplyr_1.1.4        purrr_1.2.0       
 [9] readr_2.1.6        tidyr_1.3.1        tibble_3.3.0       ggplot2_4.0.1     
[13] tidyverse_2.0.0    patchwork_1.3.2    knitr_1.50        

loaded via a namespace (and not attached):
 [1] sass_0.4.10        generics_0.1.4     lattice_0.22-7     stringi_1.8.7     
 [5] hms_1.1.4          digest_0.6.39      magrittr_2.0.4     timechange_0.3.0  
 [9] evaluate_1.0.5     grid_4.3.1         RColorBrewer_1.1-3 fastmap_1.2.0     
[13] Matrix_1.5-4.1     rprojroot_2.1.1    workflowr_1.7.2    jsonlite_2.0.0    
[17] zip_2.3.3          whisker_0.4.1      promises_1.5.0     mgcv_1.8-42       
[21] viridisLite_0.4.2  jquerylib_0.1.4    cli_3.6.5          rlang_1.1.6       
[25] splines_4.3.1      withr_3.0.2        cachem_1.1.0       yaml_2.3.11       
[29] otel_0.2.0         tools_4.3.1        tzdb_0.5.0         httpuv_1.6.16     
[33] vctrs_0.6.5        R6_2.6.1           lifecycle_1.0.4    git2r_0.36.2      
[37] fs_1.6.6           pkgconfig_2.0.3    pillar_1.11.1      bslib_0.9.0       
[41] later_1.4.4        gtable_0.3.6       glue_1.8.0         Rcpp_1.1.0        
[45] xfun_0.54          tidyselect_1.2.1   farver_2.1.2       nlme_3.1-168      
[49] htmltools_0.5.9    labeling_0.4.3     rmarkdown_2.30     compiler_4.3.1    
[53] S7_0.2.1

Skin Optics Analysis based on ISSA Data

Junhui He

2025-12-11