Assessment of Facial Skin Tone Evenness After a Microdermabrasion-AHA/TXA-Peel Protocol: Instrumental Colorimetry and Time-Resolved Expert/Novice Image Evaluation

1. Introduction

In many cultures, a uniform skin tone is regarded as a visible proxy for fitness and youthfulness. While an evenly tanned complexion may be preferred in many Western contexts and lighter facial skin is often valorized in parts of East Asia, both cultural frames converge in that facial pigmentary unevenness is commonly experienced as distressing and stigmatizing [1]. Objective image-analytic studies demonstrate that greater facial color homogeneity is associated with higher ratings of attractiveness, health, and with lower perceived age [2,3]. From the personal perspective, uneven facial pigmentation can negatively affect self-image and quality of life, particularly given the central role of facial cues in social interaction [4]. Therefore, affected individuals frequently seek evidence-based interventions to reduce hyperpigmentation.

From a clinical perspective, hyperpigmentation originates in melanogenesis: Melanin is an endogenous cutaneous pigment synthesized within epidermal melanosomes as part of an integrated photoprotective response to DNA photodamage and oxidative stress signals in skin. It is transferred to keratinocytes, where it functions as a broadband ultraviolet (UV) absorbent and radical scavenger, thereby contributing to photoprotection [5]. However, focal hyperpigmented macules can become clinically apparent when melanogenesis and/or melanin distribution is locally amplified by UV radiation, inflammatory mediators (e.g., post-inflammatory hyperpigmentation), or other metabolic and redox-related drivers of melanocyte activity, e.g. hormonal influences such as pregnancy or oral contraceptive use [6].

To address dyspigmentation, such as hyperpigmentation, different procedures for skin renewal are available in beauty salons or dermatology practices and skin clinics. While microneedling and microvibration treatments produce a stronger deep-tissue response, microdermabrasion excels at improving the skin’s surface, reducing superficial discoloration and irregularities, resulting in instantly even, clear, and smooth skin [7]. It mechanically removes the superficial epidermal layers and is known to promote skin renewal as well as to reduce scarring and pigmentary lesions, including hyperpigmented spots [8]. Across repeated regimens (e.g., 6 sessions to 12–14 weeks with typical session durations of ~10–30 min), clinical and histologic studies report improved skin texture and pigmentation, epidermal thickening/hyperplasia, partial acne-scar improvement, and altered melanosome distribution, alongside dermal remodeling with increased collagen density and variably reported elastin changes [9,10]. Mechanistically, abrasive injury activates inflammatory and wound-healing signaling (e.g., AP-1, IL-1β, TNF-α, MMPs) and can upregulate markers of neocollagenesis, with greater pathway activation after more aggressive abrasion [11,12]. Barrier perturbation appears transient: preclinical work indicates recovery of barrier function and stratum corneum within ~12–24 h, with increased permeability to topicals particularly during the first hours post-treatment, likely longer in humans [13,14]. Since evidence on enhanced active-ingredient delivery is context-dependent, penetration is known to increase for hydrophilic actives (e.g., niacinamide) [15]. This provides great potential for improving the absorption of active ingredients for hyperpigmentation and for enhancing their effectiveness.

Melanin-targeting actives have long attracted substantial interest in dermatology and the cosmetic industry, remaining the subject of intensive translational research. Established depigmenting agents include prescription-only drugs in many jurisdictions, such as hydroquinone, retinoids, topical corticosteroids, phenolic/resorcinol derivatives, and azelaic acid, which collectively address tyrosinase activity, epidermal turnover, and inflammation control [16]. Fixed triple-combination therapy (hydroquinone/tretinoin/fluocinolone acetonide) has demonstrated clinical efficacy in melasma in randomized trials, underscoring the benefit of multi-target intervention [17]. Beyond “classical” tyrosinase inhibition, contemporary concepts consider the full pigmentation cascade. Tranexamic acid is proposed to modulate upstream plasmin-driven inflammatory signaling and UV-induced melanogenic crosstalk – controlled trials support its role as monotherapy or adjunctive treatment in melasma [18]. Additional tyrosinase-directed compounds include kojic acid, arbutin, and licorice-derived flavonoids (e.g., glabridin), supported by mechanistic and preclinical evidence [19]. Ascorbates exert antioxidant and antimelanogenic effects, partly via modulation of tyrosinase-related pathways and inhibition of melanoidin oxidation [20]. Niacinamide reduces visible hyperpigmentation primarily by inhibiting melanosome transfer from melanocytes to keratinocytes, as clinical studies show [21]. Finally, chemical exfoliants (e.g. AHA, BHA, PHA) can accelerate pigment elimination via enhanced desquamation and have shown efficacy in melasma studies [22]. While the evidence supporting these ingredients varies in strength, in rational combinations, these modalities may enable comprehensive modulation of melanogenesis with acceptable tolerability making them particularly suitable for cosmetic approaches for prevention and care of dyspigmented skin.

Given this context, a holistic approach to addressing hyperpigmentation, combining device-based treatments and cosmetic products, is clearly indicated. To measure treatment effectiveness, some studies already pair objective methods (colorimeter) [23,24] with subjective assessments, such as self- and third-party rating questionnaires [25]. However, to the best of the authors’ knowledge, the literature has not yet described a triangulated framework that brings together (1) instrumental, image-based analysis, (2) investigator grading based on questionnaires, and (3) patient self-assessment. Therefore, the authors propose a double-blind, time-resolved approach to quantify pigmentation changes during long-term regimens that combine device-based treatment and cosmetic product application (serum, eye cream-, cream), integrating instrumental colorimetry with expert and lay evaluations of images, as well as self-perception and treatment acceptance/preference questionnaires.

2. Materials and Methods

2.1. Study Protocol and Participants

This work aimed to verify the reliability of a triangular image evaluation assessment. The data were collected from an uncontrolled prospective interventional study investigating the effects of microdermabrasion in combination with cosmetic treatment and aftercare in 5 adult volunteers. Participants were selected based on the following criteria: all skin types, pronounced pigmentation issues, irregular skin texture and uneven skin tone, enlarged pores, age: 35 to 60 years, without Vitamin C allergy, active acne, sensitive skin or relevant skin diseases.

Ethics committee approval was not obtained for this non-invasive study conducted in Germany. As this investigation involved the application of a complex cosmetic regimen and was not designed as a clinical trial of a medicinal product or medical device, formal review by an independent ethics committee was not sought. The procedures were performed on healthy adult volunteers after written informed consent. The study was conducted in accordance with the principles of the Declaration of Helsinki, and safety/tolerability were monitored throughout the study. Data protection was ensured through a double opt-in process on the testing platform. Consent regarding image rights was obtained using a standard form. No placebo control group was included, however, the documented state at the beginning of the study served as a baseline. Both expert and lay assessors were blinded to treatment group and time point during randomized image evaluation. All informational materials provided to the participants are included in the Appendix.

The treatment schedule and each treatment session followed a standardized, stepwise protocol. The participants attended treatment four times every 14 days. After initial skin cleansing and a preparatory peel, the skin was treated by microdermabrasion and subsequently exposed to an acid peel containing 14% AHA in combination with tranexamic acid and vitamin C. This was followed by application of a mask and a cooling massage over the mask, and a final leave-on finishing care product. The session was initiated and concluded with a company-specific ritual addressing breathing and relaxation. At-home care included a hyaluronic acid serum and an eye and face cream in the morning, as well as a vitamin C serum (weeks 1-7) or a longevity serum (week 8) and an eye and face cream in the evening. The volunteers were instructed to avoid UV exposure (vacation, tanning beds) during the study period. Full product specification related to the treatment and home-care step are presented in Table 2 (Appendix). The detailed step-by-step procedure are provided in the Appendix. Figure 1 shows the treatment schedule.

2.2. Data Collection

Standardized facial photographs were acquired before the first treatment, immediately after the first treatment, before each following treatment and 14 days after the last treatment (in sum 6 photographs) in the in-house photo studio with a fixed setup. For subjective assessment, images were anonymized, coded and presented in randomized, time-resolved order to 7 certified estheticians and 13 lay assessors on a stationary screen with consistent environmental conditions regarding light exposure and screen lighting. The questionnaire addressed the following characteristics: (1) Number of pigment spots, (2) Color intensity of pigment spots, (3) Intensity of color differences/contrast due to pigmentation disorders, (4) Size of pigment spots, and (5) Overall evenness of skin tone, using a 5-point Lickert scale and sample images as a visual reference, respectively. Colorimetric analysis was performed using the C+K VisioFace RD system (Courage + Khazaka electronic GmbH, Cologne, Germany). Calibrated facial scans were obtained at regular 14-day intervals as well, and processed using the manufacturer’s preconfigured analysis module, automatically segmenting conspicuous spot regions (displayed as red-circled areas) and reporting the proportion of conspicuous spots [%] and complexion evenness as a color-difference metric [dE] as well as distinctive spots [dL].

2.3. Statistics

We conducted a 2×2 mixed-design ANOVA with Group (novice vs. expert) as the between-subjects factor and Time (pre vs. post) and View (frontal vs. lateral) as within-subject factors, where each within-cell score was computed by averaging the five corresponding image ratings (frontal: images 1/3/5/7/9; lateral: images 2/4/6/8/10), and effects were tested using an OLS model with subjects nested within group (i.e., including a subject-within-group term) implemented in Python via statsmodels (type-III ANOVA). All pairwise within-subject comparisons were performed between weeks using paired-samples t-tests and then adjusted the resulting p-values for multiple testing with the Holm correction. The visualization was created using Microsoft® Excel® for Microsoft 365 MSO (Version 2604, Build 16.0.19929.20172), 64-bit.

3. Results

The objective of this analysis was to investigate how accurately the triangular assessment captures improvements in skin color. Across all participants, visual ratings of the time-resolved image-presentation showed significant changes in pigment-related parameters over time: the number (F (1, 54) = 62.05, p < .001, ηp² = .54), intensity (F (1, 54) = 62.05, p < .001, ηp² = .54), contrast (F (1, 54) = 83.54, p < .001, ηp² = .61), size of pigment spots (F (1, 54) = 14.70, p < .001, ηp² = .21) and skin tone evenness (F (1, 54) = 49.92, p < .001, ηp² = .48) improved between before and after treatment (see Figure 2).

No significant effects were found for group (novices vs. experts) on pigment number, intensity, contrast, or spot size, nor for any view effects (frontal vs. lateral) or interactions (group × time, group × view, time × view, or group × time × view) across the analyzed parameters (all p > .05). Only in the case of ratings for skin tone evenness significant differences were observed between expert and novice ratings (F (1, 54) = 5.24, p < .05, ηp² = .09). As Figure 3 shows, the variance was higher among novices, while the experts' ratings were more precise.

For instrumental analysis of distinctive spots (dL), the repeated-measures ANOVA showed a tendency for significance of week (F (5, 20) = 2.59, p < .10, ηp² = .39), however, none of the pairwise post-hoc comparisons remained significant after Holm correction (see Figure 4). In terms of changes in evenness (dE), neither ANOVA nor post-hoc tests revealed any significant changes.

The photo in Figure 5 illustrates exemplary the change in skin appearance between the initial condition and after the treatment routine.

4. Discussion

Visual ratings showed clear pre–post improvements across all pigment-related endpoints (Figure 2), indicating that treatment-related changes were readily detectable in the time-resolved image presentation. The absence of group, view, and interaction effects for most parameters suggests that these improvements were largely robust to rater’s expertise and viewing angle. The only exception was skin tone evenness (Figure 3), where experts and novices differed, consistent with evenness being a more holistic and potentially criterion-dependent judgment. A key limitation is that the outcomes were based on ordinal visual ratings averaged across multiple images per view, and the modest, unbalanced novice/expert sample sizes may have limited power to detect small group, view, or interaction effects. Since there was a high variability in the evenness ratings provided by non-experts and the instrumental colorimetric analysis did not reveal any significant results, we recommend evaluating evenness only by experts, as this yields the most accurate results.

Although the instrumental analysis of weekly colorimetric means of spots and evenness showed small fluctuations, the repeated-measures analyses did not provide significant evidence for a reliable time-dependent change across the measurement period, suggesting that any observed differences are more likely due to within-subject variability and the limited sample size than to a systematic effect of time. However, the identification and quantification of skin pigmentation depend strongly on the color-difference formulas used. While the dE values frequently reported in the literature describe the mathematically precise color difference as a vector in the Lab color space—accounting for the light/dark, red/green, and yellow/blue components—the “spot fraction” considers areas that vary in their gray value (dL). Using the CIELAB2000 color-difference metric, which has so far been used only to a limited extent in dermatology but is well established in colorimetry, enables a determination of color differences that most closely matches human perception, as it incorporates correction factors for the orange, blue, and saturation regions. For a more accurate assessment of melasma, evaluating the difference in the yellow component b is recommended, whereas erythema is best quantified by analysis of the red component a [23,24,26].

A subjectively or objectively increased visibility of pigmentation after an active ingredient–supported microdermabrasion treatment may have several causes. First, desquamation of the most superficial skin layers, whether induced by microdermabrasion or acid-based exfoliation, can reveal deeper-lying pigment while removing superficial pigmentation, which can be measured objectively. In addition, a reduction in baseline pigmentation and an increase in non-pigmented areas may lead to a stronger subjective perception of the remaining pigmented areas, because the contrast is enhanced and the observer visually focuses on these regions. Second, transient inflammation and a reduced intrinsic photoprotective capacity of the skin can potentiate the effects of UV radiation, which itself may exacerbate hyperpigmentation. Therefore, diligent photoprotection is essential, including during darker seasons [27].

As already described in the introduction, UV radiation is the primary trigger for pigmentation. During the study period, the UV index in the geographic region (Dortmund, Germany for the period 17 October to 22 December 2025.) ranged between 0.4 and 1.8 [28]. Participants were instructed to avoid UV exposure and use photoprotection, however, compliance could not be systematically assessed. Nevertheless, performing the procedure under different UV-index conditions may yield different results. As described in numerous microdermabrasion guidelines, treatment during the summer months is generally not recommended [29].

In this study, a standardized treatment and skincare protocol was applied to ensure comparability between participants. However, since each individual’s skin may respond differently at the clinical, structural, and inflammatory levels [7], we recommend -aiming for optimal outcomes- the investigation of biomarker substances and indicators to identify key milestones during regeneration and to address them with appropriate active ingredients.

Beyond directly pigment-modulating actives, the tested regimen also incorporated additional functional ingredient classes intended to support skin condition during repeated treatment and home care. These included antioxidants, soothing and barrier-supportive agents, regeneration-associated peptides, vitamins, hydroxy acids, humectants, and botanical oils and extracts (see Table 2 Appendix). Although the efficacy of individual formulations depends on concentration, vehicle properties, ingredient interactions, and application regimen, this classification provides a useful framework for understanding the intended cosmetic functions of the products used in the present study setting.

Antioxidants such as epigallocatechin gallate, superoxide dismutase, resveratrol, and thiotaine may help counteract oxidative stress through reactive oxygen species scavenging and support of endogenous defense systems [30,31], while polyphenolic antioxidants such as epigallocatechin gallate and resveratrol have also been discussed in the context of topical photoprotection [32,33]. Soothing and barrier-supportive ingredients such as bisabolol, Lactobacillus ferment, niacinamide, and dexpanthenol may help reduce visible irritation, support epidermal barrier function, and improve skin resilience [34,35,36,37]. Humectants including hyaluronic acid, glycerin, pentylene glycol, butylene glycol, urea, and biosaccharide gum contribute to stratum corneum hydration and may improve skin texture and viscoelasticity [38,39]. In parallel, lipid-based emollients and botanical oils may reduce transepidermal water loss and support barrier homeostasis [40,41,42], whereas peptides are commonly used to support renewal-related processes and structure-associated skin properties [43]. Hydroxy acids and related acids, including salicylic acid, glycolic acid, and tranexamic acid, address complementary cosmetic targets such as desquamation, surface smoothing, pore appearance, and tone uniformity [44,45,46]. Additional vitamins and botanical extracts may further contribute antioxidative, soothing, balancing, and skin-conditioning effects [47,48,49,50]. In addition, antioxidant plant extracts with high antioxidant capacity are not only believed to reduce cutaneous oxidative stress and skin aging [51] but also known for extending the oxidative stability of cosmetic emulsions [52].

Overall, the ingredient profile suggests a multimodal cosmetic strategy aimed not only at uneven pigmentation, but also at hydration, barrier support, antioxidative protection, soothing, and overall improvement in skin appearance.

5. Conclusions

Although the multifactorial design does not allow causal attribution to any single procedure step or active ingredient, over the course of four biweekly treatments and standardized home care, the combined microdermabrasion–cosmetic regimen was associated with visible improvements in pigment spot appearance and overall facial skin tone evenness. These changes were consistently reflected across time-resolved, randomized evaluations of standardized photographs by both trained estheticians and lay assessors, supporting the robustness of the observed improvement. However, instrumental analysis with former CIEDE calculations could not fully reproduce the consistent results. Therefore, integrating objective image-based metrics with blinded novine vs. expert perception-based ratings provides a more comprehensive representation of treatment benefit than colorimetry alone. However, instrumental colorimetry using CIEDE2000 calculation is recommended for future work. Taken together, these findings support the continued need for assessment by a trained expert, or at minimum a human evaluator, in evaluating the effectiveness of combination regimens targeting hyperpigmentation in cosmetic and dermatologic practice.