Introduction to Wood's Lamp Examination

A Wood's lamp is a diagnostic tool that emits long-wave ultraviolet A (UVA) light, typically in the 365-395 nanometer range, used extensively in dermatological practice. This handheld device creates a dark environment for examination, causing certain skin conditions to fluoresce with distinctive colors when exposed to the UVA radiation. The fundamental principle behind this phenomenon involves the interaction between UVA light and various chemical components in the skin, including porphyrins, collagen, and melanin. When these substances absorb the ultraviolet energy, they emit visible light through photochemical reactions, creating characteristic fluorescence patterns that help clinicians identify specific dermatological conditions.

The development of Wood's lamp dates back to 1903 when physicist Robert W. Wood invented the original glass filter that blocks visible light while transmitting ultraviolet radiation. Initially used in forensic science to detect forged documents and invisible inks, medical applications emerged in 1925 when Margarot and Deveze discovered its utility in diagnosing tinea capitis. The technology has evolved significantly since then, with modern Wood's lamps incorporating LED technology for improved safety and consistency. Contemporary devices feature precise wavelength control, reduced heat emission, and enhanced portability, making them indispensable in clinical wood lamp dermatology practice.

The scientific mechanism underlying Wood's lamp examination involves fluorescence - the property of certain substances to absorb light at specific wavelengths and re-emit it at longer wavelengths. In dermatological applications, UVA light penetrates the epidermis and upper dermis, interacting with various chromophores. Bacterial porphyrins from Corynebacterium minutissimum exhibit coral-red fluorescence, while Malassezia yeast in tinea versicolor produces yellow-green fluorescence due to dicarboxylic acids. Melanin abnormalities appear as enhanced contrast in hypopigmented disorders like vitiligo. The specific fluorescence patterns provide valuable diagnostic clues that are often invisible to the naked eye under normal lighting conditions.

Applications in Dermatology

Diagnosing Fungal Infections

Wood's lamp examination demonstrates exceptional utility in diagnosing superficial fungal infections, particularly those caused by dermatophytes and yeasts. In tinea capitis, Microsporum species produce a characteristic bright green fluorescence due to pteridine compounds secreted by the fungi. Hong Kong epidemiological data from the Hospital Authority reveals that approximately 15-20% of pediatric scalp infections show positive Wood's lamp fluorescence, primarily caused by M. canis and M. audouinii. For tinea versicolor, the lamp reveals golden-yellow or coppery-orange fluorescence in 80-90% of cases, resulting from azelaic acid production by Malassezia species. This immediate visualization allows for rapid diagnosis and targeted treatment, significantly reducing the need for time-consuming fungal cultures in typical presentations.

Identifying Bacterial Infections

Bacterial skin infections present distinctive fluorescence patterns under Wood's lamp examination. Erythrasma, caused by Corynebacterium minutissimum, exhibits brilliant coral-red fluorescence due to bacterial coproporphyrin III production. Studies conducted at Hong Kong's dermatology clinics indicate that Wood's lamp correctly identifies approximately 90% of erythrasma cases, with the chest, axillae, and groin being the most commonly affected areas. Pseudomonas aeruginosa infections, particularly in burn wounds or chronic ulcers, demonstrate green fluorescence resulting from pyoverdine and pyocyanin pigments. This bacterial identification capability proves crucial for differential diagnosis, as erythrasma can be mistaken for fungal infections or contact dermatitis under normal examination conditions.

Detecting Pigmentary Disorders

Pigmentation abnormalities show enhanced contrast under Wood's lamp examination due to differential melanin absorption of UVA radiation. Vitiligo lesions appear as bright blue-white or milky-white areas with sharply demarcated borders, making subclinical depigmentation visible that might be missed during routine examination. Melasma typically demonstrates enhanced pigmentation with increased epidermal melanin appearing darker brown, while dermal melanin shows less enhancement. A 2022 study from Hong Kong University's Dermatology Department found that Wood's lamp examination altered treatment approaches in 35% of pigmentary disorder cases by accurately determining melanin depth and distribution patterns. This precision enables customized therapeutic strategies based on the specific pigment localization.

Examining Acne and Sebaceous Gland Activity

Wood's lamp provides valuable insights in acne vulgaris assessment by highlighting follicular porphyrins produced by Cutibacterium acnes. These bacterial metabolites exhibit orange-red fluorescence in pilosebaceous units, indicating bacterial colonization and activity. The intensity of fluorescence often correlates with sebum production levels and inflammatory activity. In clinical practice, this helps differentiate between comedonal and inflammatory acne, monitor treatment response, and identify subclinical inflammation. Some dermatologists utilize this fluorescence to guide photodynamic therapy, as porphyrins act as endogenous photosensitizers. The examination also reveals differences in sebaceous gland distribution and activity across facial zones, informing targeted treatment approaches.

Procedure and Interpretation

Preparing for the Examination

Proper preparation ensures accurate Wood's lamp examination results. The procedure requires a completely dark room to maximize fluorescence visibility, typically achieved with blackout curtains or dedicated examination spaces. Patients must remove all makeup, moisturizers, topical medications, and deodorants at least 24 hours before examination, as these products may contain fluorescent compounds that cause false-positive results. Clothing and jewelry that might interfere with the examination should be removed from the area of interest. The Wood's lamp should warm up for approximately one minute to achieve stable UVA output, and the examiner should dark-adapt their eyes for 2-3 minutes to enhance fluorescence detection sensitivity. Documentation equipment, including cameras with appropriate filters, should be prepared to record findings for comparison during follow-up visits.

Performing the Examination: Step-by-Step Guide

The Wood's lamp examination follows a systematic approach to ensure comprehensive assessment. Begin by positioning the lamp 10-15 centimeters from the skin surface at a 45-degree angle to minimize glare. Methodically scan the area of concern using slow, overlapping motions to ensure complete coverage. Compare affected areas with adjacent normal skin and contralateral body regions to establish baseline fluorescence. For scalp examinations, part the hair in multiple directions to visualize the entire scalp surface. Examination duration should not exceed 5 minutes continuously to minimize UVA exposure. When using a dermatologist microscope in conjunction with Wood's lamp, initial gross examination precedes detailed microscopic assessment of specific fluorescent areas. Document findings with descriptive terminology and photographic evidence when possible.

Interpreting Results: Different Colors and Their Meanings

Fluorescence interpretation requires understanding the correlation between colors and underlying pathological processes:

• Blue-white: Vitiligo, tuberous sclerosis (ash-leaf spots), hypopigmented scars
• Bright green: Tinea capitis (Microsporum species), pseudomonas infection
• Golden-yellow: Tinea versicolor, pityriasis versicolor
• Coral-red: Erythrasma, porphyria cutanea tarda
• Orange-red: Acne vulgaris (follicular porphyrins), topical medication residues
• Purple: Normal skin fluorescence
• Brown-black: Enhanced epidermal pigmentation (lentigines, ephelides)
• Pink-orange: Increased collagen visibility in scars or sclerotic conditions

Approximately 5-10% of healthy individuals exhibit minimal background fluorescence, primarily in seborrheic areas. Interpretation should always consider clinical context, as some fluorescence patterns overlap between conditions.

Limitations of Wood's Lamp Examination

Despite its diagnostic value, Wood's lamp examination presents several limitations. False-negative results occur in tinea capitis caused by Trichophyton species, which don't produce fluorescent metabolites, representing approximately 70% of cases in Hong Kong according to recent surveillance data. Sun exposure, recent washing, or application of quenching substances may diminish fluorescence intensity. Pigmented skin may show reduced fluorescence contrast, particularly in mild pigmentary disorders. The examination has limited penetration depth, primarily assessing superficial epidermal conditions. Some medications, chemicals, and fabrics produce confounding fluorescence. Additionally, operator experience significantly impacts interpretation accuracy, with studies showing 20-30% variation in findings between novice and experienced practitioners. These limitations necessitate correlation with clinical history and additional diagnostic methods when indicated.

Wood's Lamp vs. Other Diagnostic Tools

Comparison with Dermatoscopy and Biopsy

Wood's lamp, dermatoscopy, and biopsy represent complementary diagnostic approaches with distinct advantages and applications. While Wood's lamp excels at detecting specific fluorescent substances and pigment contrast, dermatoscopy provides detailed morphological analysis of skin structures. dermal nevi dermoscopy enables visualization of pigment networks, vascular patterns, and specific features like blue-white veil or maple-leaf-like structures that help differentiate benign from malignant lesions. Dermatoscopy achieves approximately 90% sensitivity in melanoma detection compared to 65-70% with naked-eye examination alone. Biopsy remains the gold standard for histopathological confirmation but is invasive, time-consuming, and carries infection and scarring risks. The table below compares these diagnostic modalities:

Feature	Wood's Lamp	Dermatoscopy	Biopsy
Invasiveness	Non-invasive	Non-invasive	Invasive
Immediate Results	Yes	Yes	No (3-7 days)
Cost	Low	Moderate	High
Primary Application	Fluorescence detection	Morphological analysis	Histopathological diagnosis
Operator Dependency	Moderate	High	Low (pathologist interpretation)

When to Use a Wood's Lamp vs. Other Methods

Wood's lamp serves as an excellent first-line diagnostic tool for specific indications where fluorescence patterns provide immediate diagnostic information. It proves particularly valuable for screening suspected tinea capitis in school outbreaks, rapid erythrasma diagnosis in intertriginous areas, and vitiligo assessment for subclinical lesions. Dermatoscopy becomes preferable when evaluating pigmented lesions, vascular anomalies, or inflammatory dermatoses requiring detailed morphological analysis. Biopsy remains necessary for definitive diagnosis of neoplasms, bullous disorders, or when therapeutic decisions depend on histopathological confirmation. In many clinical scenarios, sequential use of these modalities provides optimal diagnostic accuracy - beginning with Wood's lamp for initial screening, followed by dermatoscopy for detailed examination of suspicious findings, and reserving biopsy for cases requiring pathological confirmation. This multimodal approach maximizes diagnostic yield while minimizing unnecessary invasive procedures.

The Role of Wood's Lamp in Modern Dermatology

Despite technological advancements in dermatological diagnostics, Wood's lamp maintains significant clinical relevance in contemporary practice. Its unique ability to detect specific fluorescent patterns provides immediate diagnostic information that complements newer technologies. The device offers exceptional cost-effectiveness, with initial equipment costs approximately 80-90% lower than advanced imaging systems, making it accessible across diverse healthcare settings. Portability enables use in outpatient clinics, bedside examinations, and remote medical services. Modern iterations incorporate LED technology with consistent UVA output, reduced heat generation, and improved safety profiles. Integration with digital imaging systems allows for documentation and serial comparison of fluorescent findings. While advanced modalities like reflectance confocal microscopy provide superior resolution, Wood's lamp remains unsurpassed for specific applications like porphyrin detection and rapid screening of fungal infections. Its continued inclusion in dermatological training programs worldwide underscores its enduring utility. As dermatology embraces technological innovation, Wood's lamp persists as a valuable tool that bridges clinical observation with specialized diagnostic capabilities, maintaining its position as an essential component of comprehensive dermatological assessment.

By:Jane