What is a Wood's Lamp?

What is a Wood's Lamp?

A Wood's lamp is a dermatological diagnostic device that generates long-wave ultraviolet light (UV-A) with a wavelength of 320-400 nanometers by using a filter containing nickel oxide. Often referred to as a "microscope" for the skin, it aids in observing lesions in the epidermis and superficial dermis.

The device operates based on the differential fluorescent reactions of various skin conditions under ultraviolet light. For example, Cutibacterium acnes exhibits a brick-red fluorescence, vitiligo lesions appear pure white, and active tinea versicolor shows a yellowish-white or copper-orange fluorescence. The examination is non-contact, offering advantages of being non-invasive and rapid. It is applicable in scenarios such as defining the borders of vitiligo, classifying melasma, differentiating tinea capitis, and assisting in the diagnosis of skin tumors. Furthermore, its fluorescent characteristics can help assess the degree of melanin loss, distinguish between complete and incomplete vitiligo, and aid in locating tumor boundaries for photodynamic therapy.

Scope of Use for Wood's Lamp

As a detection tool in clinical dermatology, its applications are extensive, including: acne, tinea capitis, white piedra, tinea versicolor, erythrasma, trichomycosis axillaris; certain pigmentary disorders such as melasma, age spots, and café-au-lait spots; as well as neurofibromatosis and skin tumors.

Known as the skin's "microscope," when used for acne detection, it can accurately determine the aggregation degree of C. acnes. C. acnes produces protoporphyrin, which emits a brick-red fluorescence under Wood's lamp. This fluorescence is detectable, particularly more easily observable in the follicles of the nasolabial folds.

The Wood's lamp is a standard instrument for identifying vitiligo. It can accurately detect the amount of melanin loss, distinguish between complete and incomplete vitiligo, and also reveal lesions that have already occurred but are invisible to the naked eye.

Detection Principle

The Wood's lamp utilizes long-wave ultraviolet light (320-400 nm) obtained by passing light through a filter containing nickel oxide (or nickel hydride) to examine certain skin diseases, aiding in their diagnosis and treatment. The affected area is irradiated with this long-wave UV light. Areas with reduced melanin reflect light more strongly, appearing lighter, while areas with increased melanin reflect light weakly, appearing darker.

The Wood's lamp assists in the diagnosis of pigmentary skin diseases, including vitiligo. It is sometimes clinically difficult to detect faint spots on normal skin, especially fair skin, with the naked eye. The Wood's lamp can easily detect vitiligo and differentiate it from other hypopigmented conditions. This is particularly useful when follicular repigmentation begins in a vitiligo patch, as the early stages of repigmentation may not be apparent under natural light but can be confirmed using the Wood's lamp.

Under the Wood's lamp, vitiligo lesions appear as pure white, contrasting sharply with the surrounding normal skin, with clear borders. In contrast, achromic nevus, pityriasis alba, tuberous sclerosis, post-inflammatory hypopigmentation, and leprosy hypopigmented patches appear yellowish-white or grayish-white under Wood's lamp; tinea versicolor appears brownish-yellow or yellowish-white; the faint white patches of nevus anemicus do not become apparent.

Operation Procedure

1. Connect the Wood's lamp power cord to the power source. Press the switch, and the lamp will light up after a few seconds.

2. The examination should be conducted in a dark room, often viewed through a magnifying lens.

Examination Methods

1. Conditions like achromic nevus, pityriasis alba, tuberous sclerosis, post-inflammatory hypopigmentation, and leprosy hypopigmented patches appear yellowish-white or grayish-white under Wood's lamp. Tinea versicolor appears brownish-yellow or yellowish-white. The faint lesions of nevus anemicus do not become visible.

2. When friction causes the skin to redden, observe whether the white patch reddens similarly or if it appears more prominent against the red background. The latter suggests nevus anemicus.

3. Perform skin sensation tests, including temperature, pain, and touch. Sensation is typically normal in patients with vitiligo, whereas hypopigmented patches in leprosy often involve impaired superficial sensation, including diminished or absent sensation.

4. The Koebner phenomenon (isomorphic response) in vitiligo can be tested. On normally pigmented skin (e.g., the deltoid area), after routine disinfection with alcohol, make a crisscross scratch about 1 cm in size with a sterilized vaccination needle. Examine the site after one month. Depigmentation indicates a positive result, while no change indicates negative. A positive Koebner reaction often suggests a spreading type of vitiligo, which may respond less well to photochemotherapy but better to corticosteroid treatment.

Advantages

1. The procedure does not involve direct contact with the examination site, offering advantages such as being painless and non-infectious, making it suitable for patients of all ages.

2. Hailed as the skin's "microscope," the Wood's lamp can accurately detect the degree of melanin loss, distinguish between complete and incomplete vitiligo, and screen for nascent lesions invisible to the naked eye.

3. The Wood's lamp simplifies the diagnosis of vitiligo. The confirmation process involves no blood drawing, is painless, and provides fast and accurate results.

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