A Woods lamp examination is a diagnostic test used to detect certain skin conditions. During this test, a doctor shines an ultraviolet lamp, called a Woods lamp, on areas of a patient’s skin. The lamp emits long wave ultraviolet light that causes certain substances or organisms in the skin to fluoresce or glow. This fluorescence helps the doctor identify features that are often not visible under normal light. An orange color under the Woods lamp has specific meaning and indicates certain skin findings.
What is a Woods Lamp?
A Woods lamp is a handheld ultraviolet lamp that emits long wave UV light at a wavelength of 360-400 nanometers. It was invented in 1903 by Robert Williams Wood, an American physicist and inventor. The lamp is also known as a blacklight. The Woods lamp produces ultraviolet light just beyond the visible violet end of the light spectrum. When shone on some materials, this long wave UV light causes them to absorb the energy and re-emit it as visible light, resulting in fluorescence. While the lamp emits UV light, the visible fluorescent light it produces allows it to be used to examine the skin safely.
How Does a Woods Lamp Work?
When a Woods lamp is held near the skin and turned on, the UV light it produces interacts with certain substances in or on the skin in different ways:
- Some skin components like oils and proteins absorb the UV light and emit fluorescence, glowing visibly under the lamp.
- Other substances like bacteria or fungi reflect the UV light, also becoming fluorescent.
- Normal skin generally does not fluoresce under the lamp.
- The color and intensity of the fluorescence depends on the particular substance.
This fluorescence makes the substance easier to see, helping diagnose some skin conditions. The color of the fluorescence provides clues to identify the specific skin finding.
Why is the Woods Lamp Used?
The Woods lamp allows visualization of certain features of skin lesions that cannot be seen under normal white light. It is useful for diagnosing:
- Fungal infections – Some fungal species fluoresce green, red, orange, or yellow.
- Bacterial infections – Some bacteria glow blue, yellow, or red under the lamp.
- Certain pigment disorders – Hypopigmented patches like vitiligo lose pigment and may glow bright white or blue.
- Some skin cancers – Basal cell carcinomas often glow pale yellow.
- Scabies mite infestations – Mite droppings and eggs fluoresce yellow-green.
- Sebum deposits – Skin oils may fluoresce orange.
- Porphyrins – These compounds may build up in lesions and fluoresce red, pink, or coral.
By revealing these features, the Woods lamp allows easier, faster diagnosis of many dermatological conditions without needing a biopsy or culture.
What Does Orange Mean Under a Woods Lamp?
An orange color under the Woods lamp has two main possible causes:
1. Porphyrins
Porphyrins are compounds produced as waste products of protein metabolism. They can build up abnormally in the outer layer of skin and fluoresce orange or pink under UV light. Increased porphyrins cause lesions to glow orange under a Woods lamp. Some conditions that may cause excess porphyrins include:
- Bacterial infections like impetigo or cellulitis
- Fungal infections like tinea versicolor
- Viral infections like herpes simplex or varicella zoster (shingles)
- Acne and acne scarring
- Skin injuries like abrasions, burns, or contact dermatitis
Orange porphyrin fluorescence in lesions often indicates an inflammatory process or infection that should be evaluated and treated.
2. Sebum
Sebum is an oily, waxy substance secreted by sebaceous glands in the skin. It helps moisturize and protect the skin and hair. Sebum is made up of lipids like triglycerides, fatty acids, and squalene. It may accumulate excessively in clogged pores and sebaceous cysts. The lipids in sebum fluoresce orange when illuminated under a Woods lamp.
Areas of orange sebum fluorescence may indicate:
- Oily skin or seborrheic dermatitis
- Open or closed comedones (whiteheads and blackheads)
- Cysts – sebum-filled sacs under the skin
Orange sebum fluorescence is generally benign. But it can signal blocked pores, follicles, or oil glands that may become infected or inflamed.
How is Orange Fluorescence Interpreted?
Doctors use the pattern, location, and shade of orange fluorescence under a Woods lamp together with the patient’s symptoms and medical history for diagnosis. Some key points of interpretation include:
- Pattern – Splashes of orange indicate sebum, while a diffuse orange glow suggests porphyrins.
- Location – Orange on the nose, cheeks and central face points to sebum, while widespread orange suggests porphyrins.
- Shade – Dark or bright orange implies porphyrins while pale orange signifies sebum.
- Lesion features – Sebum correlates with bumps and plugged pores, porphyrins with sores, blisters, or scabs.
Other aspects of the skin exam are considered as well. The following table summarizes interpretation of orange fluorescence with a Woods lamp:
| Fluorescence Pattern | Likely Source | Possible Diagnoses |
|---|---|---|
| Splattered or patchy orange | Sebum | Oily skin, clogged pores, comedones, cysts |
| Diffuse weak orange | Porphyrins | Bacterial, viral, or fungal infections |
| Intense orange | Porphyrins | Skin injuries, ulcers, inflammation |
Uses and Limitations of Orange Fluorescence
Identifying orange fluorescence on the skin can assist with diagnosing some conditions. However, the Woods lamp has limitations. Orange fluorescence alone does not confirm a definite diagnosis. It must be interpreted in context with examination findings and patient factors. Non-fluorescing conditions may still be present. Skin biopsy may still be needed for a final diagnosis. And some normal structures like eyebrows, eyelashes, moles, veins, or hair follicles may fluoresce orange as well.
Conclusion
Orange fluorescence seen when examining the skin under a Woods lamp has two main causes – porphyrins and sebum. Porphyrins produce an orange glow indicating infections, inflammation or skin injury that requires treatment. Sebum fluoresces orange due to lipids and fats in clogged pores, comedones or oil cysts. The pattern, distribution, exact shade, and associated skin features provide clues to determine whether orange fluorescence is due to porphyrins or sebum buildup. This can help diagnose some skin problems. But Woods lamp findings alone are not definitive, and other aspects of the exam and the patient’s clinical presentation must be considered as well.
