**Core Concept**
Wood's lamp is a diagnostic tool that emits ultraviolet (UV) light, used to detect various conditions such as fungal infections, vitiligo, and porphyria. It is essential to understand the composition of the lamp to appreciate its uses.

**Why the Correct Answer is Right**
The correct answer, **D. 9% nickel oxide with barium silicate**, is the composition of the filter used in a Wood's lamp. The filter is designed to produce a specific wavelength of UV light that is not too intense, making it safe for use on human skin. The combination of nickel oxide and barium silicate is crucial for achieving the desired spectral output. The presence of nickel oxide helps to reduce the intensity of the UV light, while barium silicate enhances the emission of the desired wavelength.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the composition of the filter is not 9% nickel oxide with 6% BuSO4 (potassium sulfate). The presence of potassium sulfate is not necessary for the functioning of the Wood's lamp.
**Option B:** This option is incorrect because the composition of the filter is not 9% nickel oxide with 9% BuSO4. The percentage of BuSO4 is not relevant to the functioning of the Wood's lamp.
**Option C:** This option is incorrect because it lacks specific details about the composition of the filter. The correct answer requires a specific combination of nickel oxide and barium silicate.

**Clinical Pearl / High-Yield Fact**
When using a Wood's lamp, it is essential to remember that the lamp emits UV light, which can cause damage to the skin if used excessively. Therefore, it is crucial to follow proper safety protocols when using the lamp.

**Correct Answer:**
✓ Correct Answer: D. 9% nickel oxide with barium silicate