Spectral Power Distribution (SPD) curves are essential tools in lighting science. They show the power emitted by a light source at each wavelength across the visible spectrum, typically from 380 nm to 780 nm. Understanding SPD curves helps designers, engineers, and enthusiasts evaluate light quality, color rendering, and application suitability.

Natural daylight has a smooth, continuous SPD with balanced energy across the spectrum, producing excellent color rendering. Incandescent bulbs emit a warm, continuous spectrum skewed toward red and infrared, giving objects a warm appearance. Fluorescent lights have a spiky SPD with narrow peaks from mercury vapor lines, often causing poor color accuracy despite high efficiency.

LEDs offer customizable SPDs. Many white LEDs combine a blue pump with a yellow phosphor, creating a broad peak around 450 nm and a secondary hump near 560 nm. This design yields decent color rendering, but higher-quality LEDs use multiple phosphors to fill spectral gaps, improving the Color Rendering Index (CRI).

Interpreting an SPD curve involves noting peak positions, spectral gaps, and overall shape. A curve with high intensity in the blue region may appear cool, while strong red content gives a warm feel. Gaps around 500 nm can cause a greenish hue, while missing red weakens red object appearance.

In horticulture, SPD guides plant growth—blue light promotes foliage, red light supports flowering. In museums, full-spectrum SPD preserves artwork. For general lighting, a smooth curve with high CRI values (above 90) is preferred.

By mastering SPD interpretation, you can choose or design lights that match your visual and functional needs. Always check the SPD curve, not just color temperature or lumens, for true light quality assessment.