Light and Color
Spectral Power Distribution

SPD Curves–Precise Lamp Profiles
A spectral power distribution (SPD) curve shows the precise color output of a given light source by charting the level of energy present at each wavelength across the visible spectrum. An SPD diagram of sunlight at midday, for example, clearly shows it to be an exceptionally balanced light source—all wavelengths of visible light are present in nearly equal quantities. Logically, an even curve like this indicates outstanding color rendering abilities. When compared to artificial light sources, sunlight exhibits large amounts of energy in the blue and green portions of the spectrum, making it a cool light source with a high color temperature (5500K). SPD diagrams can also be very useful in understanding how various lamps differ in the color composition of their light output. 

Incandescent Technology
All incandescent light is produced by heating a solid object—the filament—until it radiates light. In a sense, this is the way light is produced by the sun. Therefore both standard incandescent and halogen lamps exhibit smooth, even SPD curves, not unlike sunlight. Because the blackbody radiator used to measure color rendering also generates light in this fashion, incandescent lamps score very high in CRI ratings. This does not mean, however, that they render all colors in an identical manner. Standard incandescent lamps produce very little radiant energy in the short wavelength end of the spectrum and therefore do not render blues very well. Halogen lamps have a flatter curve with more energy at lower wavelengths, so they brighten blue and violet colors.

HID Technology
The electric arc that powers any gaseous discharge light source tends to produce narrow bands of energy at specific wavelengths. The SPD curves for HID lamps therefore feature spectral energy spikes related to these “resonance lines.” The addition of halides to the arc stream of a METALARC® 150W lamp allows it to achieve a fairly balanced curve. However, note that it is still somewhat uneven. The SPD diagram explains why standard metal halide lamps achieve good, but not exceptional, CRI ratings. In actual use, metal halide lamps often provide better color response than their CRI measurements indicate. The SPD curve for a LUMALUX® lamp shows virtually all of the spectral energy in the yellow/orange range. Clearly, this lamp cannot render a broad range of colors, which is why high pressure sodium lamps are not recommended unless color rendition is not critical.