Eye Sensitivity under Scotopic (Rod) and Photopic (cone) Conditions The Figure below shows the eye sensitivity under scotopic and photopic conditions. Eye sensitivity shift depending on lighting conditions. The rods are most sensitive to the blue-green portion of the visible spectrum (wavelength λ=450-570 nm). So, in Scotopic vision (low light or ”night”) conditions when rods are fully responsible for vision, blue-green light is most effective.

The cones are most sensitive to the green-yellow portion of the visible spectrum (wavelength λ=500-620 nm). . So, in photopic vision (high light or “day”) conditions when cones are primarily responsible for vision, green-yellow light is most effect. NB: Before specifying the most efficient lamps available, their color rendering and spectral power distribution (SPD) should be assessed first.

Spectral Power Distribution (SPD): A measure of the power or intensity of electromagnetic energy present in a given light source. Typically reported in graph form showing the relative intensity of the various wavelengths of energy produced by a given light source. Some light sources like, daylight on a sunny day, have a rather complete or full spectrum of visible (all colors of light are present). Other light sources like blacklights, have a very limited spectrum of light (mostly ultraviolet and deep violet electromagnetic energy is present in the SPD of blacklights). Therefore lighting designers should recognize the implications on how people perceive color. If a blacklight (rich in violet light ) is aimed onto red surface, the red surface will appear black since the red pigment is capable of reflecting only red light that strikes it, yet the black light only produces violet light. SPD is responsible, then, for how colors are rendered.

SPD Curve for GE’s 830 fluorescent Lamp (Triphosphor Lamp). The SPD curve shows a spectrum plot of relative power (or intensity) of wavelengths versus wavelength. The lamp composed of three phosphor layers. Each layer produces a specific color range of light (in this case, orange, yellow-green, and blue). The orange, yellow-green, and blue combine to make a relatively full spectrum white light. Because of little violet and red light, violet and red materials will appear dulled.

Color Temperature: It’s a measure of the whiteness of light produced by a given lamp with 0K as black (producing no light). A candle flame has a color temperature of about 2000K. A standard incandescent lamp has a color temperature of about 2700K, while the new, crisp white halogen lamps typically have a color temperature of about 3000K. Ceramic Metal Halide (CMH) Lamps: Are a part of the HID (high intensity discharge) family of lamps. CMH lamps are the most efficacious white light point sources today exhibiting excellent color rendering, long life, and available in a warm white 3000 K color temperature.