correlated color temperature Electric light sources are rated for visual color appearance in degrees Kelvin, i.e., 3000°K. This rating is called ‘Correlated Color 9000° Temperature’ (CCT). This is a Northlight / relative comparison with the Blue Sky color given off by an incandes- 8500° cent tungsten lighting filament when heated to that temperature. 8000° This makes it possible to create a standard color reference 7500° Overcast for describing the color of light Sky from a lamp. When a lamp is 7000° designated as being ‘3000°K’, it does not mean anything in the 6500° lamp is that temperature. It only "Daylight" means the light is the same color Fluorescent as an incandescent filament at Mercury 6000° that temperature. Electric lights now come in 5500° Summer CCTs ranging from 1900°K to Sunlight 20,000°K. Choosing a CCT is 5000° "Cool White" partly design, and partly behav- Fluorescent ioral science. CCT choices greatly Metal 4500° (4200°K) influence how a space will ‘feel’. Halide 4100°K Lower CCTs have more red 4000° 4100°K light, while higher CCTs have more blue light. Some designers 3500°K Tungsten 3500°K may refer to redder sources as 3500° Halogen ‘warm’, and bluer sources as 3000°K ‘cool’, causing some confusion 3000° At right is a chart showing comparisons of CCTs of a range 2500° of light sources. 40 W High Incandescent 2000° Pressure Sodium

Candle 1500° Color color rendering index (cri)

‘Color rendering’ (CRI) describes the effect a light source has on an object’s color. Light sources with low (poor) CRI will distort or Color Rendering Index wash out colors. Higher CRI light sources will show a fuller color depth of the object. In most cases, CRI is a scale from 50 to 100. When CRI was invented, ‘warm white’ fluorescent lamps were the Halogen bottom of the scale, and incandescent sources were the top. (Incandescent) 100 In rating a lamp for CRI, reference colors are viewed under a HQR and ‘C’ type 95 light source, and the distortions in the reference colors are measured. fluorescent These distortions are used to create an overall rating of color accuracy 90 for that light source, in relation to other light sources of the same Deluxe Cool White Correlated Color Temperature (CCT). CRI comparisons are not valid 85 for lamps of different CCTs.

The higher the CRI, the lower the level of distortion of colors un- RE 80 lamps 80 der the light. The CRI superiority of incandescent sources is no longer as great today as it was in the past. Fluorescent and HID sources are 75 now available with CRI ratings at or above 95, narrowly approaching the 100 of incandescent sources. RE 70 lamps 70 The CRI system is far from perfect — CRI comparisons between different color temperatures are not technically meaningful. Short- Standard 65 comings of a light source are not included (for instance, the 100 CRI Metal Halide 60 of incandescent is noticeably lacking in blue frequencies). Choosing, Deluxe HPS and especially matching, colors can become a tremendously compli- cated process. 55 ‘Cool White’ At the deepest levels of inquiry into the nature of color qualities, Fluorescent 50 there are few straightforward answers to some questions. The color that you ‘see’ is produced by the interaction of 3 variables: ‘Warm White’ 45 • The color frequencies in the light falling on an object Fluorescent • The color qualities of the object. (What light frequencies does the 40 object reflect, and which does it absorb?) Mercury Vapor 35 • How well the observer’s visual system processes and interprets color (Are you capable of discerning color?) 30 As a general rule, lamps that provide very high levels of color ac- curacy have traditionally done so at the cost of energy efficiency. One 25 of the breakthroughs of rare earth phosphor lamps (such as compact fluorescent and T-8’s) is that they not only provide high lumens per 20 watt performance, but very good CRI as well. Plus, research indicates that higher CRI (RE80 and RE90) lamps provide greater task visibility 15 than lower CRI lamps. The most fundamental advice on color temperature and CRI is 10 — pick the most flattering (or suitable) color temperature for your Standard HPS space, and then buy the highest CRI lamp in that color temperature 5 that your budget can afford.

sponsored by: northwest energy efficiency alliance seattle city light u.s. department of energy british columbia hydro Low Pressure state of alaska Sodium (LPS) — 44