At least one national building supplies chain -- Atlanta-based Home Depot -- sells Philips D50 fluorescent lamps and standard fixtures at comparatively low prices. The Philips Colortone 48-inch 5,000°K fluorescent tube (F40T12/C50) meets the ISO specifications for D50. Unfortunately, as of this publication's deadline, Home Depot inexplicably stocked a different brand of D50/5,000°K lamp in the 24-inch (F20T12) and 18-inch (F15T8) sizes that are useful in smaller ceiling and under-cabinet luminaires -- the General Electric Sunshine lamps. According to General Electric, the Sunshine lamps (F20T12/C50/24 inch and F15T8/C50/18 inch) offered by Home Depot do not meet the D50 illuminant specifications
Figure 4: D50 and standard fluorescent 5,000°K bulbs can deliver a cost-effective daylight-balanced lighting solution.
Though a convenient source, Home Depot's use of two different brands of 5,000°K fluorescent lamps makes it difficult to standardize lighting when different lamp sizes are needed. You also should consider that individual commercial lamps from a mass-market assembly line might vary from nominal specifications unless every lamp is inspected or tested. This level of control is costly to implement, and it is one reason why the D50 lamps offered by companies such as Gretag-MacBeth are more expensive than ordinary 5,000°K lamps.
To some extent, replacing mass-market lamps long before usage reaches the 10,000- or 20,000-hour limits noted on packages can minimize this problem. In other words, for a low-cost solution that approximates D50 lighting, go with standard 5,000°K fluorescent lamps but change them out often -- say, every few thousand hours.
