Light output can be measured using the "flash energy" method, and a distinction can be made between halogen and strobe lights. Tests indicate that halogen lights meet or exceed the total flash energy of strobe signals. Another factor in determining the effectiveness of a warning light is its color. A variable to consider when selecting the most effective color is transmittance, the amount of light that will pass through a colored filter. An amber filter allows 60 percent of the halogen light to pass through, red filters allow about 25 percent, and blue filters allow about 15 percent. Surveys have shown that the human eye is more sensitive to red in the day and to blue at night, although amber light should not be overlooked as an effective warning signal. Color temperature describes how many of the various colors in the light spectrum are contained in a given light source. Halogen light has a color temperature in the 2,000 to 3,000 degree Kelvin range. Strobe light sources, on the other hand, have a much higher color temperature. As a result, strobe lights tend to blend with daylight and often appear very dim in bright daylight. For maximum impact, a warning light signal needs to contain a very high flash energy to allow the light to be seen from a distance and a very high activity level so that it can be quickly picked up by peripheral vision. The design of effective halogen and strobe light systems is discussed and compared.