Why the large recent increase in attention to LEDs and solid state lighting? The cost of energy and the amount of carbon emissions is one answer. Lighting consumes 22% of the electricity produced in the US and 8% of the energy. The impact of SSL and other alternative lighting sources by 2025 is projected to be a 50% decrease of energy consumption by lighting and a 10% reduction in carbon emissions.1 The Department of Energy and many manufacturers are very interested in increasing the efficiency conversion of electrical power to light output. This is the key performance factor that, depending on the application, determines the level of energy-efficiency.
For applications where LEDs are used as sources of light that are viewed directly, the term luminance intensity is used (i.e. units of lumen/steradian, also known as candela). For example, in a traffic intersection signal light, the important parameter is the light emitted into a solid cone angle. For applications where LEDs are used as general illumination, the total light emitted in all directions, the total luminous flux (units of lumen), is the metric of interest. The efficiency of the conversion of electrical power into light has a specific term, luminous efficacy, and a defined relationship of measurable parameters: 1) optical power in watts, 2) electrical power in watts, and 3)luminous flux in lumens.2
New developments in the detectorbased standards have improved the ability to determine efficiency and luminous efficacy by lowering the uncertainty of easy-to-use working standards.3 These detector-based standards consist of a high-precision current-to-voltage amplifier, temperature-stabilized, silicon photodiode and temperature- stabilized spectral filter that makes the filter-detector combination very closely match the CIE 1931 standard photometric observer function. The new working standard improved the performance of components contributing to the total measurement uncertainty by factors of between 2 and 5; this has led to a reduction of total uncertainty level in the candela by a factor of 3. These detector-based standards can be used to directly determine the average LED intensity of sources using the CIE publication 127 recommended geometries.4